Probably at some point you have seen the way some animal’s eyes sort of light up when a car’s headlights have swept past. Perhaps it was a neighborhood cat in the night or maybe a coyote, but when the light shined in their face you saw a bright glow from their eyes.
That glow is of course the light of the headlights reflecting back at you. A thing that has that reflective characteristic is called a retroreflector. In this case the retroreflector is the animal’s eyes.
Another retroreflector you’re familiar with is the bicycle reflector. If you’ve ever looked at one very closely you will see the surface of the reflector is composed of many, many small pyramid shapes. These are positioned and angled such that they bounce any light shone at them straight back at its source.
A retroreflector you may not have heard of is one that went with the astronauts to the moon on an Apollo mission. It was left on the surface of the moon pointing earthward. In this way, engineers on Earth were able to shine a laser at it from these thousand of miles away and have that laser reflected directly back. By timing very precisely the length of time it took the laser to travel to the moon and back, we now know the distance to the moon to within a few centimeters.
But now here’s where it gets maybe a little creepy. Did you know that spiders have remarkably retroreflective eyes? If you take a flashlight and go out at night in an area known to have spiders you will see their eyes reflecting very brightly back at you. The only trick to doing this is to make sure the flashlight is held as closely to in-line with your eyes as you can. That is, you want your eyes right behind the flashlight so that you see the light from the spider’s eyes that is being reflected straight back at the flashlight.
Below: Some folks showing you how to find spiders in the night with a flashlight.