Chapter 5 AstroPhysics
Refracting telescopes are so called for which of the following reasons?
The light is bent to a focus.
Which of the following is the first thing light hits when it enters a telescope?
the primary lens or mirror
In a telescope, which of the following is the optical element with the shortest focal length?
the eyepiece
What is the biggest advantage of a reflecting telescope?
The light never passes through glass.
Which of the following is a reason that reflecting telescopes have shorter tubes than refracting telescopes?
The light goes up the tube and then back down.
Why are short tubes an advantage for large telescopes?
Short tubes can fit inside a smaller observatory building.
Short tubes can be supported with less vibration.
Short tubes weigh less than those of refracting telescopes
Which of the following is true of reflecting telescopes?
They focus light by bouncing it off of optical surfaces.
Based on what you learned about light, select all of the correct statements from the following list.
A photon is a particle of light.
High frequency photons carry more energy than long wavelength photons.
Light can act both as a wave and a particle.
All electromagnetic waves travel at the speed of light.
Based on what you learned about telescopes, select all of the correct statements from the following list.
Recently built modern telescopes are reflectors.
Based on what you learned about radio telescopes, select all of the correct statements from the following list.
Radio telescopes have to be larger than optical telescopes, since radio waves are longer than visible light waves.
Radio waves can travel through obstacles that block out waves of visible light.
Radio telescopes are used to detect cool hydrogen.
Based on what you learned about astronomy from space, select all of the correct statements from the following list.
Most types of electromagnetic waves are not visible to ground-based telescopes.
The Compton, Chandra and Spitzer telescopes are all examples of space telescopes.
The visible part of the electromagnetic spectrum can be divided into seven color bands: red, orange, yellow, green, blue, indigo, and violet (from long to short wavelength). A single photon of which of these colors has the greatest amount of energy?
violet
The entire electromagnetic spectrum can be divided into seven bands: radio, microwave, infrared, visible, ultraviolet, X ray, and gamma ray (from longest to shortest wavelength). To which of these two bands is Earth’s atmosphere the most transparent?
visible and radio
In this figure you can see that different wavelengths of light are refracted (bent) by different amounts. Which wavelengths are bent by the greatest amount?
short wavelengths
Why do the pupils of a cat’s eyes open wider at night?
To increase light gathering power.
Why have no large refracting telescopes been built since 1900?
Refracting telescopes suffer from chromatic aberration.
Making large glass lenses without interior defects is difficult.
Refracting telescopes have several surfaces to shape and polish.
Large glass lenses are more difficult to support than large mirrors
Of the two telescopes shown here, which will allow you to see dimmer objects?
The telescope on the left.
Which power of a telescope is the least important?
magnifying power
What can radio telescopes do that optical telescopes cannot?
Find the location of cool hydrogen gas and see through dust clouds
What is a disadvantage of radio telescopes compared to optical telescopes?
Radio photons have lower energy, thus radio waves have low intensity.
interference from nearby sources of radio waves
poor resolving power
Radio photons have lower energy, thus radio waves have low intensity and interference from nearby sources of radio waves.
Radio telescopes are often connected together to do interferometry. What is the primary problem overcome by radio interferometry?
poor resolving power
Why are near-infrared telescopes located on mountaintops and ultraviolet telescopes in Earth orbit?
The primary infrared blocker, water vapor, is mostly in the lower atmosphere and the primary ultraviolet blocker, ozone, is located high in the atmosphere, far above mountaintops.
Why must far-infrared telescopes be cooled to a low temperature?
To reduce interfering heat radiation emitted by the telescope.
Rank the following colors of visible light in order of increasing wavelength: blue, green, red, yellow.
least blue
green
yellow
greatest red
green
yellow
greatest red
Rank the following colors of visible light in order of increasing frequency: green, red, violet, yellow.
least red
yellow
green
greatest violet
Rank the following types of electromagnetic radiation in order of decreasing wavelength: infrared, gamma-ray, radio, ultraviolet, visible.
greatest radio
infrared
visible
ultraviolet
least gamma-ray
(a) Rank telescopes designed for the following specific types of electromagnetic waves in order of the minimum altitude at which they would be useful: infrared, radio, visible, x-ray.
least radio
visible
infrared
greatest x-ray
Rank telescopes designed for the following specific types of electromagnetic waves in decreasing order of the altitudes at which they would be useful: infrared, radio, ultraviolet, visible.
greatest ultraviolet
infrared
visible
least radio
Wavelengths of light are measured in units of
nanometers (nm)
Visible light has wavelengths
between 400-700 nm.
Astronomical Telescopes
• Primary objective:
• Primary objective:
to gather large amounts
of light
of light
distance from the center of the lens to the image formed.
Focal length
Different wavelengths are
focused at different focal
lengths (prism effect)
• Can be corrected, but not
eliminated, by second
lens out of different
material
• Second lens is difficult and
expensive to produce; all
surfaces must be perfectly
shaped; glass must be
flawless; lens can only be
supported at the edges
focused at different focal
lengths (prism effect)
• Can be corrected, but not
eliminated, by second
lens out of different
material
• Second lens is difficult and
expensive to produce; all
surfaces must be perfectly
shaped; glass must be
flawless; lens can only be
supported at the edges
Disadvantages of
refracting telescopes
refracting telescopes
The Best Location for a Visible light telescopes?
Far away from civilization-to avoid light pollution.
The Best Location for a Visible light Telescope?
On high mountaintops-to avoid atmospheric turbulence (→ seeing) and other weather effects.
Best location for radio
telescopes?
telescopes?
Located far from populated areas (to avoid TV, radio, radar) à often in valleys to shield them
→ Combine the signals from several smaller
telescopes to simulate one big mirror →
telescopes to simulate one big mirror →
Interferometry
Best location for Infrared
Telescopes
Telescopes
Infrared observations can
also be performed form high-
flying aircraft.
Best locations for X-Ray
Telescopes •
Telescopes •
Must be placed in space on satellites