Storm in a Teacup: The Physics of Everyday Life(41)
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.
Put the coin flat on the bottom of the mug so that it’s touching the side closest to you. Now bend down until the edge of the mug just hides the coin from you. Light travels in straight lines, and at this point there is no straight line that can get from the coin to your eyes. Now, without moving your head or the mug, fill the mug up with water. The coin will appear. It hasn’t moved, but the light from it changed direction as it left the water and now it can reach your eyes. It’s an indirect demonstration that water slows down light. As the light meets the air, it speeds up again and so the wave is bent through an angle as it crosses the boundary. We call this refraction. And it’s not just water that does this; everything light passes through slows it down, but by different amounts. The “speed of light” means its speed in a vacuum, when light is traveling through nothingness. Water slows light down to 75 percent of that speed, glass to 66 percent, and light in diamond is dawdling along at 41 percent of its maximum speed. The more it’s slowed down, the bigger the bend at the boundary with the air. This is why diamonds are so much more sparkly than most gems— they slow light down much more than the others.? And that bending is the only reason that you can actually see glass, water, or diamonds. The material itself is transparent, so we don’t see it directly. What we see is that something is messing about with light coming from behind it, and we interpret that something as a transparent object.
It’s nice that we can see diamonds (and will come as a relief to anyone who has shelled out for one), but refraction isn’t just about aesthetics. Refraction gives us lenses. And lenses opened the doors to a huge chunk of science: microscopy to discover germs and the cells that we’re made of, telescopes to explore the cosmos, and cameras to record the details permanently. If light waves always traveled at the speed of light, we would have none of those things. We live in a bath of light waves, and those waves are constantly being reflected and refracted, slowed down and sped up as they travel. Just like the chaos of the stormy ocean surface, overlapping light waves of different sizes are traveling in every possible direction around us. But by selecting and refracting, keeping some waves out and slowing others down, our eyes marshal a tiny fraction of that light so that we can make sense of it. The Hawaiian queen standing on the cliff was watching water waves by using light waves, and the same physics applies to both.
That’s all fine if some waves have arrived for you to see after being reflected or refracted. But what if they never reach you at all?
One of life’s little oddities is that if you give a child some crayons and tell them to draw water coming out of a tap, the water in their picture is blue. But no one has ever seen blue water coming out of a tap. Tap water has no color (if yours has, I suggest you seek advice from a plumber). If you did see blue water coming out of a tap, you certainly wouldn’t drink it. But the water in pictures is always blue.
On satellite pictures of the Earth, the oceans are definitely blue. It’s not because of the salt—there are ponds of salt-free meltwater on top of glaciers, and they are also a stunningly deep, spectacular blue. They almost look like someone has filled pockets in the ice with blue food coloring. But where water is trickling over the ice to join the rest of the meltwater, it has no color. What matters for the color isn’t what’s in the water, but how much water you have.