Astronomy-Chapter 6

A Newtonian telescope consists of which of the following combinations of optical elements?

one concave and one flat mirror

A department store sells an “astronomical telescope” with an objective lens of 30 cm focal length and an eyepiece lens of focal length 5 mm. What is the magnifying power of this telescope?

60×

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?

the same angle α on the opposite side of the perpendicular to the incident ray

A particular reflecting telescope has an objective mirror with a focal length of 1.2 m and an eyepiece lens of focal length 6 mm. What is the magnifying power of this telescope?

200×

What is a radio telescope similar to?

is very similar to a reflecting optical telescope.

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?

400

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?

16×

A spherical mirror suffers from spherical aberration because

different parts of the mirror focus the light at different distances from the mirror.

A typical refracting telescope is made up of

a long-focal-length lens at the front and a short-focal-length lens at the rear

An electromagnetic wave has a wavelength of 80 cm. This wave is

a radio wave.

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.)

160 m

By what factor is the amount of light gathered by the 10-m diameter Keck telescope on Mauna Kea, Hawaii, greater than that gathered by the 2.5-m diameter Mount Wilson telescope?

16

Chromatic aberration occurs in a refracting telescope when

all colors of light do not come to the same focal point.

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

less.

High energy telescopes detect the radiation at the high-energy end of the electromagnetic spectrum, namely

X rays and gamma rays.

How does angular resolution for a given diameter of telescope depend on wavelength?

Angular resolution worsens as wavelength increases.

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

4 × 10⁶

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)

0.0125 arcsec

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

The blue light converges to a point that is slightly closer to the lens than the point to which the red light converges.

Light enters a plane-parallel block of glass from the vacuum of space and exits through the opposite side (e.g., in a space-borne instrument). What is the speed of light after it exits the glass?

the same as when it entered the glass

Many of the telescopes sent into space have to be cooled to liquid helium temperatures. Why is this?

The telescope will radiate as a blackbody, and this radiation will obscure the astronomical signal. This blackbody radiation is reduced at cooler temperatures.

One major difference between radio waves and light is that

radio waves have lower frequencies.

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

make the dish size larger.

Over what range of wavelengths does the Hubble Space Telescope operate?

infrared, visible, and ultraviolet

The constituents of the atmosphere that absorb most strongly at infrared wavelengths, limiting our ability to observe astronomical infrared objects from the surface of Earth, is (are)

water vapor, H₂O.

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

The telescope with 0.1 arcsecond has better resolution.

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

sag too much under their own weight.

The light-gathering power of a telescope is related directly to the

area of its primary mirror or lens.

The main reason for placing astronomical telescopes and detectors on satellites is to

get above the absorption in Earth's atmosphere.

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

blue

The shortest wavelength observed by HST is 115 nm with objective mirror 2.4m. What is its angular resolution at this wavelength?

0.01 arcsec

The two ranges of electromagnetic radiation for which Earth's atmosphere is reasonably transparent are

visible and radio radiation.

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

Different colors are refracted at different angles to produce a spectrum.

What is a charge-coupled device (CCD)?

an array of electronic sensors that record the charge buildup due to photon absorption

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?

160

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

to obtain much sharper images of sources

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

Angular resolution gets worse as wavelength increases.

What is the refraction of light?

 

the change in direction of a light ray as it crosses from a less dense, transparent material to a more dense one

When a ray of light strikes a smooth mirror surface at an angle to the perpendicular, the ray is reflected

on the “other” side of the perpendicular but at the same angle as the incoming ray.

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

shorter wavelengths