IF YOU WANDER past the fish counter at a supermarket and look at what’s on offer, what you see is mostly silver. The exceptions to the rule are tropical fish like red mullet and red snapper, and the bottom-dwelling fish such as sole and flounder. But mostly, you’re looking at fish that swim in the open ocean in big schools, like herring, sardines, and mackerel. Silver is interesting because it isn’t really a color. It’s just our word for something that acts as a trampoline for light, bouncing it back out into the world. All waves can be reflected, and almost all materials reflect some light. What’s special about silver is that it sends everything back indiscriminately. Every color is treated in the same way, no exceptions. Polished metal is really good at this trick, and it’s useful because the angle at which the light arrives is the angle at which it leaves. If you take an image of the world and use a mirror to bounce it in a different direction, the relative angles of all those light rays stay the same. It’s difficult to polish metal perfectly enough to get a perfect image, and mirrors have been very highly prized in human history. And yet we take silver fish for granted. The fish can’t even use metal; in order to be silvery, they’ve got to build structures that do the same job out of organic molecules. That’s complicated, and therefore expensive in evolutionary terms. If you’re a herring, why do you bother?
Herring roam the seas in schools, feeding on small shrimp-like creatures and hoping to avoid the big carnivores: dolphins, tuna, cod, whales, and sea lions. But the oceans are huge, empty places with nowhere to hide. The only solution is invisibility, or the closest that nature can come to it: camouflage. So should fish be blue, to match the watery background? The problem with that is that the exact hue depends on the time of day and what’s in the water, so it changes all the time. But the herring absolutely must look like the water behind them, in order to survive. So they turn themselves into swimming mirrors, because the empty ocean behind them looks exactly like the empty ocean in front of them. They can reflect 90 percent of all the light that falls on them, similar to a high-quality aluminum mirror. By bouncing light waves back out into the eyes of potential predators, a herring can swim about behind a shield made of light.
Reflection isn’t always this perfect. Quite often, only some of the light is reflected by an object. But that’s fantastically useful if two objects are sitting next to each other and we want to tell them apart. The one reflecting blue light is my tea mug, and the one reflecting red is my sister’s. So reflection matters when a wave hits a surface. But it’s not the only thing that can happen when a wave meets a boundary. Refraction can shunt waves about in a more subtle way, altering how they travel.
When a Hawaiian queen stood on a cliff overlooking the coast, watching the surf build, she would have noticed that even though the swell out on the open ocean was approaching from a different direction each day, at the point the water waves reach the shore, they are always parallel with the beach. Waves don’t ever come in sideways, whatever direction the coast faces. That’s because the speed of water waves depends on the depth of the water, and waves in deeper water will travel faster. Imagine a long, straight beach with swell coming in from a direction that’s slightly to the left of straight-on. The part of the wave crest that’s on the right, farther away from the shore, is in deeper water. So it travels faster, catching up the closer part of the wave, and the whole wave crest turns clockwise as it moves toward the shore, lining it up with the beach. By the time the wave breaks, the wave crest is parallel to the shore. So you can change the direction that a wave is traveling in by changing the speed of some parts of the wave crest relative to others. This is called refraction.
It’s easy to imagine changing the speed of a water wave, but what about light? Physicists are always talking about “the speed of light.” It’s an unimaginably gigantic speed, and a crucially important fixture in Einstein’s most famous legacies: the Theories of Special and General Relativity. The discovery that there is a constant “speed of light” was controversial and difficult to accept and brilliant. So it feels a bit like spoiling the party to tell you that you have never in your life detected a light wave that was traveling at the speed of light. Even water will slow light down, and you can confirm this for yourself with a coin and a mug.