Over the past hundred years or so, physics experiments have demonstrated that light has a dual nature. In many instances, it is convenient to represent light as a particle phenomenon, thinking of light as discrete packets of energy that we call photons. Now in this way of thinking, not all photons are created equal, at least in terms of how much energy they contain. Each photon of X ray light contains a lot of energy in comparison with, say, an optical or radio photon. It is this energy content per photon that is one of the distinguishing characteristics of the different ranges of light described above. Even though it is not strictly correct, it is hard not to think of a beam of light as a collection of little light bullets all strung together in a row.

A different way of representing light is as a wave phenomenon. This is somewhat more difficult for most people to understand, but perhaps an analogy with sound waves will be useful. When you play a high note and a low note on the piano, they both produce sound, but the main thing that is different between the two notes is the frequency of the vibrating string producing the sound waves the faster the vibration the higher the pitch of the note. If we now shift our focus to the sound waves themselves instead of the vibrating string, we would find that the higher pitched notes have shorter wavelengths, or distances between each successive wave. Restricting ourselves to optical light for the moment, blue light and red light are both just light, but the blue light has a higher frequency of vibration or a shorter wavelength than the red light.

The colors of the familiar rainbow of visible light correspond to differing wavelengths of the light, here shown on a nanometer scale. The wavelengths get successively larger as one moves from left to right. Optical light runs from about 400 to 700 nanometers. It is the same way as we move throughout the electromagnetic spectrum. Each range of light we have defined above corresponds to a range of frequencies or wavelengths of light vibrations. These wavelengths are one of the primary indicators we use to describe light and spectra. For instance, the rainbow of color is what you see when you pass white light through a prism. What may not be obvious, however, is that the intensity or brightness of the light is also changing along with the colors.

The role of light in optics is very essential. It is the scientific study of light. Physical optics is concerned with the creation, nature, and properties of light. Psychological optics pertains to the role of light in vision. Geometrical optics deals with the properties of reflection and refraction of light, as part of the study of mirrors, lenses, and optical fibers. The study of optics falls into three general categories, light itself, what it is and how it behaves, how we perceive light through the sense of sight, and how light can be manipulated through such processes as reflection and refraction. Although we talk of these categories as if they were separate and distinct, actually the whole reality of sight and vision constantly involves all of these categories in various combinations. Examples of optics and light in nature are all around us are rainbows, glories, sundogs, iridescence on animals, halos, and the shadow of the earth. These are just a few examples of the myriad of natural optical phenomena that happen everyday, in your backyard or right over your head.