MadSci Network: Earth Sciences

Re: How do cats eyes in the road, reflectors on a bike and optic fibres work?

Date: Sun Dec 24 10:41:27 2000
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Earth Sciences
ID: 975842356.Es

Hi Craig! All eyes reflect light well because the path for light always works in both directions. In other words, if incoming light is focused by the cat's cornea lens and forms a tiny bright spot on the retina... then that tiny bright spot can send light out through the cornea-lens, and beam it back to the original source. If you shine light at a cat, ONLY YOU will see the glowing eyes, since the cat's eyes are beaming the reflected light back towards the flashlight (and you are seeing some spill-light). People standing nearby might not see the glow at all. All eyes will "glow" like this. However certain animals' eyes glow very brightly because their retina has a reflective layer called the tapetum. (The tapetum improves their night vision by doubling the amount of light that hits retinal cells, and by getting rid of diffuse light inside the eye by throwing it back out through the pupil so bright moonlight won't cause glary washed-out nightime images.)

To REALLY make cats' eyes glow brightly, hold the flashlight near your face (or clamp it in your mouth) and gaze past it into the distance.

Human eyes do the same as cats, and that's where "red-eye" in flash photographs comes from. The eyes send the light from the camera flash back towards the camera. To get rid of red-eye, move the camera flash far from the camera lens. Tiny cameras create "red-eye" because the photoflash is too close to the camera lens, and the camera "sees" the light that the eyes are beaming back to the flash tube.

Very old railroad reflectors used "cats eye" reflectors in the form of glass lenses with a curved mirror in place of the "retina". If you ever find an old ball of red glass by the side of the road that has silver on one side, it's an ancient "cats eye" safety reflector.

Road reflectors on a bike are based on something entirely different. If you place two mirrors together at a 90deg angle, then all incoming light will bounce twice and then retrace approximately the same path on its way out. (Try it, and you'll find that the reflection that you see in the mirror-pair is NOT REVERSED as it is in a single mirror!) And if you put THREE mirrors together and look into the corner of them, you'll see an upside- down, unreversed image of your face. And no matter how you twist the mirrors or move your head, the image of your face will stay in the same spot. This device is called a CORNER-CUBE REFLECTOR. It returns incoming light back to its source.

Bicycle reflectors are composed of hundreds of tiny Corner Cube reflectors formed into the plastic. (Call this device a "Corner-Cube Array.") When you look at a bicycle reflector close up, notice that it looks black. The black color is actually the upside- down image of your eye's dark pupil! If the reflector facets were lots bigger, you'd see an image of your eye within each one. Gaze at the reflector while slowly moving the edge of a white piece of paper across your eye, and just before it blocks your vision, you'll see small white bits appear in the facets of the bicycle reflector.

If you take apart a bicycle reflector, you'll find that the faceted back of the plastic is NOT a metal-coated mirror. In fact, if it was metallized, it would only reflect about 80% of the light; same as normal mirrors. Without the metal, it reflects 100% of the light. This strange phenomenon occurs because the light is INSIDE THE PLASTIC when it strikes the tilted facets, and the 100% reflection is known as "Total Internal Reflection." When light within a transparent material strikes the inner surface of that material at a glancing angle, it reflects totally. Total Internal Reflection causes the surface of water to look silver when viewed from underwater. It causes the bottom of a glass cup to look silver except where you touch it with wet fingers. (Try searching the www using keywords "total internal reflection".) (Try dipping a bicycle reflector into an aquarium or other flat, water- filled container. Will it still reflect? Or will it become transparent?)

Optical fibers are just reflective tubes. Shine light down the tube, and it keeps going because it bounces from the walls. But if you've ever looked at optical fibers, you'll notice that they are NOT METALLIZED like a mirror, they have no silvery coating. They look like transparent fishing line. I bet you're way ahead of me now. Yes, that's right, they use "Total Internal Reflection," just like the Corner Cubes on your bike reflector. That's why optical fibers can guide light for many miles: it's because the walls of the fiber don't absorb light at all, instead they reflect 100%.

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