PFE015: Mirage

This was inspired by driving down the highway (I've been driving around a lot lately).

If you have ever been on a long road trip on a sunny day and you've looked ahead on the highway, you may have noticed a shimmer or a reflection. It sort of looks like there is water on the road, but even as you're driving 70 miles per hour, you can't seem to reach it. Where does this phenomenon come from?

It turns out to be a trick of the eyes.

Before we can discuss this, let's talk about the way light and our minds behave. First, imagine you are seeing something both in a mirror and normally. We know that the light from the same object hits your eyes twice, after traveling along two different paths. Since I assume that light travels in a straight line it looks like there is a second object behind the mirror. Luckily, I (along with most people and some chimps) have experience with mirrors and know that the light is really bouncing off it.

There is another fascinating property of light, and that is that it bends as it passes through different substances. Consider a (straight) straw in a glass of water. The straw actually looks bent at the point where it enters the water even though we know it is actually straight. This is because the speed of light actually changes in the water creating a bending effect. "But, I thought the speed of light was always constant?" you protest. Once again, your teachers have lied misinformed you. The speed of light is constant in a vacuum, but is slower in other things like glass, water, even air a little bit. And as light changes speed, it bends.

Now we know all of the physics to understand the glimmer at the edge of vision. On a sunny day (it doesn't necessarily have to be warm) the sun will heat up the pavement which will in turn heat up the air. But this effect has a limit in that only the air up to about a foot or two will be significantly warmer than the rest of the air. This difference in temperature, you guessed it, causes the light to bend. But unlike with the water where there's a kink, the bend is smoother and curvier because the temperature of the air changes smoothly.

(I think I have one of those awards coming for my graphic artistry.)

So there appear to be two images of the car, the normal one, straight ahead, and another one from below. But since we naturally assume that light travels in a straight line, our eyes see the second image as a reflection.

Desert mirages are actually the same thing, but they should be differentiated from hallucinations. Mirages are actual images (they show up on a camera) that remind us of water. You don't have to be crazy to see them. The other kind, the hallucinations, does require some loss of sanity.

That's a mirage.