81 Physics of Light

Basic Properties of Light

We define light as the electromagnetic rays that interact with the photoreceptors in our eyes. A honeybee would have a different definition of light, because it can see ultraviolet rays (200-400nm), where we just think of UV as a sunburn hazard.

Light is a form of electromagnetic radiation. Electromagnetic radiation is sinusoidal electric and magnetic fields that are oriented at right angles (orthogonal) to each other, out of phase (one gets large while the other gets small, then they trade off). Because the fields feed off of each others energy, they can propagate through a vacuum. The rate at which the fields rise and fall as they travel is the frequency; the distance that it takes for one cycle to happen as the fields travel is wavelength. All electromagnetic radiation travels at the same speed (300 million meters per second in a vacuum), and speed = wavelength x frequency, so light with longer wavelengths has electromagnetic fields that oscillate at lower frequencies.

These waves come in a wide variety of wavelengths called the electromagnetic spectrum. The portion of the electromagnetic spectrum that we call light is the portion that has wavelengths between 400 and 700 nanometers — these are the frequencies that are absorbed by the photoreceptors in our eyes.

One property of light is that it is quantized, which means it can only take on specific values. Even though light propagates as a continuous dance between electric and magnetic fields, the energy is quantized—a photon is the smallest amount of light that can be generated or transmitted.

Figure 8.1.1.. Electromagnetic spectrum. Humans are only able to perceive wavelengths of light between 400nm and 700nm. The wavelength is directly related to the perceived color that is seen. Shorter wavelengths are perceived as violet, where are longer wavelengths are perceived as red. Credit: EM_spectrumrevised © Philip Ronan, Gringer is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license

Factors that Determine Color

The color of an object is determined by which wavelengths the object reflects and which it absorbs. Since different light sources provide different wavelengths of light, illumination can change the amount of light reflected and absorbed at each wavelength. This may or may not change our perception of the color — later sections of this book will talk about color perception and how it depends on context!