Astronomers have learned most of what we know about stars and galaxies by analyzing the electromagnetic radiation coming from them. Electromagnetic radiation, of which the light that our eyes respond to is one part, is a form of energy. Without any material aspects, it is energy that can move through the empty reaches of the universe.
Light As Wave
Historically, this conceptual view began in 1862, when the Scottish physicist James Clerk Maxwell (1831-1879) showed that light is energy carried in the form of a traveling wave composed of electric and magnetic fields. The electric and magnetic fields vary in intensity, and they are at right angles to each other and to the direction in which the wave is propagating. The electric and magnetic fields continually interact, forming an electromagnetic wave. These fields maintain themselves and continue to propagate until the energy of the wave is converted to some other form of energy. At that point the electromagnetic wave ceases to exist. Maxwell's proposal that light is an electromagnetic wave, as we shall soon see, was not the last word in explaining the physical nature of light. But visualizing light as waves spreading out from a radiating source helps us to understand many aspects of it.
The speed of light measured in empty space is 299,792 kilometers per second (3 x 105 kilometers per second in round figures, or 186,300 miles per second). This appears to be the speed limit for all energy transported in the universe. As we discussed in the last chapter, the speed of light is a fundamental constant of nature and apparently has the same value throughout the universe.
Electromagnetic waves possess a range of wavelengths, the distance between successive crests or troughs, and a range of frequencies. The product of the wavelength and frequency gives the speed at which the electromagnetic wave travels. The amount of energy the wave transports is proportional to the square of the wave's amplitude.
