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#2 Re: This is Cool » Miscellany » Today 00:25:47

417) X-rays

X-rays are types of electromagnetic radiation probably most well-known for their ability to see through a person's skin and reveal images of the bones beneath it. Advances in technology have led to more powerful and focused X-ray beams as well as ever greater applications of these light waves, from imaging teensy biological cells and structural components of materials like cement to killing cancer cells. 

X-rays are roughly classified into soft X-rays and hard X-rays. Soft X-rays have relatively short wavelengths of about 10 nanometers (a nanometer is one-billionth of a meter), and so they fall in the range of the electromagnetic (EM) spectrum between ultraviolet (UV) light and gamma-rays. Hard X-rays have wavelengths of about 100 picometers (a picometer is one-trillionth of a meter). These electromagnetic waves occupy the same region of the EM spectrum as gamma-rays. The only difference between them is their source: X-rays are produced by accelerating electrons, whereas gamma-rays are produced by atomic nuclei in one of four nuclear reactions.

History of X-rays

X-rays were discovered in 1895 by Wilhelm Conrad Röentgen, a professor at Würzburg University in Germany. According to the Nondestructive Resource Center's "History of Radiography," Röentgen noticed crystals near a high-voltage cathode-ray tube exhibiting a fluorescent glow, even when he shielded them with dark paper. Some form of energy was being produced by the tube that was penetrating the paper and causing the crystals to glow. Röentgen called the unknown energy "X-radiation."

Experiments showed that this radiation could penetrate soft tissues but not bone, and would produce shadow images on photographic plates.

For this discovery, Röentgen was awarded the very first Nobel Prize in physics, in 1901.

X-ray sources and effects

X-rays can be produced on Earth by sending a high-energy beam of electrons smashing into an atom like copper or gallium, according to Kelly Gaffney, director of the Stanford Synchrotron Radiation Lightsource. When the beam hits the atom, the electrons in the inner shell, called the s-shell, get jostled, and sometimes flung out of their orbit. Without that electron, or electrons, the atom becomes unstable, and so for the atom to "relax" or go back to equilibrium, Gaffney said, an electron in the so-called 1p shell drops in to fill the gap. The result? An X-ray gets released.

"The problem with that is the fluorescence [or X-ray light given off] goes in all directions," Gaffney told Live Science. "They aren't directional and not focusable. It's not a very easy way to make a high-energy, bright source of X-rays."

Enter a synchrotron, a type of particle accelerator that accelerates charged particles like electrons inside a closed, circular path. Basic physics suggests that any time you accelerate a charged particle, it gives off light. The type of light depends on the energy of the electrons (or other charged particles) and the magnetic field that pushes them around the circle, Gaffney said.

Since the synchrotron electrons are pushed to near the speed of light, they give off enormous amounts of energy, particularly X-ray energy. And not just any X-rays, but a very powerful beam of focused X-ray light.

Synchrotron radiation was seen for the first time at General Electric in the United States in 1947, according to the European Synchrotron Radiation Facility. This radiation was considered a nuisance because it caused the particles to lose energy, but it was later recognized in the 1960s as light with exceptional properties that overcame the shortcomings of X-ray tubes. One interesting feature of synchrotron radiation is that it is polarized; that is, the electric and magnetic fields of the photons all oscillate in the same direction, which can be either linear or circular.

"Because the electrons are relativistic [or moving at near light-speed], when they give off light, it ends up being focused in the forward direction," Gaffney said. "This means you get not just the right color of light X-rays and not just a lot of them because you have a lot of electrons stored, they're also preferentially emitted in the forward direction."

X-ray imaging

Due to their ability to penetrate certain materials, X-rays are used for several nondestructive evaluation and testing applications, particularly for identifying flaws or cracks in structural components. According to the NDT Resource Center, "Radiation is directed through a part and onto [a] film or other detector. The resulting shadowgraph shows the internal features" and whether the part is sound. This is the same technique used in doctors' and dentists' offices to create X-ray images of bones and teeth, respectively.

X-rays are also essential for transportation security inspections of cargo, luggage and passengers. Electronic imaging detectors allow for real-time visualization of the content of packages and other passenger items.

The original use of X-rays was for imaging bones, which were easily distinguishable from soft tissues on the film that was available at that time. However, more accurate focusing systems and more sensitive detection methods, such as improved photographic films and electronic imaging sensors, have made it possible to distinguish increasingly fine detail and subtle differences in tissue density, while using much lower exposure levels.

Additionally, computed tomography (CT) combines multiple X-ray images into a 3D model of a region of interest.

Similar to CT, synchrotron tomography can reveal three-dimensional images of interior structures of objects like engineering components, according to the Helmholtz Center for Materials and Energy.

X-ray therapy

Radiation therapy uses high-energy radiation to kill cancer cells by damaging their DNA. Since the treatment can also damage normal cells, the National Cancer Institute recommends that treatment be carefully planned to minimize side effects.

According to the U.S. Environmental Protection Agency, so-called ionizing radiation from X-rays zaps a focused area with enough energy to completely strip electrons from atoms and molecules, thus altering their properties. In sufficient doses, this can damage or destroy cells. While this cell damage can cause cancer, it can also be used to fight it. By directing X-rays at cancerous tumors, it can demolish those abnormal cells.

X-ray astronomy

According to Robert Patterson, professor of astronomy at Missouri State University, celestial sources of X-rays include close binary systems containing black holes or neutron stars. In these systems, the more massive and compact stellar remnant can strip material from its companion star to form a disk of extremely hot X-ray-emitting gas as it spirals inward. Additionally, supermassive black holes at the centers of spiral galaxies can emit X-rays as they absorb stars and gas clouds that fall within their gravitational reach.

X-ray telescopes use low-angle reflections to focus these high-energy photons (light) that would otherwise pass through normal telescope mirrors. Because Earth's atmosphere blocks most X-rays, observations are typically conducted using high-altitude balloons or orbiting telescopes.


#3 Jokes » Burger Jokes - 3 » Today 00:05:05

Replies: 0

Q: Why did the man climb to the roof of the fast food restaurant? 
A: They told him the meal was on the house!
* * *
Q: Where are the best tacos served?
A: In the Gulp of Mexico!
* * *
Q: What did the frog order at McDonald's?
A: French flies and a diet Croak.
* * *
Q: Would octopus make a good fast food?
A: You must be squidding!
* * *
Q: Where do burgers like to dance?
A: At a meat ball!
* * *
Q: Did you hear about the time Billy Crystal took Meg Ryan to McDonalds?
A: It's "When Harry Fed Sally".
* * *
Q: How did the hamburger introduce his wife?
A: Meet patty (meat patty).
* * *
Q: What do you get when you cross a hamburger with a computer?
A: A big mac!
* * *
Q: What did Sushi A say to Sushi B?
A: Wasabi!
* * *

#4 Re: Ganesh's Puzzles » Mensuration » Yesterday 14:33:35

Hi 666 bro,


The solution M # 577 is correct. Brilliant!

#M # 578. The edge of a cube is 12 centimeters. Find the volume of the largest sphere that can be carved out from it. (In terms of pi).

#5 Re: Introductions » Hello! » Yesterday 13:01:30

Hi seercalf,

Welcome to the forum!

#6 Re: Ganesh's Puzzles » Mensuration » 2019-10-16 14:49:57


The solution M # 576 is correct. Excellent, 666 bro!

M # 577. The ratio of the base radii of two cones is 3 : 5 and their heights are in the ratio 2 : 3. What is the ratio of their volumes?

#7 Jokes » Burger Jokes - 2 » 2019-10-16 00:32:05

Replies: 0

Q: Why did the french fry win the race?
A: Because it was fast food!
* * *
Q: What did Little Caesars say to Wendys?
A: You'll always have a pizza my heart.
* * *
Q: Why is it called "Fast Food"?
A: It's called "fast" food because you're supposed to eat it really fast. Otherwise, you might actually taste it.
* * *
Q: Why is Fast Food increasing illegal immigration?
A: "Fast" food slows you down when it hits your stomach, parks there, and lets the fat have time to get off and apply for citizenship. 
* * *
Q: Where do they hold prizefights in Fastfoodland? 
A: In an onion ring!
* * *
Q: Why do hamburgers go to the gym?
A: To get better buns.
* * *

#8 Re: This is Cool » Miscellany » 2019-10-16 00:04:42

416) Grasshopper

Grasshopper, any of a group of jumping insects (order Orthoptera) that are found in a variety of habitats. Grasshoppers occur in greatest numbers in lowland tropical forests, semiarid regions, and grasslands. They range in colour from green to olive or brown and may have yellow or red markings.

The grasshopper senses touch through organs located in various parts of its body, including antennae and palps on the head, cerci on the abdomen, and receptors on the legs. Organs for taste are located in the mouth, and those for smell are on the antennae. The grasshopper hears by means of a tympanal organ situated either at the base of the abdomen (Acrididae) or at the base of each front tibia (Tettigoniidae). Its sense of vision is in the compound eyes, while change in light intensity is perceived in the simple eyes (or ocelli). Although most grasshoppers are herbivorous, only a few species are important economically as crop pests.

The femur region of the upper hindlegs is greatly enlarged and contains large muscles that make the legs well adapted for leaping. The male can produce a buzzing sound either by rubbing its front wings together (Tettigoniidae) or by rubbing toothlike ridges on the hind femurs against a raised vein on each closed front wing (Acrididae).

Some grasshoppers are adapted to specialized habitats. The South American grasshoppers of Pauliniidae spend most of their lives on floating vegetation and actively swim and lay eggs on underwater aquatic plants. Grasshoppers generally are large, with some exceeding 11 cm (4 inches) in length (e.g., Tropidacris of South America).

In certain parts of the world, grasshoppers are eaten as food. They are often dried, jellied, roasted and dipped in honey or ground into a meal. Grasshoppers are controlled in nature by predators such as birds, frogs, and snakes. Humans use insecticides and poison baits to control them when they become crop pests.

The short-horned grasshopper (family Acrididae, formerly Locustidae) includes both inoffensive nonmigratory species and the often-destructive, swarming, migratory species known as locust. The long-horned grasshopper (family Tettigoniidae) is represented by the katydid, the meadow grasshopper, the cone-headed grasshopper, and the shield-backed grasshopper.

Other orthopterans are also sometimes known as grasshoppers. The pygmy grasshopper (family Tetrigidae) is sometimes called the grouse, or pygmy, locust. The leaf-rolling grasshopper (family Gryllacrididae) is usually wingless and lacks hearing organs.


#9 Re: Ganesh's Puzzles » Oral puzzles » 2019-10-15 13:35:29

Hi math9maniac,


#4661. Find the area of a parallelogram whose length is 30 cm, width is 20 cm, and one diagonal is 40 cm.

#10 Re: This is Cool » Miscellany » 2019-10-10 00:03:57

415) Gyroscope

Gyroscope, device containing a rapidly spinning wheel or circulating beam of light that is used to detect the deviation of an object from its desired orientation. Gyroscopes are used in compasses and automatic pilots on ships and aircraft, in the steering mechanisms of torpedoes, and in the inertial guidance systems installed in space launch vehicles, ballistic missiles, and orbiting satellites.

Mechanical Gyroscopes

Mechanical gyroscopes are based on a principle discovered in the 19th century by Jean-Bernard-Léon Foucault, a French physicist who gave the name gyroscope to a wheel, or rotor, mounted in gimbal rings. The angular momentum of the spinning rotor caused it to maintain its attitude even when the gimbal assembly was tilted. During the 1850s Foucault conducted an experiment using such a rotor and demonstrated that the spinning wheel maintained its original orientation in space regardless of Earth’s rotation. This ability suggested a number of applications for the gyroscope as a direction indicator, and in 1908 the first workable gyrocompass was developed by German inventor H. Anschütz-Kaempfe for use in a submersible. In 1909 American inventor Elmer A. Sperry built the first automatic pilot using a gyroscope to maintain an aircraft on course. The first automatic pilot for ships was installed in a Danish passenger ship by a German company in 1916, and in that same year a gyroscope was used in the design of the first artificial horizon for aircraft.

Gyroscopes have been used for automatic steering and to correct turn and pitch motion in cruise and ballistic missiles since the German V-1 missile and V-2 missile of World War II. Also during that war, the ability of gyroscopes to define direction with a great degree of accuracy, used in conjunction with sophisticated control mechanisms, led to the development of stabilized gunsights, bombsights, and platforms to carry guns and radar antennas aboard ships. The inertial guidance systems used by orbital spacecraft require a small platform that is stabilized to an extraordinary degree of precision; this is still done by traditional gyroscopes. Larger and heavier devices called momentum wheels (or reaction wheels) also are used in the attitude control systems of some satellites.

Optical Gyroscopes

Optical gyroscopes, with virtually no moving parts, are used in commercial jetliners, booster rockets, and orbiting satellites. Such devices are based on the Sagnac effect, first demonstrated by French scientist Georges Sagnac in 1913. In Sagnac’s demonstration, a beam of light was split such that part traveled clockwise and part counterclockwise around a rotating platform. Although both beams traveled within a closed loop, the beam traveling in the direction of rotation of the platform returned to the point of origin slightly after the beam traveling opposite to the rotation. As a result, a “fringe interference” pattern (alternate bands of light and dark) was detected that depended on the precise rate of rotation of the turntable.

Gyroscopes utilizing the Sagnac effect began to appear in the 1960s, following the invention of the laser and the development of fibre optics. In the ring laser gyroscope, laser beams are split and then directed on opposite paths through three mutually perpendicular hollow rings attached to a vehicle. In reality, the “rings” are usually triangles, squares, or rectangles filled with inert gases through which the beams are reflected by mirrors. As the vehicle executes a turning or pitching motion, interference patterns created in the corresponding rings of the gyroscope are measured by photoelectric cells. The patterns of all three rings are then numerically integrated in order to determine the turning rate of the craft in three dimensions. Another type of optical gyroscope is the fibre-optic gyroscope, which dispenses with hollow tubes and mirrors in favour of routing the light through thin fibres wound tightly around a small spool.


#11 Re: Ganesh's Puzzles » Oral puzzles » 2019-10-07 15:45:24


The solution #4659 is correct. Neat work, Monox D. I-Fly!

#4660. A boatman goes 2 kilometers against the current of the stream in 1 hour and goes 1 kilometer along the current in 10 minutes. How long will it take to go 5 kilometers in stationary water?

#12 Re: Dark Discussions at Cafe Infinity » crème de la crème » 2019-10-06 00:00:17

586) Benjamin Eisenstadt


Benjamin Eisenstadt (December 7, 1906 – April 8, 1996) was the designer of the modern sugar packet and developer of Sweet'N Low. He was the founder of the Cumberland Packing Corporation and a notable philanthropist.

Personal life

Benjamin Eisenstadt was born in New York City on December 7, 1906. His family was Jewish. He attended Brooklyn College.

He married Betty Gellman (1910-2001) on October 27, 1931 while living at 1250 44th Street in Brooklyn. Their children were Marvin Eisenstadt, who married Barbara; Gladys Eisenstadt; Ira Eisenstadt, who married Deirdre Howley; and Ellen Eisenstadt, who married Herbert Cohen.

Business and philanthropy

After college, Eisnstadt operated a cafeteria across from the Brooklyn Navy Yard. He switched to making tea bags after his cafeteria business declined.

In the mid 1940s, he invented the idea of single servings of table sugar to utilize his tea bag machinery. He proposed the idea to the major sugar producers, but was unsuccessful in attracting their interest. Since he had not secured a patent before shopping the idea around, sugar producers were then free to use his idea without paying royalties, and they did so.

In 1957 he came up with a formula for a powdered saccharin sweetener. Previously saccharin was sold as liquid drops, or tiny tablets. He mixed the saccharin with dextrose to bulk it up to a teaspoon sized portion, added cream of tartar, and calcium silicate as anti-caking agents. His Cumberland Packing Corporation marketed the product, called Sweet'N Low, in bright pink packets so that the saccharin packets would not be confused with sugar packets at restaurants.

His company was also the first to package soy sauce and other single serving condiments.

After the Cumberland Packing Corporation was on a financially successful footing, Eisenstadt devoted a part of his wealth to medical philanthropy. He became chairman of the board of the foundation for Maimonides Medical Center. During his 20 year tenure as a trustee and benefactor of this institution, he also served as secretary, and vice chairman of the board.


Benjamin died at age 89 after complications from open heart surgery. When Betty died in 2001 she had removed Ellen and her children from her will.


Maimonides Medical Center has the Eisenstadt Administration Building and the Gellman Pavilion. The Gellman Pavilion was named in memory of Dr. Abraham Gellman, the brother of Betty Gellman (1910-2001).


#13 Re: Ganesh's Puzzles » 10 second questions » 2019-10-05 23:29:42


#7542. Find the average of first nine prime numbers.

#14 Re: Ganesh's Puzzles » Oral puzzles » 2019-10-05 23:04:36


#4659. If 50% of x = 30% of y, find x : y.

#15 Re: This is Cool » Miscellany » 2019-10-05 00:45:37

414) Differences in Microphones & Speakers

Although at first glance, microphones and speakers appear to be very different kinds of devices, they are in fact closely related. Speakers and microphones are both transducers -- components which transform energy from one type to another. A speaker turns electrical currents into sound waves; a microphone converts sound into electrical energy. The main differences between them lie in the way audio designers have optimized each to perform its particular task efficiently.


A speaker produces sound when you drive it with an amplifier connected to an audio source. Most speakers use a electromagnet design in which a permanent magnet is situated in a metal frame that holds a cone made of paper or plastic. A wire coil attached to the end of the cone produces attractive and repulsive forces when electrical current flows through it; the pushing and pulling against the cone generates sound waves. The cone's size dictates the general frequency range it reproduces most efficiently: large cones produce low frequencies, and small ones generate high frequencies.


When you speak or sing into a microphone, the sound waves of your voice produce vibrations in a diaphragm inside the mike. Although they have a variety of basic designs, a common type called the dynamic microphone uses a magnetic principle similar to that used in speakers. The diaphragm carries a lightweight wire coil made of fine wire; the coil moves through a magnetic field and produces electrical currents which mirror the incoming sound waves. Another popular design, called the condenser microphone, places the diaphragm on one of two metal plates separated by an insulator. The vibrations in the diaphragm produce changes in the electrical capacitance between the two plates. Condenser mics require a battery, as the capacitance effect doesn't produce electrical currents by itself.


The dynamic microphone and standard speaker both employ a moving coil in a magnetic field, producing electrical currents from sound vibrations or vice-versa. It is possible, although risky, to connect a dynamic microphone to a speaker output and hear sound from the mic. As the microphone is not designed to handle electrical inputs, a loud amp setting can destroy the mic if used in this manner. In the same way, you can connect a speaker to a microphone input, but because a speaker doesn't make an ideal mike, you must yell into it to produce a detectable signal. Walkie-talkies and room intercom systems use a single speaker-microphone device that performs both functions moderately well.


Microphones produce a relatively weak output that requires preamplification to bring the signal to a standard line level. Because the signals are weak, microphone cables have shielding that reduces electrical noise picked up from fluorescent lights and appliances. A microphone picks up a wide range of frequencies with great sensitivity. The loudspeaker's purpose is to fill a room with high-fidelity sound. This means handling large amounts of power from an amplifier -- up to several hundred watts for some types of speakers. To manage the power, the speaker has a robust, heavy design. For good fidelity, a single speaker cabinet may have two or more separate speaker drivers, each suited to a particular frequency range; a single speaker does not have the wide range that a microphone has.


#16 Re: Ganesh's Puzzles » English language puzzles » 2019-10-04 23:59:50


The Answer #3345 is correct. Neat work, Monox D. I-Fly!

#3347. What does the noun dogma mean?

#3348. What does the noun doily mean?

#17 Re: Dark Discussions at Cafe Infinity » crème de la crème » 2019-10-04 00:48:58

585) Hans Christian Gram

Hans Christian Joachim Gram (13 September 1853 – 14 November 1938) was a Danish bacteriologist noted for his development of the Gram stain, still a standard technique to classify bacteria and make them more visible under a microscope.

Early life and education

Gram was the son of Frederik Terkel Julius Gram, a professor of jurisprudence, and Louise Christiane Roulund.

He studied at the University of Copenhagen, and was an assistant in botany to the zoologist Japetus Steenstrup. His study of plants introduced him to the fundamentals of pharmacology and the use of the microscope.

Gram entered medical school in 1878 and graduated in 1883. He travelled throughout Europe between 1878 and 1885.


Gram stain

In Berlin, in 1884, Gram developed a method for distinguishing between two major classes of bacteria. This technique, the Gram stain, continues to be a standard procedure in medical microbiology. This work gained Gram an international reputation. The stain later played a major role in classifying bacteria. Gram was a modest man, and in his initial publication he remarked, "I have therefore published the method, although I am aware that as yet it is very defective and imperfect; but it is hoped that also in the hands of other investigators it will turn out to be useful."

A Gram stain is made using a primary stain of crystal violet and a counterstain of safranin. Bacteria that turn purple when stained are called 'Gram-positive', while those that turn red when counterstained are called 'Gram-negative'.

Other work

Gram's initial work concerned the study of red blood cells in men. He was among the first to recognise that macrocytes were characteristic of pernicious anaemia.
In 1891, Gram taught pharmacology, and later that year was appointed professor at the University of Copenhagen. In 1900, he resigned his chair in pharmacology to become professor of medicine. As a professor, he published four volumes of clinical lectures which became widely used in Denmark. He retired from the University of Copenhagen in 1923, and died in 1938.

Popular recognition

On 13 September 2019, Google commemorated the anniversary of his birth with a Doodle for Canada, Peru, Argentina, Australia, New Zealand, Israel, India and some European countries.


#18 Re: Ganesh's Puzzles » English language puzzles » 2019-10-04 00:39:46


#3345. What does the noun dodecahedron mean?

#3346. What does the adjective dogged mean?

#19 Re: Ganesh's Puzzles » General Quiz » 2019-10-04 00:18:07


#7355. What is 'Pulitzer Prize' awarded for?

#7356. What is 'Hyatt Hotels Corporation' known for?

#20 Re: Introductions » Hello! » 2019-10-03 13:26:53

Hi stivemorgan,

Welcome to the forum!

#21 Re: This is Cool » Miscellany » 2019-10-03 00:15:00

413) Telescope

The telescope is an instrument that collects and analyzes the radiation emitted by distant sources. The most common type is the optical telescope, a collection of lenses and/or mirrors that is used to allow the viewer to see distant objects more clearly by magnifying them or to increase the effective brightness of a faint object. In a broader sense, telescopes can operate at most frequencies of the electromagnetic spectrum, from radio waves to gamma rays. The one characteristic all telescopes have in common is the ability to make distant objects appear to be closer. The word telescope is derived from the Greek tele meaning far, and skopein meaning to view.
The first optical telescope was probably constructed by German-born Dutch lensmaker Hans Lippershey (1570–1619), in 1608. The following year, Italian astronomer and physicist Galileo Galilei (1564– 1642) built the first astronomical telescope, from a tube containing two lenses of different focal lengths aligned on a single axis (the elements of this telescope are still on display in Florence, Italy). With this telescope and several following versions, Galileo made the first telescopic observations of the sky and discovered lunar mountains, four of Jupiter’s moons, sunspots, and the starry nature of the Milky Way galaxy. Since then, telescopes have increased in size and improved in image quality. Computers are now used to aid in the design of large, complex telescope systems.

Operation Of A Telescope

Light gathering

The primary function of a telescope is that of radiation gathering, in many cases light gathering. As will be seen below, resolution limits on telescopes would not call for an aperture much larger than about 30 in (76 cm). However, there are many telescopes around the world with diameters several times this value. The reason for this occurrence is that larger telescopes can see further because they can collect more light. The 200 in (508 cm) diameter reflecting telescope at Mt. Palomar, California, for instance can gather 25 times more light than the 40 in (102 cm) Yerkes telescope at Williams Bay, Wisconsin, the largest refracting telescope in the world. The light gathering power grows as the area of the objective increases, or the square of its diameter if it is circular. The more light a telescope can gather, the more distant the objects it can detect, and therefore larger telescopes increase the size of the observable universe.


The resolution, or resolving power, of a telescope is defined as being the minimum angular separation between two different objects that can be detected.

Unfortunately, astronomers are not able to increase the resolution of a telescope simply by increasing the size of the light gathering aperture to as large a size as is need. Disturbances and non-uniformities in the atmosphere limit the resolution of telescopes positioned on the surface of Earth to somewhere in the range 0.5 to 2 arc seconds, depending on the location of the telescope. Telescope sights on top of mountains are popular since the light reaching the instrument has to travel through less air, and consequently the image has a higher resolution. However, a limit of 0.5 arc seconds corresponds to an aperture of only 12 in (30 cm) for visible light: larger telescopes do not provide increased resolution but only gather more light.


Magnification is not the most important characteristic of telescopes as is commonly thought. The magnifying power of a telescope is dependent on the type and quality of eyepiece being used. The magnification is given simply by the ratio of the focal lengths of the objective and eyepiece. Thus, a 0.8 in (2 cm) focal length eyepiece used in conjunction with a 39 in (100 cm) focal length objective will give a magnification of 50. If the field of view of the eyepiece is 20°, the true field of view will be 0.4°.

Types Of Telescope

Most large telescopes built before the twentieth century were refracting telescopes because techniques were readily available to polish lenses. Not until the latter part of the nineteenth century were techniques developed to coat large mirrors, which allowed the construction of large reflecting telescopes.

Refracting telescopes

The parallel light from a distant object enters the objective, of focal length f1, from the left. The light then comes to a focus at a distance f1 from the objective. The eyepiece, with focal length f2, is situated a distance f1+ f2 from the objective such that the light exiting the eyepiece is parallel. Light coming from a second object (dashed lines) exits the eyepiece at an angle equal to f1/f2 times the angle of the light entering.

Refracting telescopes, i.e., telescopes that use lenses, can suffer from problems of chromatic and other aberrations, which reduce the quality of the image. In order to correct for these, multiple lenses are required, much like the multiple lens systems in a camera lens unit. The advantages of the refracting telescope include having no central stop or other diffracting element in the path of light as it enters the telescope, and the stability of the alignment and transmission characteristics over long periods of time. However, the refracting telescope can have low overall transmission due to reflection at the surface of all the optical elements. In addition, the largest refractor ever built has a diameter of only 40 in (102 cm): lenses of a larger diameter will tend to distort under their own weight and give a poor image. Additionally, each lens needs to have both sides polished perfectly and be made from material that is of highly uniform optical quality throughout its entire volume.

Reflecting telescopes

All large telescopes, both existing and planned, are of the reflecting variety. Reflecting telescopes have several advantages over refracting designs. First, the reflecting material (usually aluminum), deposited on a polished surface, has no chromatic aberration. Second, the whole system can be kept relatively short by folding the light path, as shown in the Newtonian and Cassegrain designs below. Third, the objectives can be made very large, since there is only one optical surface to be polished to high tolerance, the optical quality of the mirror substrate is unimportant and the mirror can be supported from the back to prevent bending. The disadvantages of reflecting systems are 1) alignment is more critical than in refracting systems, resulting in the use of complex adjustments for aligning the mirrors and the use of temperature insensitive mirror substrates and 2) the secondary or other auxiliary mirrors are mounted on a support structure that occludes part of the primary mirror and causes diffraction.

These are a) the prime focus, where the detector is simply placed at the prime focus of the mirror; b) the Newtonian, where a small, flat mirror reflects the light out to the side of the telescope; c) the Cassegrain, where the focus is located behind the plane of the primary mirror through a hole in its center and d) the Coudè, where the two flat mirrors provide a long focal length path as shown.

Catadioptric telescopes

Catadioptric telescopes use a combination of lenses and mirrors in order to obtain some of the advantages of both. The best-known type of catadioptric is the Schmidt telescope or camera, which is usually used to image a wide field of view for large area searches. The lens in this system is very weak and is commonly referred to as a corrector-plate.

Overcoming Resolution Limitations

The limits to the resolution of a telescope are, as described above, a result of the passage of the light from the distant body through the atmosphere, which is optically non-uniform. Stars appear to twinkle because of constantly fluctuating optical paths through the atmosphere, which results in a variation in both brightness and apparent position. Consequently, much information is lost to astronomers simply because they do not have sufficient resolution from their measurements. There are three ways of overcoming this limitation, namely setting the telescope out in space in order to avoid the atmosphere altogether, compensating for the distortion on a ground-based telescope, and/or stellar interferometry. The first two methods are innovations of the 1990s and have lead to a new era in observational astronomy.

Space Telescopes

The best known and biggest orbiting optical telescope is the Hubble Space Telescope (HST), which has an 8 ft (2.4 m) primary mirror and five major instruments for examining various characteristics of distant bodies. After a much-publicized problem with the focusing of the telescope and the installation of a package of corrective optics in 1993, the HST has proved to be the finest of all telescopes ever produced. The data collected from HST is of such a high quality that researchers can solve problems that have been in question for years, often with a single photograph. The resolution of the HST is 0.02 arc seconds, close to the theoretical limit since there is no atmospheric distortion, and a factor of around twenty times better than was previously possible. An example of the significant improvement in imaging that space-based systems have given is the Doradus 30 nebula, which prior to the HST was thought to have consisted of a small number of very bright stars. In a photograph taken by the HST it now appears that the central region has over 3,000 stars.

Another advantage of using a telescope in orbit about Earth is that the telescope can detect wavelengths such as the ultraviolet and various portions of the infrared, which are absorbed by the atmosphere and not detectable by ground-based telescopes.

Adaptive Optics

In 1991, the United States government declassified adaptive optics systems (systems that remove atmospheric effects), which had been developed under the Strategic Defense Initiative for ensuring that a laser beam could penetrate the atmosphere without significant distortion.

A laser beam is transmitted from the telescope into a layer of mesospheric sodium at 56–62 mi (90– 100 km) altitude. The laser beam is resonantly backscattered from the volume of excited sodium atoms and acts as a guide-star whose position and shape are well defined except for the atmospheric distortion. The telescope collects the light from the guide-star and a wavefront sensor determines the distortion caused by the atmosphere. This information is then fed back to a deformable mirror, or an array of many small mirrors, which compensates for the distortion. As a result, stars that are located close to the guide-star come into a focus, which is many times better than can be achieved without compensation. Telescopes have operated at the theoretical resolution limit for infrared wavelengths and have shown an improvement in the visible region of more than ten times. Atmospheric distortions are constantly changing, so the deformable mirror has to be updated every five milliseconds, which is easily achieved with modern computer technology.

Recording Telescope Data

Telescopes collect light largely for two types of analysis, imaging and spectrometry. The better known is imaging, the goal of which is simply to produce an accurate picture of the objects that are being examined. In past years, the only means of recording an image was to take a photograph. For long exposure times, the telescope had to track the sky by rotating at the same speed as the Earth, but in the opposite direction. This is still the case today, but the modern telescope no longer uses photographic film but a charge-coupled device (CCD) array. The CCD is a semiconductor light detector, which is fifty times more sensitive than photographic film, and is able to detect single photons. Being fabricated using semiconductor techniques, the CCD can be made to be very small, and an array typically has a spacing of 15 microns between CCD pixels. A typical array for imaging in telescopes will have a few million pixels. There are many advantages of using the CCD over photographic film or plates, including the lack of a developing stage and the output from the CCD can be read directly into a computer and the data analyzed and manipulated with relative ease.

The second type of analysis is spectrometry, which means that the researcher wants to know what wavelengths of light are being emitted by a particular object. The reason behind this is that different atoms and molecules emit different wavelengths of light—measuring the spectrum of light emitted by an object can yield information as to its constituents. When performing spectrometry, the output of the telescope is directed to a spectrometer, which is usually an instrument containing a diffraction grating for separating the wavelengths of light. The diffracted light at the output is commonly detected by a CCD array and the data read into a computer.

Modern Optical Telescopes

For almost 40 years the Hale telescope at Mt. Palomar (San Diego, California) was the world’s largest with a primary mirror diameter of 200 in (5.1 m). During that time, improvements were made primarily in detection techniques, which reached fundamental limits of sensitivity in the late 1980s. In order to observe fainter objects, it became imperative to build larger telescopes, and so a new generation of telescopes is being developed for the 2000s and beyond. These telescopes use revolutionary designs in order to increase the collecting area; 2,260 square feet (210 square meters) is being used for the European Southern Observatory (ESO), which operates observatories in Chile; the organization is headquartered near Munich, Germany.

This new generation of telescopes does not use the solid, heavy primary mirror of previous designs, whose thickness was between one-sixth and one-eighth of the mirror diameter. Instead, it uses a variety of approaches to reduce the mirror weight and improve its thermal and mechanical stability, including using many hexagonal mirror elements forming a coherent array; a single large meniscus mirror (with a thickness one-fortieth of the diameter), with many active support points which bend the mirror into the correct shape; and, a single large mirror formed from a honeycomb sandwich. In 2005, one of the first pictures taken by ESO was of 2M1207b, an exo-solar planet (a planet orbiting a star other than the sun) orbiting a brown dwarf star about 260 light-years away (where one light-year is the distance that light travels in vacuum in one year). These new telescopes, combined with quantum-limited detectors, distortion reduction techniques, and coherent array operation allow astronomers to see objects more distant than have been observed before.

One of this new generation, the Keck telescope located on Mauna Kea in Hawaii, is currently the largest operating optical/infrared telescope, using a 32 ft (10 m) effective diameter hyperbolic primary mirror constructed from 36 6-ft (1.8-m) hexagonal mirrors. The mirrors are held to relative positions of less than 50 nm using active sensors and actuators in order to maintain a clear image at the detector.

Because of its location at over 14,000 ft (4,270 m), the Keck is useful for collecting light over the range of 300 nm to 30 æm. In the late 1990s, Keck I was joined by an identical twin, Keck II. Then, in 2001, the two telescopes were linked together through the use of interferometry for an effective mirror diameter of 279 ft (85 m).

Alternative Wavelengths

Most of the discussion so far has been concerned with optical telescopes operating in the range from 300 to 1,100 nanometers (nm). However, valuable information is contained in the radiation reaching Earth at different wavelengths and telescopes have been built to cover wide ranges of operation, including radio and millimeter waves, infrared, ultraviolet, x rays, and gamma rays.

Infrared telescopes

Infrared telescopes are particularly useful for examining the emissions from gas clouds. Since water vapor in the atmosphere can absorb some of this radiation, it is especially important to locate infrared telescopes in high altitudes or in space. In 1983, NASA launched the highly successful Infrared Astronomical Satellite, which performed an all-sky survey, revealing a wide variety of sources and opening up new avenues of astrophysical discovery. With the improvement in infrared detection technology in the 1980s, the 1990s saw several new infrared telescopes, including the Infrared Optimized Telescope, a 26 ft (8 m) diameter facility, on Mauna Kea, Hawaii. In August 2003, NASA launched the Spitzer Space Telescope (formerly the Space Infrared Telescope Facility; named after Lyman Spitzer, Jr., who first suggested placing telescopes in orbit in the 1940s). It is in orbit about the sun (a heliocentric orbit), in which it follows behind Earth’s orbit about the sun, slowly receding away from Earth each year. Its primary mirror is about 2.8 ft (85 cm) in diameter, with a focal length that is twelve times the diameter of the primary mirror.
Several methods are used to reduce the large thermal background that makes viewing infrared difficult, including the use of cooled detectors and dithering the secondary mirror. This latter technique involves pointing the secondary mirror alternatively at the object in question and then at a patch of empty sky. Subtracting the second signal from the first results in the removal of most of the background thermal (infrared) noise received from the sky and the telescope itself, thus allowing the construction of a clear signal.

Radio telescopes

Radio astronomy was developed following World War II, using the recently developed radio technology to look at radio emissions from the sky. The first radio telescopes were very simple, using an array of wires as the antenna. In the 1950s, the now familiar collecting dish was introduced and has been widely used ever since.
Radio waves are not susceptible to atmospheric disturbances like optical waves are, and so the development of radio telescopes over the past forty years has seen a continued improvement in both the detection of faint sources as well as in resolution. Despite the fact that radio waves can have wavelengths which are meters long, the resolution achieved has been to the sub-arc second level through the use of many radio telescopes working together in an interferometer array, the largest of which stretches from Hawaii to the United States Virgin Islands (known as the Very Long Baseline Array). The largest working radio telescope is the Giant Meterwave Radio Telescope in India. It contains 14 telescopes arranged around a central square and another 16 positioned within three arms of a Y-shaped array. Its total interferometric baseline is about 15.5 mi (25 km). Construction is being made on the Low Frequency Array (LOFAR), which is a series of radio telescopes located across the Netherlands and Germany. As of September 2006, LOFAR has been constructed and is in the testing stage. When operational, it will have a total collecting area of around 0.4


Chromatic aberration —The reduction in image quality arising from the fact that the refractive index in varies across the spectrum.
Objective —The large light collecting lens used in a refracting telescope.
Reflecting telescope —A telescope that uses only reflecting elements, i.e., mirrors.
Refracting telescope —A telescope that uses only refracting elements; i.e., lenses.
Spectrometry —The measurement of the relative strengths of different wavelength components that make up a light signal.


#22 Re: Ganesh's Puzzles » Oral puzzles » 2019-10-02 15:33:53


The solution #4657 is correct, Excellent, Monox D. I-Fly!

#4658. A 260 meter long train crosses a platform thrice its length in 80 seconds. What is the speed of the train in kilometers per hour?

#23 Re: Dark Discussions at Cafe Infinity » crème de la crème » 2019-10-02 00:21:08

584) Gordon Gould

Gordon Gould, in full Richard Gordon Gould, (born July 17, 1920, New York, N.Y., U.S.—died Sept. 16, 2005, New York), American physicist who played an important role in early laser research and coined the word laser (light amplification by stimulated emission of radiation).

Gould received a bachelor’s degree in physics from Union College in Schenectady, N.Y., in 1941 and a master’s degree in physics from Yale University two years later. He then worked on the Manhattan Project but was released from the project because of his membership in a communist political group (which he left in 1948). He started teaching physics at the City College of New York in 1946, and he entered graduate school at Columbia University, New York City, in 1949.

He came up with the idea of the laser and its name in 1957. He had discussed the idea with physicist Charles Townes, who had invented the maser, which amplified microwave radiation. Gould took Townes’s advice that he should write down his ideas and notarize them as a first step of applying for a patent. Gould left Columbia and joined the defense research firm Technical Research Group (TRG) in 1958 to work on building a laser. Believing that he first needed to have a working prototype, he waited until 1959 to apply for a patent, but by that time Townes and physicist Arthur Schawlow had filed such an application and his was rejected. With the initial support of TRG and with his notarized notebook as his main piece of evidence, Gould fought Townes and Schawlow’s award of the laser patent. After many years of litigation, he prevailed, and in 1977 he was issued the first of the four U.S. basic laser patents that he was eventually granted. The laser industry then fought the award of patents to Gould to avoid paying him millions of dollars in royalties, but he finally prevailed in 1987.

During the legal struggle over the laser patents, Gould taught at the Polytechnic Institute of New York from 1967 to 1973, and he founded an optical communications company, Optelecom, in 1973. He retired from Optelecom in 1985, and he was inducted into the (U.S.) National Inventors Hall of Fame in 1991.


#24 Re: Ganesh's Puzzles » Oral puzzles » 2019-10-02 00:14:22



#4657. Find the value of m.

#25 Re: Ganesh's Puzzles » 10 second questions » 2019-10-02 00:01:40


#7541. What is the least number to be added to 4523 to make it a perfect square?

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