A light wave is an electromagnetic wave, a self-sustaining oscillation of the electromagnetic field. Other electromagnetic waves, such as radio waves, microwaves, and ultraviolet light, have the same physical characteristics as light waves even though we cannot sense them with our eyes. It is easy to demonstrate that light will pass unaffected through a container from which all the air has been removed, and light reaches us from distant stars through the vacuum of interstellar space. Such observations raise interesting but difficult questions. If light can travel through a region in which there is no matter, then what is the medium of a light wave? What is it that is waving?

It took scientists over 50 years, most of the 19th century, to answer this question. We will examine the answers in more detail in Chapter 31 after we introduce the ideas of electric and magnetic fields. For now we can say that light waves are a “self-sustaining oscillation of the electromagnetic field.” That is, the displacement D is an electric or magnetic field. Being self-sustaining means that electromagnetic waves require no material medium in order to travel; hence electromagnetic waves are not mechanical waves. Fortunately, we can learn about the wave properties of light without having to understand electromagnetic fields.

It was predicted theoretically in the late 19th century, and has been subsequently confirmed, that all electromagnetic waves travel through vacuum with the same speed, called the speed of light. The value of the speed of light is

where the special symbol c is used to designate the speed of light. (This is the c in Einstein’s famous formula $E=m{c}^2\text{.})$ Now this is really moving—about one million times faster than the speed of sound in air!

The wavelengths of light are extremely small. You will learn in Chapter 33 how these wavelengths are determined, but for now we will note that visible light is an electromagnetic wave with a wavelength (in air) in the range of roughly $400\text{nm}(400\times {10}^{-9}\text{m})$ to $700\text{nm}(700\times {10}^{-9}\text{m})$. Each wavelength is perceived as a different color, with the longer wavelengths seen as orange or red light and the shorter wavelengths seen as blue or violet light. A prism is able to spread the different wavelengths apart, from which we learn that “white light” is all the colors, or wavelengths, combined. The spread of colors seen with a prism, or seen in a rainbow, is called the visible spectrum.

If the wavelengths of light are unbelievably small, the oscillation frequencies are unbelievably large. The frequency for a 600 nm wavelength of light (orange) is

The frequencies of light waves are roughly a factor of a trillion $\left({10}^{12}\right)$ higher than sound frequencies.

Electromagnetic waves exist at many frequencies other than the rather limited range that our eyes detect. One of the major technological advances of the 20th century was learning to generate and detect electromagnetic waves at many frequencies, ranging from low-frequency radio waves to the extraordinarily high frequencies of x rays. FIGURE 16.20 shows that the visible spectrum is a small slice of the much broader electromagnetic spectrum.

FIGURE 16.20 The electromagnetic spectrum from ${10}^6\text{Hz}$ to ${10}^{18}\text{Hz}$.