It was a dark and stormy night.
(I've always wanted to start off with that line.)
You were driving home from work. The rain had finally stopped, but the roads were wet and shiny, making it difficult to see the lane markers painted on the surface. It seemed like the headlights only made things worse. But after turning onto another road, everything got better. You could clearly see the lines on the street, as though battery-powered lights were illuminating your way. But that light was actually coming from slightly raised markers called retroreflectors. These are optical devices that always reflect light back to the source from which it came—no matter which orientation the device has.
How do these help you navigate at night? Let's start with some basics.
How Do You See?
Whenever you look around, you can see two types of objects. The first produces its own light, like an incandescent bulb, a candle flame, or the screen on your television. The other type doesn’t. Instead, light reflects off the object. Imagine a pencil on your desk. If the desk lamp is on, you can see the pencil, because it’s reflecting light from the lamp. If the lamp’s off, you can’t see it.
But whether or not an object is shiny also affects how well you can see it in low-light conditions. When a beam of light hits a super-shiny object like a mirror, it reflects off it in a very particular way.
Here are two lasers (a green and a red) aimed at a flat mirror. I’m using two different colors to show two different paths of light. There’s nothing particular special about these two colors, except that they look cool.
For both of them, their light hits the mirror at the same angle that their light also reflects from the mirror—but in the opposite direction. (Both angles are measured from a line perpendicular to the mirror.) Physicists say that "the angle of incidence is equal to the angle of reflection." Here is a diagram that should help you visualize this:
In this diagram, θi is the angle of incidence and θr is the angle of reflection. We call this the “law of reflection.”
(A note about lasers: In most cases, you can't really see a laser beam all by itself. The only way to see the beam is to have something in its path reflect its light. In the image above, I used an aerosol spray can, which shoots tiny particles into the beam’s path. Then the laser reflects off these particles, so you can see it.)
Unlike a mirror, non-shiny objects have what is called “specular reflection.” If the object is not shiny, different parts of a light beam will hit different parts of the surface and reflect in many directions. Here is what happens when I put a paper card in front of the mirror:
Notice that the light from the laser doesn't just reflect in one direction, but in all directions. Yes, the paper looks smooth, and feels smooth enough to your fingers, but not smooth enough that light all reflects the same way. In fact, the light scatters so much that you can barely see the red laser. (The camera picks up the green better, and the green is a little brighter, anyway.)
If you’re driving on a clear night on a dry road, this is how you can see the lane lines: They are not perfectly smooth or shiny, so the light scatters. Some of the light from the car’s headlights hits the painted lines and reflects back at you.
But what about a wet road? The water on the surface makes it shiny, and that means that most of the light from the headlights reflects off the road away from the car, just like the light from those lasers bounced away from the flat mirror. (Remember: The angle of incidence is equal to the angle of reflection.) That means little light gets reflected back to you, the driver, and you just see black and can’t tell where the road ends.
Here is a diagram showing the difference between a wet and dry road.
A couple of notes: In the diagram, I just drew a human with a flashlight instead of a car with headlights. (I'm sure we can all agree that it's basically the same thing, right?) For the dry road, I drew a few random rays of light, not all the different ways the light can bounce. The important thing is that one of these rays goes back to the human so that the road is visible to them. However, notice that the total amount of reflected light can't be more than the incident light, or the light going towards an object. I tried to show this by drawing lighter-colored reflected rays. In other words, the light reflected back to the viewer is always dimmer than the amount of light originally emitted from the source.
For the wet road, all of the light reflects away from the flashlight. If you were in a car approaching from the other side of the road, this light would be glaringly bright. That’s why other cars on the road can make it doubly hard to see when you’re driving in the rain.
Retroreflectors are not only used not only for lane markers, but also traffic signs and bike safety gear, to make them more visible to drivers.
A retroreflector doesn't reflect light in all directions like a dry road, and it doesn't reflect light away from the source like a flat mirror. Instead, a retroreflector directs the light back towards the source. Yes, if you shine a light at a mirror with a 0° angle of incidence—right towards the mirror—it will come straight back at the source. But with a retroreflector, the light will go back to the source no matter which way it comes from.
While you’re driving, your eyes are on a plane that’s not that far above the one the headlights are on. That means the light bounces from headlight to lane marker and back to the level of your eyes, and you see these reflectors as bright—and that is good, because it helps you stay on the road.
But if you were on the side of the road and a car passed by, you wouldn’t see the lane markers as bright. None of the light from the car’s headlights would reflect sideways and into your eyes.
Here are two images of roads. The top image has lines painted and the bottom has lines with retroreflectors. Can you notice the difference?
How to Make a Retroreflector
There are different designs for retroreflectors, but I'm going to show you the two simplest ways to get one to work. This first method uses three flat mirrors (they can be as large or small as you like) connected at right angles. Together, these mirrors form the corner of a box. When a light ray enters this retroreflector, it will make multiple reflections, bouncing among the mirrors and ultimately coming back out the way it went in.
Here's what that looks like using the red and green lasers. I put the two lasers at different angles so that you can see that each beam of light comes back the same way it entered.
If you can find three small mirrors, this isn't too difficult to build yourself. Put it in a dark room and hold a flashlight near your head. Now, no matter where you go in the room, the light will reflect back to you. I mean, that's not just science—it's art.
There's another way to make a retroreflector: You can use a clear sphere (like glass) with a mirror covering one side of it. But in the following laser demo, instead of a sphere, I'm going to use a glass cylinder so it's easier to take a photo. In this case, the glass container is filled with corn syrup. Why syrup? Corn syrup has optical properties (the index of refraction) very close to that of glass, so filling the cylinder with syrup makes it act like it's solid glass, instead of a hollow tube. I also put some shiny aluminum foil on the back of the cylinder. Here's what it looks like with a laser aimed into it:
(There is a bit of scattering with the green laser, but that's because of the aluminum foil. I think it still shows the basic idea.)
Other Real-Life Retroreflectors
Here is an everyday example of a sphere-based retroreflector: the back of the pedal on a bike.
You can see the small spheres in the retroreflector. This makes that pedal super shiny to anyone passing by in a car, so that the driver will know to give the cyclist some space.
You can also make tiny spherical beads and put them in fabric to give it increased visibility; you often see this kind of material in the stripes on running shoes and jackets.
Do you know what else is very much like a glass sphere with shiny material on the back? The eyes of some animals, like dogs, cats, racoons, and monsters in the woods. If you shine a light at these animals and they are looking straight at you, the light will reflect right back, so it will look like their eyes are glowing in the dark.
(Don't worry, these are just the two family dogs and not monsters.)
And you don't have to use a flashlight. Imagine you are sleeping outside near a campfire. You look away from the fire into the dark woods and see the glowing eyes of an animal. The fire behind you is acting just like a flashlight. This phenomenon of animals having retroreflector eyes has been experienced by humans since the invention of fire—maybe that's why it's common for cartoons to display spooky glowing eyes in scenes of total darkness?
And finally, NASA built a retroreflector and put it on the moon; check it out. Their version has a bunch of small inner-cube mirrors, just like my three-mirror example. They use it to measure the distance between the Earth and our satellite by directing a laser beam at the moon. Then they clock the time it takes the light to travel there and back and calculate the distance using the known value for the speed of light.
But they have to use a retroreflector; if they just used a regular mirror, the receiver on Earth would have to be in a very particular location to detect the reflected beam. This detector position would have to change as the moon moves and the angle of the mirror changes. In fact, it’s possible this detector would have to be in space to sense the reflected beam.
So, technically, when you shine a flashlight at the moon, some of that light hits the retroreflector and bounces back to you—even though it’s almost 240,000 miles away.