The study of light is called optics. But what is light? The first Greek scientists did not make a distinction between light and vision. Light, to them, was inseparable from seeing. But gradually there arose a view that light actually “exists,” that light is some sort of physical entity that is present regardless of whether or not someone is looking. But if light is a physical entity, what is it?

Newton, in addition to his pioneering work in mathematics and mechanics in the 1660s, investigated the nature of light. Newton knew that a water wave, after passing through an opening, spreads out to fill the space behind the opening. You can see this in FIGURE 33.1a, where plane waves, approaching from the left, spread out in circular arcs after passing through a hole in a barrier. This inexorable spreading of waves is the phenomenon called diffraction. Diffraction is a sure sign that whatever is passing through the hole is a wave.

FIGURE 33.1 Water waves spread out behind a small hole in a barrier, but light passing through an archway makes a sharp-edged shadow.

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In contrast, FIGURE 33.1b shows that sunlight makes sharp-edged shadows. We don’t see the light spreading out in circular arcs after passing through an opening. This behavior is what you would expect if light consists of noninteracting particles traveling in straight lines. Some particles would pass through the openings to make bright areas on the floor, others would be blocked and cause the well-defined shadow. This reasoning led Newton to the conclusion that light consists of very small, light, fast particles that he called corpuscles.

The situation changed dramatically in 1801, when the English scientist Thomas Young announced that he had produced interference of light. Young’s experiment, which we will analyze in the next section, quickly settled the debate in favor of a wave theory of light because interference is a distinctly wave-like phenomenon. But if light is a wave, what is waving? It was ultimately established that light is an electromagnetic wave, an oscillation of the electromagnetic field requiring no material medium in which to travel.

But this satisfying conclusion was soon undermined by new discoveries at the start of the 20th century. Albert Einstein’s introduction of the concept of the photon—a wave having certain particle-like characteristics—marked the end of classical physics and the beginning of a new era called quantum physics. Equally important, Einstein’s theory marked yet another shift in our age-old effort to understand light.