Deck 6: Optics and Telescopes

Question 1

Compared to the speed of visible light in a vacuum (or in space), its speed in glass is:

A)greater.
B)less.
C)exactly the same.
D)much greater.

Answer 1

B

Question 2

Lenses are used in applications like microscopes and telescopes. What does a lens do in these applications?

A)Lenses sort out the entering light by color (or wavelength) and absorb the colors not required for the image.
B)A lens captures all the light emitted by a source and focuses it to form an image.
C)A lens reduces the chromatic aberration found in light emitted naturally from a source.
D)Light that would not otherwise form an image enters a lens and has its direction changed so that it focuses to form an image.

Answer 2

D

Question 3

If the Sun shines on a simple converging lens made of glass, which of the following can you infer?

A)Light of all colors converges to precisely the same point.
B)The red light converges to a point that is slightly closer to the lens than the point to which the blue light converges.
C)The blue light converges to a point that is slightly closer to the lens than the point to which the red light converges.
D)The image of the Sun will be distorted, being more red toward the bottom and somewhat green near the top.

Answer 3

C

Question 4

When light passes from air into glass it is refracted. This means that:

A)the intensity of the light wave is reduced.
B)the frequency of the wave increases.
C)the wave slows down.
D)the entire wave turns around and returns to the source.

Question 5

What is the refraction of light?

A)the change in direction of a light ray as it reflects from a more dense material than the one in which it is traveling
B)the absorption of light as it traverses a dense, transparent material
C)the breaking of white light into its composite colors
D)the change in direction of a light ray as it crosses from a less dense, transparent material to a more dense one

Question 6

Refraction of light at a surface between two transparent substances occurs because:

A)the speed of light is different in the two materials.
B)some light is reflected at the surface.
C)the amount of absorption of the light changes at the surface.
D)the frequency of the light changes at the surface.

Question 7

The phenomenon of refraction, the change in direction of a light beam as it enters a dense but transparent material, is caused by the:

A)slowdown of light in denser material.
B)speedup of light as it enters denser material.
C)change in the color or wavelength of light as it enters denser material.
D)reflection of part of the light at the surface.

Question 8

A refracting telescope has an objective lens of focal length 80 cm, a diameter of 10 cm, and an eyepiece of focal length 5 cm and diameter 1 cm. How much more light will this telescope collect than the average unaided eye, whose focal length is 2 cm and diameter is 0.5 cm?

A)4
B)1600
C)20
D)400

Question 9

How many times more light can the 10-m diameter Keck telescope on Mauna Kea in Hawaii collect than an average unaided human eye, which has a typical aperture diameter of 5 mm?

A)2 × 10⁴
B)2000
C)4 × 10⁶
D)4

Question 10

A refracting telescope has an objective lens of focal length 80 cm, a diameter of 10 cm, and an eyepiece of focal length 5 cm and diameter 1 cm. What is the magnifying power of this telescope?

A)10×
B)8×
C)16×
D)80×

Question 11

A particular reflecting telescope has a primary mirror 0.4 m in diameter, 2.0 m focal length, and an eyepiece lens 1.0 cm in diameter and 0.5 cm focal length. What is the magnifying power of this telescope?

A)400×
B)40×
C)4×
D)80×

Question 12

What determines the magnification of a telescope image?

A)the size of the objective mirror or lens
B)only the focal length of the objective
C)only the focal length of the eyepiece
D)the focal lengths of both the objective and the eyepiece

Question 13

In a correctly focused refracting telescope, the objective lens and the eyepiece lens are separated by a distance equal to the

A)focal length of the eyepiece.
B)focal length of the objective minus the focal length of the eyepiece.
C)sum of the focal lengths of objective and eyepiece.
D)focal length of the objective.

Question 14

What happens when a beam of white light (containing all colors) is passed through a prism of glass?

A)Different colors are refracted at different angles to produce a spectrum.
B)Selected colors are absorbed so that the remaining light that leaves the prism is colored.
C)The speed of the red light that leaves the prism is higher than that of the blue light, leading to a colored beam.
D)Selected colors are reflected from the outer faces of the prism, and the light that passes through is colored.

Question 15

When white light passes through a prism or a lens, which wavelengths of light are deflected most by the glass?

A)The directions of all wavelengths are changed by the same amount.
B)The intermediate wavelengths.
C)The longer wavelengths.
D)The shorter wavelengths.

Question 16

Which of the following types of telescope will suffer from chromatic aberration unless very expensive measures are taken to avoid it?

A)radio telescope
B)Cassegrain telescope
C)reflecting telescope
D)refracting telescope

Question 17

Which of the following is not a defect or problem that occurs in a refracting telescope?

A)opaqueness of the glass lens to certain wavelengths of light
B)bubbles in the glass lens that scatters light
C)chromatic aberration, focusing light of different wavelengths to different foci
D)spherical aberration at the primary reflecting surface

Question 18

When used for photography, which of the following simple telescopes (with film or detector placed at the appropriate focus) will suffer from chromatic aberration?

A)refracting
B)Newtonian reflecting
C)reflecting, prime focus
D)Cassegrain reflecting

Question 19

The largest refracting telescope in the world is the 102-cm (40-in.) diameter telescope at Yerkes Observatory, built in 1897. Refracting telescopes with larger diameter have never been built because they would:

A)sag too much under their own weight.
B)give too much magnification.
C)have too little chromatic aberration.
D)be too thick, and suffer too much spherical aberration.

Question 20

To correct for chromatic aberration in a refracting telescope, a corrective lens is mounted next to the objective lens. In this corrective lens:

A)red light bends more than blue light.
B)blue light bends more than red light.
C)all colors bend the same amount.
D)no bending is experienced by any of the colors.

Question 21

A thick lens is able to bend light more than a thin lens. What, if anything, can you say about the focal lengths?

A)The thick lens has the longer focal length.
B)The thin lens has the longer focal length.
C)Focal length is determined by the focal point for parallel light and has nothing to do with lens thickness.
D)To determine relative focal length, you would have to know both the lens thickness and the light wavelength.

Question 22

If someone says they are using an 8-inch reflecting telescope, what do they mean?

A)The length of the telescope tube is 8 inches.
B)The radius of the mirror is 8 inches.
C)The diameter of the mirror is 8 inches.
D)The focal length of the mirror is 8 inches.

Question 23

At the mirror of a reflecting telescope, the light reflects from a coated surface rather than refracting through a lens. This eliminates:

A)spherical aberration.
B)chromatic aberration.
C)coma.
D)astigmatism.

Question 24

In the reflection of a light beam from a flat mirror, the angle between the incident and reflected beams relative to the perpendicular to the surface of the mirror is:

A)equal to the angle between the incident beam and the perpendicular.
B)equal to 1/2 the angle between the incident beam and the perpendicular.
C)always a right angle, or 90°.
D)twice the angle between incident beam and the perpendicular.

Question 25

In the reflection of a beam of light from a flat surface, the relationship between the angle of incidence i between the perpendicular to the surface and the incident beam and the angle of reflection r between the perpendicular and the reflected beam is:

A)r is equal to 2i.
B)the reflected ray always follows the perpendicular direction from the reflecting surface.
C)r is equal to i/2.
D)r is equal to i.

Question 26

A light ray strikes a smooth surface of glass at an angle α to the perpendicular. What angle will the reflected ray make with the perpendicular to the surface?

A)2α on the opposite side of the perpendicular to the incident ray
B)0°; the ray will always reflect along the perpendicular, a condition necessary for the focusing of light.
C)the same angle α on the opposite side of the perpendicular to the incident ray
D)the same angle α on the same side of the perpendicular, returning along the incident direction

Question 27

A Newtonian telescope uses:

A)two curved mirrors.
B)several mirrors, some curved, some flat, to guide light to a fixed focus with respect to Earth.
C)one curved mirror and one flat mirror at a 45° angle to the curved mirror.
D)only one mirror.

Question 28

Placing a small 45° mirror in the path of the light from the objective mirror in a reflecting telescope (as Newton did) does all of the following except one. Which one is not a consequence of this method?

A)It makes the telescope easier to use compared to a prime focus.
B)It introduces the possibility of distortion in the secondary mirror.
C)It decreases the amount of light reaching the objective mirror.
D)It causes a blank spot in the image where the beam has been interrupted by the secondary mirror.

Question 29

A reflecting telescope in which light is reflected by one curved mirror and a second plane mirror at 45° to the original beam, to reach a focus at the side of the telescope, is being used at its:

A)Newtonian focus.
B)out-of-focus position.
C)prime focus.
D)Cassegrain focus.

Question 30

The prime focus cage of a telescope whose primary mirror is 3 m in diameter has a diameter of 0.5 m. What fraction of the incoming light is obstructed by this cage?

A)about 3%
B)about 44%
C)about 1/6, or 17%
D)about 36%

Question 31

If part of the primary mirror of a reflecting telescope is blocked off by a small screen, which of the following characteristics of the final image is affected?

A)Although the brightness is unaffected, the size of the image will be reduced.
B)A part of the image is missing.
C)Only the brightness is reduced.
D)Depending on which part of the mirror is obscured, the color of the image is affected.

Question 32

A particular telescope is set up to use the Coudé focus. What arrangement of mirrors is used to bring the light to this focus?

A)A concave primary mirror and concave secondary mirror reflect light back through a hole in the primary mirror.
B)A series of mirrors channel the light to a remote and fixed location.
C)A concave primary mirror and flat, diagonal secondary mirror are used.
D)A concave primary mirror and convex secondary mirror reflect light back through a hole in the primary mirror.

Question 33

The magnifying power of a reflecting telescope, in terms of the focal length F of the primary mirror and the focal length f the eyepiece, (with F and f measured in the same units), is:

A)fF.
B)f/F.
C)F/f ².
D)F/f.

Question 34

What is the magnification of a Newtonian telescope with a primary mirror of diameter 0.25 m and focal length of 4 m when used with an eyepiece of focal length 25 mm and an optical diameter of 2.5 mm?

A)100
B)160
C)10
D)1600

Question 35

To produce the sharpest images of very distant objects, the best shape for the polished surface of a large astronomical mirror should be:

A)elliptical.
B)spherical.
C)perfectly flat and smooth.
D)parabolic.

Question 36

At the present time (2018), the largest optical telescope in the world has a primary mirror with a diameter of about:

A)6.0 m.
B)11.0 m.
C)30.0 m.
D)6.5 m.

Question 37

In order to correct for spherical aberration:

A)a spherical mirror is used with a correcting lens.
B)a parabolic mirror is used with a correcting lens.
C)an elliptical mirror is used without a correcting lens.
D)an elliptical mirror is used with a correcting lens.

Question 38

If a beam of waves passes through a narrow opening, like the optics of a telescope, the beam will be seen to diverge and not just continue in the original direction. This is a consequence of:

A)diffraction.
B)refraction.
C)reflection.
D)chromatic aberration.

Question 39

In telescopes, the resolution is worse for:

A)larger diameter lenses or mirrors and shorter wavelength light (or other electromagnetic radiation).
B)smaller diameter lenses or mirrors and longer wavelength light (or other electromagnetic radiation).
C)smaller diameter lenses or mirrors and shorter wavelength light (or other electromagnetic radiation).
D)larger diameter lenses or mirrors and longer wavelength light (or other electromagnetic radiation).

Question 40

Assuming that all effects due to variations in Earth's atmosphere could be removed (with adaptive optics) from images produced by a telescope, what diameter of a telescope would be needed to see two bright (laser) headlights of wavelength 650 nm separated by 2 m on a vehicle on the Moon? (Assume Earth-Moon distance to be 400,000 km.)

A)0.16 m
B)1600 m
C)1.6 m
D)160 m

Question 41

If all effects caused by Earth's atmospheric variations (seeing) could be removed from the visible image of a star on one of the 10-m diameter Keck telescopes on Hawaii, what would be the angular resolution achievable by this telescope in arcseconds for light of wavelength 500 nm? (1 nm = 10^-9 m)

A)0.00125 arcsec
B)0.125 arcsec
C)12.5 arcsec
D)0.0125 arcsec

Question 42

To compensate for atmospheric distortion using adaptive optics, a bright object must serve as a target. How is this accomplished?

A)Adaptive optics can only be used when a bright star is in the field of view.
B)Sunlight reflecting from artificial satellites serves as the target.
C)Adaptive optics can only be used on very powerful telescopes for which even faint stars can be used as targets.
D)Lasers are used to excite sodium atoms in the upper atmosphere to glow and serve as a target.

Question 43

The diffraction-limited angular resolution of one telescope is 0.1 arcsec and that of another is 0.2 arcsec. What does this mean?

A)The telescope with 0.1 arcsec has better resolution.
B)The telescope with 0.2 arcsec has better resolution.
C)It really doesn't mean anything, because telescopes are never limited by diffraction.
D)It is relatively unimportant, because angular resolution is only one of many forms of resolution that must be taken into account.

Question 44

What do adaptive optics actuators do?

A)They deform the telescope mirror to compensate for turbulence in the atmosphere.
B)They deform the telescope mirror to compensate for the mirror's tendency to sag under its own weight.
C)They add false color to the images.
D)They compensate for Earth's rotation.

Question 45

Recently, it has become possible to equip telescope mirrors with mechanical devices called actuators, which can move a portion of the mirror small distances very rapidly. Three of the statements below describe consequences of employing this technique, and one does not. Which statement is the exception?

A)active optics - the ability to change the shape of the mirror to compensate for sagging and for expansion/contraction as the mirror temperature changes
B)adaptive optics - the ability to change the shape of the mirror to compensate for atmospheric changes that would otherwise distort the image
C)the capability of creating segmented mirrors much larger than any single-piece mirror
D)the ability to build a single mirror that can detect electromagnetic radiation all across the spectrum, from gamma rays to radio waves

Question 46

During the past few decades a number of techniques have been developed to enhance the performance of optical and radio telescopes. These include all of the following except one. Which one is the exception?

A)placing optical telescopes in orbit above the atmosphere
B)building lens telescopes comparable in size to the largest mirror telescopes (approximately 10 m)
C)computer connections allowing radio telescopes thousands of miles apart to be used in unison
D)adaptive optics, allowing the shape of a telescope mirror to be changed rapidly to compensate for atmospheric turbulence

Question 47

Each of the following is a technique used by astronomers to increase the amount of detail that can be seen or photographed through telescopes except one. Which is the exception?

A)antireflective coatings, where the mirror is coated with a substance, such as fluorite, to reduce the amount of reflected light
B)increased size, where mirrors are made far larger than was possible before, e. g., 8 to 10 m in diameter and larger
C)adaptive optics, where the tilt and shape of mirrors in the telescope are changed many times per second to compensate for atmospheric turbulence
D)multiple-mirror telescopes, where several mirrors are mounted together in a single telescope to simulate the performance of a single, very large mirror

Question 48

How are the Keck telescopes on Mauna Kea designed to work?

A)A single, thin primary mirror that has a 20-m diameter has its shape changed and controlled by a computer.
B)The light from two 10-m telescopes, which are 85 m apart, will eventually be combined into a single image.
C)A dish of mercury 100 m in diameter is rotated and thus forms a very large parabolic mirror.
D)Six thin mirrors with diameters of 10 m are arranged in a circle to simulate a single mirror with a diameter of 30 m.

Question 49

What angular resolution is possible with the Keck telescopes when light from both telescopes is combined? (Assume observations are made in visible light at 500 nm wavelength.)

A)1/20,000 arcsec
B)1/200 arcsec
C)1/2000 arcsec
D)1/5 arcsec (limited by atmospheric turbulence, or "seeing")

Question 50

What factor has seriously reduced the effectiveness of the telescopes around the world, such as those at Mt. Palomar in California and at Kitt Peak, near Tucson, Arizona, over the last few years?

A)tarnishing of the mirror surface by air pollution
B)cracking of the mirror by earthquakes
C)bending of the mirror surface from repeated exposure to the cold night air
D)light scattering in the atmosphere from nearby cities

Question 51

You are using a telescope with a diameter of 20 cm, and you are observing with light of wavelength 650 nm. You are looking at the word "astronomy" on a distant signboard. What maximum angular spacing (in arcseconds) between adjacent letters will ensure that the entire word can be seen clearly?

A)0.25 arcsec
B)0.50 arcsec
C)0.75 arcsec
D)1.00 arcsec

Question 52

Suppose the four 8.2-m mirrors of the Very Large Telescope were replaced with 10-m mirrors, but they were still spread out over a distance of 200 m. The combined light-gathering power would be equivalent to that of a single telescope of what diameter?

A)16.4 m
B)20 m
C)40 m
D)200 m

Question 53

The resolution in a telescope is best for which segment of the visible spectrum?

A)red
B)green
C)blue
D)The resolution is the same all across the visible spectrum.

Question 54

At the present time (2018), the telescope mirror with the largest diameter in the world is in:

A)the Gran Telescopio Canarias.
B)the European Southern Observatory.
C)Gemini North.
D)the Keck II telescope on Mauna Kea, Hawaii.

Question 55

What is a charge-coupled device (CCD)?

A)a very fine grained photographic film that can be "read" electronically
B)an extremely sensitive thermocouple that records the temperature increase every time a photon hits it
C)a spectrometer that has been made especially sensitive by using a grating with over one million lines per centimeter
D)an array of electronic sensors that record the charge buildup due to photon absorption

Question 56

The primary reason CCDs can more efficiently observe faint stars than photographic plates of film can is:

A)the information on CCDs can be stored electronically.
B)CCDs are much more efficient at recording the light that falls on them.
C)CCDs are cheaper to build.
D)CCDs reduce the diffraction-limited angular resolution.

Question 57

Approximately what percentage of the light falling on a typical photographic plate from a telescope is actually used to record an image on this plate?

A)25%
B)50%
C)98%
D)2%

Question 58

What happens if adjacent pixels in a CCD become so close together that the angular separation between them is smaller than the resolution limit of the telescope being used?

A)The electronic signals from these adjacent pixels will really represent a single signal representing the average intensities of this two-pixel region.
B)The resolution of the picture will improve dramatically.
C)The CCD can no longer be used on this instrument.
D)Only the long-wavelength images will remain sharp.

Question 59

A spectrograph is usually used in astronomy to measure the:

A)variation of the mass of an object as it moves through space.
B)distribution of light intensity among the various colors.
C)vibration of Earth following an earthquake.
D)brightness of light at one specific color.

Question 60

In a spectrograph, a glass prism has a number of disadvantages compared to a diffraction grating. Which of the following is not one of these disadvantages?

A)A prism cannot be used with a CCD detector and a computer.
B)A prism is opaque to ultraviolet light.
C)A prism reduces the light intensity unevenly over the spectrum.
D)A prism does not disperse the colors uniformly.

Question 61

In spectroscopy, a diffraction grating is a:

A)screen placed at the focus of a telescope to provide a calibrated grid on photographs.
B)source of light that provides calibration lines alongside the measured spectrum.
C)piece of glass or metal on which many closely spaced grooves have been cut.
D)prism of very pure glass through which light can pass.

Question 62

When making a spectrum of a distant galaxy, it is usual practice to allow light from a laboratory sample of some element, say neon, to fall upon the spectrum as well. Why is this done?

A)The known positions of the neon spectrum lines allow the spectrum to be calibrated so that the wavelengths of the lines from the galaxy can be determined.
B)The emission lines from the laboratory neon will fill in the absorption lines from neon in our atmosphere, thus getting rid of these lines so that they are not confused with lines from the galaxy.
C)The laboratory neon lines show the maximum velocity position of neon. These laboratory neon lines can then be compared with neon lines from the galaxy spectrum so the Doppler shift based on neon can be calculated.
D)The procedure is just a test to assure that the system is working properly without distortion before the galaxy spectrum is analyzed.

Question 63

The best diffraction gratings have about how many lines per centimeter?

A)100
B)1000
C)10,000
D)100,000

Question 64

One major difference between radio waves and light is that:

A)radio waves have shorter wavelengths.
B)radio waves are electromagnetic, unlike light.
C)light waves are electromagnetic, unlike radio waves.
D)radio waves have lower frequencies.

Question 65

The first true radio telescope, built by Grote Reber to map radio emissions from the center of the Milky Way, was built in:

A)1980.
B)1946.
C)1932.
D)1936.

Question 66

Astronomers began to use radio telescopes to investigate the universe:

A)during the 1920s.
B)during the 1980s, with the development of CCD detectors.
C)in the early 1960s, with the development of satellites.
D)in the late 1940s, shortly after World War II.

Question 67

One way to improve the angular resolution of a radio telescope is to:

A)increase the exposure time.
B)observe at a longer wavelength.
C)observe from high mountains where atmospheric distortion is less.
D)make the dish size larger.

Question 68

The quality of images produced by a large radio telescope is limited mostly by:

A)atmospheric distortion.
B)light pollution.
C)diffraction.
D)cloud cover.

Question 69

The angular resolution attainable with a radio telescope, compared to that attainable with an optical telescope of the same diameter, is significantly inferior because:

A)the wavelength of radio waves is larger than that of visible light.
B)it is difficult to make a reflector for radio waves since these waves penetrate most materials.
C)Earth's atmosphere disturbs radio waves from space much more than it does visible light.
D)radio wavelengths are smaller than visible wavelengths, making it difficult to produce a reflector sufficiently smooth to produce images.

Question 70

Why is the angular resolution of a 20-m-diameter radio telescope worse than that of a 0.5-m-diameter optical telescope?

A)The ratio of wavelength to telescope diameter is larger for the radio telescope.
B)It is difficult to make a radio dish sufficiently smooth to reflect radio waves.
C)Atmospheric turbulence disturbs radio waves more than it does visible light.
D)The ratio of wavelength to telescope diameter is smaller for the radio telescope.

Question 71

How does the angular resolution of the 305-m Arecibo radio telescope in Puerto Rico compare with the angular resolution of the 10-m Keck optical telescope on Mauna Kea, Hawaii?

A)They have about the same angular resolution, since angular resolution is limited by turbulence in Earth's atmosphere, not by mirror diameter.
B)The 305-m telescope has much worse angular resolution than the 10-m Keck telescope.
C)It is not possible to compare their angular resolution, since they work in different wavelength ranges.
D)The 305-m telescope has much better angular resolution than the 10-m Keck telescope.

Question 72

Which of the following systems will produce the best resolution?

A)a single radio telescope with a 100-m dish
B)two 25-m radio telescopes separated by 200 m
C)two 50-m radio telescopes separated by 200 m
D)two 25-m radio telescopes separated by 380 m

Question 73

A 10-m-diameter radio telescope has worse angular resolution than a 1-m-diameter optical telescope because:

A)radio waves have much longer wavelengths than visible light.
B)optical mirrors suffer from chromatic aberration.
C)angular resolution gets worse as mirror size increases.
D)radio waves have much shorter wavelengths than visible light.

Question 74

What is the main reason for combining many radio telescopes together into an interferometer with large distances between telescopes?

A)to obtain much sharper images of sources
B)to ensure that at least one of the telescopes is in a radio interference-free zone
C)to ensure that observations are uninterrupted by the failure of one or two telescopes
D)to collect more radiation from very faint sources

Question 75

Using the combined signals from radio telescopes on opposite sides of Earth, the highest resolution attainable on astronomical images from Earth's surface by Very Long Baseline Array is:

A)1 arcsec.
B)0.001 arcsec.
C)0.1 arcsec.
D)1 arcmin.

Question 76

The Very Large Array (VLA) built in New Mexico provides very sharp radio images of astronomical sources by combining the signals from many individual telescopes. It consists of:

A)85 units.
B)2 units.
C)3 units.
D)27 units.

Question 77

The angular resolution of a radio telescope with a baseline equal to the diameter of Earth is given as 0.001 arcsec. What wavelength is being used for the calculation?

A)0.02 m
B)0.05 m
C)0.5 m
D)20 m

Question 78

An optical telescope using 650-nm light has a diameter of 1 m. A radio telescope using radio waves of wavelength 6.5 cm has a diameter of 100 m. The resolution of the optical telescope is:

A)better than that of the radio telescope by a factor of 1000.
B)worse than that of the radio telescope by a factor of 1000.
C)better than that of the radio telescope by a factor of 100,000.
D)worse than that of the radio telescope by a factor of 100,000.

Question 79

One important fact to keep in mind when comparing visible and infrared images of Saturn is that:

A)neither visible nor IR is emitted by Saturn. What we see is entirely reflected from the Sun.
B)visible radiation is emitted by Saturn but the IR is entirely reflected from the Sun.
C)IR radiation is emitted by Saturn but the visible is entirely reflected from the Sun.
D)both visible and IR are emitted by Saturn in substantial amounts.

Question 80

A high mountaintop such as Mauna Kea in Hawaii is a good site for an infrared observatory because it is:

A)above most of the moisture in Earth's atmosphere.
B)farther from city lights.
C)closer to the stratospheric ozone layer.
D)above most of the turbulence in Earth's atmosphere.