|MadSci Network: Earth Sciences|
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-
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
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
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%.
Try the links in the MadSci Library for more information on Earth Sciences.