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 radioemitting objects. Long baseline interferometers can produce much better resolution than optical telescopes.
