Celestial objects other than Solar System bodies are so far away that the light emitted by them is practically the only means of understanding their true nature. Light can behave as a wave or as a stream of particles called photons. Some of the light-related phenomena can be interpreted by using wave nature whereas others can be interpreted by using particle nature of the light.

A basic property of light is its wavelength which is defined as the distance between consecutive wavecrests.

Visible light just a limited portion of the electromagnetic spectrum, which extends from the gamma-rays to radio wavelengths. Actually both ends of the electromagnetic spectrum extend without a limit.

White light is composed of various light waves of different wavelengths. When we pass light from a prism, it spread out its component wavelengths or colors. The science of analyzing the spectra is called spectroscopy.

An electromagnetic wave consists of varying electric and magnetic fields. These fields propagate through space with a velocity of approximately 300 000 km per second. The wavelength of visible light is so small that we use a special unit called Angstrom.

1 Angstrom=10^-8 cm

Visible light lies between 4000 Æ and 7000 Æ in wavelength. Other properties of a light wave is its frequency and energy which are defined through

f=c/l and E=hc/l

where c is the speed of light in the medium considered ( i.e., empty space)