What is the Electromagnetic Spectrum?

Electromagnetic radiation is a form of energy emitted by all objects. Light rays are a form of electromagnetic radiation. Other rays in the electromagnetic spectrum are invisible to humans, such as television and radio waves, microwaves, X-rays, and gamma rays.

The electromagnetic spectrum is made up of waves of different lengths, forming bands. All of these waves have their own distinct set of measurements or properties: Speed, amplitude, wavelength and frequency are used to measure a wave. All electromagnetic waves travel at the same speed, 186,000 miles (300,000 km) per second, through a vacuum. This includes visible light.

A wave’s amplitude is the distance from the middle of the wave (or its resting position) to the crest or trough. The amplitude squared is proportional to the amount of energy the wave carries. In other words, a wave with twice the amplitude of another carries four times the energy.

The wavelength of a wave is measured from the crest of one wave to the crest of the next one, or from the trough of one to the trough of the next. The crest is the highest point of the wave and the trough is the lowest point of the wave. Radio waves are long waves, and can measure thousands of yards long from crest to crest. Microwaves have wavelengths of only a few millimeters, and cosmic rays are only a tiny fraction of a millimeter in length.

The frequency of a wave is its rate of vibration, or put another way the frequency of a wave is the number of crests (troughs) that pass a given point in one second. One wave per second equals one hertz. If four water waves pass the edge of a pier in 20 seconds, the frequency is four waves divided by 20 seconds, or 0.2 hertz. Waves with short wavelengths have high frequencies, while waves with long wavelengths have low frequencies.

Visible light, that which we see, forms the middle band of the spectrum. Light is unusual in that it behaves as a wave and a particle. The particles of light are made of energy, called photons, and they are able to knock electrons out of place in an atom just as a billiard ball knocks another out of place in a game of pool. Light also travels in a straight line as a wave made of a magnetic field and an electric field, which are oriented at right angles to each other.

Most objects look the color they are because of the way their structure reflects light. An object that reflects light of all wavelengths in equal amounts looks white. An object that absorbs shorter wavelengths of light but reflects longer waves looks red. An object that absorbs all wavelengths in equal amounts appears black.

Illustrations by Diane Boyack, 2000.