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.
Light waves hitting the water surface are either reflected back up into the sky or pass through and travel down into the depths. But sometimes, a tiny particle or even the water itself acts as an obstacle, sending the wave off in a new direction. This redirection may happen to the same light wave enough times that it eventually makes its way back out to the air. And on that long journey, the water has filtered the light. The light waves coming from the Sun are a mixture of lots of different wavelengths, all the colors of the rainbow. But the water can absorb light, and it absorbs some colors more than others. The first to go is the red light—a few yards of water is enough to get rid of most of that. And then the yellows and greens follow after a few tens of yards. But blue light is hardly absorbed at all—it can travel for huge distances. And so by the time the light is on its way out of the ocean, most of what’s left is blue. The reason tap water is colorless is that there isn’t enough of it to make a difference. Tap water does have a color, the same color as all the other water in the world. But that color is so faint that you need a huge amount of water all together to actually see the effect that the water is having on the waves going through it.# When you do see it, it’s spectacular, and bright blue crayon really is the right choice. But you’d never learn that from a tap.
So as waves travel, they can be absorbed by whatever they’re passing through. It’s a very slow process of attrition, sneaking away wave energy tiny bit by tiny bit. The amount that’s lost depends on what type of wave it is and also its wavelength. All this variability means there’s a huge richness in what waves are doing and what they can tell us. We can see and hear some of the contrasts in one of my favorite atmospheric phenomena: the thunderstorm.
A thunderstorm is a magnificent spectacle, a dramatic reminder that air is far more than an invisible filler for the sky. Our atmosphere is host to vast quantities of water and energy, and usually these hefty commodities are shunted around slowly and peacefully. The thundercloud, the mighty cumulonimbus, develops in order to rebalance the atmosphere when peaceful shunting is no longer enough. The system starts when buoyant, warm, moist air near the ground shoves upward into the cooler air above, taking huge amounts of energy with it. In the center of the vast cloud, hot, humid air rises rapidly, churning the atmosphere above it and liberating huge raindrops. Most dramatic of all, the churning causes electrical charges to be separated and redistributed to different parts of the clouds. The charges accumulate until nearby clouds or the Earth itself are stabbed by giant pulses of electrical current, carrying the excess electrical charge away. Each lightning bolt lasts for less than a millisecond, but the thunder echoes across the landscape for far longer. I love thunder and lightning, both for the theatrical spectacle and for the glimpse it gives us into the atmospheric engine. Thunderstorms produce such unlikely opposites: the sharp, shocking flash of lightning contrasting with the deep, drawn-out rumble of thunder. But both are beautiful examples of the versatility of waves.
The lightning bolt is temporary. The electrical connection is a superheated tube of atmosphere, stretching from the thundercloud to the Earth or perhaps to another cloud. It’s a corridor full of molecules that have been blown apart by the energy rushing past them. For a brief instant, the temperature in that tube may reach 90,000°F, and so it blazes blue-white. A giant pulse of light waves whooshes outward from the tube, filling the landscape, but they rush away at such an enormous speed that they’re gone in an instant. As the superheated tube carrying the electrical current heats up, it expands sideways, thumping into the air around it. This gigantic pressure pulse ripples outward through the air, following the light, but much more slowly. These are sound waves, and this is the thunder. We know that lightning bolts exist because they make both light and sound waves.