* Waves are regular series of disturbances that move through space. Wavelength (the distance between crests of the wave), frequency (the rate at which waves pass), and energy describe a wave.
* Light and other forms of electromagnetic radiation are oscillating
electric and magnetic fields produced by accelerating electric
charges. The different forms of electromagnetic radiation all
travel at the speed of light, but they differ greatly in wavelength
and frequency.
* The Doppler effect is a change in the wavelength and frequency
of a wave as a consequence of motion of the source of the wave
and the observer toward or away from each other. The amount of
change in wavelength is proportional to the speed of the source
relative to the observer.
* Some properties of light are best explained if light is a stream
of massless particles called photons. The frequency of a photon
determines its energy.
* An electromagnetic wave can be reflected from a surface or refracted
when it passes from one material to another. Dispersion occurs
when the amount of refraction depends on wavelength.
* Refracting telescopes collect light using lenses. Reflecting
telescopes use mirrors to bring light to a focus. All large telescopes
are reflectors because large lenses are expensive to make and
because large lenses sag under their own weight.
* The brightness of the image formed by a telescope depends on
its focal ratio, the ratio of the focal length to the diameter
of its main lens or mirror. The area of the main lens or mirror
of a telescope determines its light-gathering power.
* The resolution of a telescope, the finest detail that it can
distinguish, improves as its diameter increases. Resolution worsens,
however, with increasing wavelength.
* After light has been focused by a telescope, its properties
can be measured using detectors, which can determine the brightness
of an image over a large range of wavelengths or produce a spectrum
in which the light can be measured at many wavelengths.
* All large modern telescopes, including the 10-m Keck telescopes,
are reflectors. Other large reflectors are now being built or
planned. Many of these telescopes will use segmented mirrors or
thin mirrors to reduce costs and improve optical performance.
* Astronomers build observatories on mountaintops to benefit from
clear skies, atmospheric transparency, good seeing, and the absence
of light pollution. The best sites are those at which the blurring
of images resulting from atmospheric turbulence is at a minimum.
Astronomers are using adaptive optics to overcome the blurring
effects of atmospheric turbulence.
* The Earth's atmosphere blocks incoming X-ray, gamma ray, ultraviolet,
and most infrared radiation. Consequently, space observatories
have been put in orbit to observe in these parts of the electromagnetic
spectrum and to obtain optical images that are free of atmospheric
distortion.
* Because individual radio telescopes have poor resolution, radio
astronomers use interferometers to measure the interference patterns
of radio-emitting objects. Long baseline interferometers can produce
much better resolution than optical telescopes.
adaptive optics,charge coupled devices, (CCD) detector, dispersion Doppler effect, electromagnetic wave energy, flux, focal length, focal plane, focal point, frequency, index of refraction, interferometry light-gathering power, objective, photon, pixel, reflection, reflectivity, reflector, refraction, refractor resolution, seeing, spectrograph, spectroscopy, wave wavelength
1. Which of the following is true about the frequency and velocity of electromagnetic waves? as frequency increases, wavelength decreases
2. The wavelength of a wave is 8 meters. The frequency of the
wave is 2 Hz. What is the speed of the wave? 16 m/s
3. Which of the following are electromagnetic waves? all of these
4. Compared with light waves, radio waves have longer wavelengths
5. The focal ratio of a lens or mirror is the ratio of its focal
length to diameter
6. How does the light gathering power of a 100 inch telescope
compare to that of a 20 inch telescope? 25 times as great
7. Telescope A has a mirror which is 5 times larger than the mirror
of telescope B. Telescope A can resolve an angle as small as 0.1
seconds of arc. What is the smallest angle which can be resolved
by telescope B? 0.5 seconds of arc
8. What is the primary reason that radio astronomers use arrays
of telescopes to observe celestial objects? to obtain high angular
resolution
9. Imagine you are a graduate student in astronomy. Your Ph.D.
thesis is to determine if there is any krypton gas on a newly
discovered planet. You should outfit your telescope with a spectrometer
10. Suppose a source of electromagnetic waves is moving away from
us at 2\% of the speed of light. What can be said of the wavelength
of the waves we receive from the source? they are redshifted by
2\% The correct answer is: all of these
4. Compared with light waves, radio waves have longer wavelengths
5. The focal ratio of a lens or mirror is the ratio of its focal
length to diameter
6. How does the light gathering power of a 100 inch telescope
compare to that of a 20 inch telescope? 25 times as great
7. Telescope A has a mirror which is 5 times larger than the mirror
of telescope B. Telescope A can resolve an angle as small as 0.1
seconds of arc. What is the smallest angle which can be resolved
by telescope B? 0.5 seconds of arc
8. What is the primary reason that radio astronomers use arrays
of telescopes to observe celestial object? to obtain high angular
resolution
9. Imagine you are a graduate student in astronomy. Your Ph.D.
thesis is to determine if there is any krypton gas on a newly
discovered planet. You should outfit your telescope with a spectrometer
10. Suppose a source of electromagnetic waves is moving away from
us at 2\% of the speed of light. What can be said of the wavelength
of the waves we receive from the source? they are redshifted by
21%