We might wish to ask ourselves "What is light?" The simplest answer to this question is that light is a form of energy. A better question, however, might be "What is the nature of light?" The answer to this question is twofold.

First, light can be considered a wave. When you think of a wave, you generally think of a water wave or possibly a sound wave. Begin by considering the shape of a water wave. It curves up and down in a repetitive fashion. The top portion of the wave is considered a peak, the bottom portion of the wave is called a trough. The distance from one peak to the next peak is called the wavelength. If you are on a boat through which this water wave was passing, the number of waves passing under the boat each second would be the frequency of the wave.

In this same fashion, light is a wave. It has the characteristics of frequency and wavelength. Another wave with which we are familiar is the sound wave. We are familiar with the concept of frequency as it pertains to sound waves. High frequency sound waves are also called high-pitched sound waves. Such waves may be produced by a musical instrument such as a flute. Low frequency sound waves would be produced by a tuba. If we consider the size of these two musical instruments, we can see that a tuba is a large instrument and a flute is a small instrument. The size of the instrument is directly proportional to the wavelength of the sound wave. So a high frequency flute sound wave has a small wavelength. Conversely, a low frequency tuba sound wave has a large wavelength. I find that pipe organs are an even better representation of a musical instrument showing wavelengths and frequencies. Large pipes have large wavelengths and produced low frequencies, small pipes have small wavelengths and produce high frequencies.

The same arguments hold for light waves as well. High frequency light waves have small wavelengths, low frequency light waves have large wavelengths. But what do we mean when we discuss small and large wavelength of light? When we consider sound waves, various wavelengths of sound produce various frequencies, or what we call "pitches." In the case of light waves, various frequencies (and therefore wavelengths) are interpreted by humans as various colors. Red light has a long wavelength and blue light has a short wavelength. The distribution of colors between red and blue is what is referred to as the spectrum of light.

There exists colors of light which the human eye cannot see, just as there are sounds we cannot hear (dog whistles for example). These colors are invisible. They include such colors as infrared, ultraviolet, gamma rays, and radio waves plus others. Astronomers are careful not to ignore the invisible forms of light. Although these forms of light have different frequencies, they share the same basic nature. They are fluctuations which propagate through electric and magnetic fields filling all space. For this reason, they are often referred to as electromagnetic waves.

In addition to having wave characteristics, light seems to also have the characteristics of particles. For many years this was a contradiction in science. Any form of energy or matter either had characteristics of a particle or the characteristics of a wave but never both. Light was the first form of matter or energy discovered to have both characteristics. Since this discovery, particles such as electrons, protons, and neutrons have been discovered to contain both wave and particle characteristics. Discussion of these characteristics will be left to future courses. The particle nature of light is referred to as a photon. In general we shall look at light as a wave, but on occasion we will call light a photon and be discussing its particle characteristics.

We shall consider two important sources of light or radiation. The first of these are thermal sources or black body radiation. An incandescent bulb is an example of this type of radiation. The second of these will be atomic radiation. A fluorescent lamp is an example of this type of radiation. Let's first address black body radiation.

Thermal Radiation

Atomic Radiation

This page was last updated on 06/13/01.