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This week’s question was asked by a friend.

QUESTION: When you shine deer at night, why do their eyes light up?

ANSWER: The round black center of our eye is the pupil. The pupil is actually a hole that lets light into the eye. The pupil is like the diaphragm or aperture of a camera that adjusts the amount of light that gets to the film of the eye called the retina.

Our eyes don’t glow in the dark like a cat or a deer because there is a layer of dark pigment behind our retina that absorbs or soaks up light. Very little light reflects back out of the eye.

The process functions similarly to the way the moon shines, reflecting light from an outside source. Many animals, including cats and deer, have a reflective layer of cells behind the retina, called the tapetum lucidum, which in Latin means “bright tapestry.” This reflective membrane is composed of 15 layers of cells directly behind the retina.

The cat, deer and many nocturnal animals have this special membrane layer that reflects light, even of low intensity, onto the retina. This makes their vision far more sensitive than ours but somewhat blurry. They see better in the dark, but they can’t see fine detail. When you and I see an animal’s eyes glowing in the dark, it is because we are seeing some of the reflected light bouncing back from that reflective layer of cells.

This thin membrane allows tigers and lions to hunt during their peak hours of activity, the nighttime hours from dusk to dawn.

By the way, you may have noticed that a cat’s pupil is slit-shaped. The cat has to be able to see well in dark but also needs to “stop down” or block out bright daylight glare. Our round pupil eye can’t close down as much as a cat’s slit-shaped pupil. The cat is able to block more light from going into the eye.

As a kid growing up on the farm outside of Seneca in the heart of Crawford County, I wondered if our horses could see in color. Research has shown that horses can see red and blue but are unable to tell green from gray.

A recent account in the British press was particularly intriguing. A drunken horse-and-trap driver was hauled into Barnsley magistrates’ court for driving his horse, Fred, through a set of red traffic lights. In England, a trap is a light two-wheeled carriage pulled by a single horse. The inebriated driver claimed his horse was color-blind and didn’t know he should have stopped.

Cats and dogs can only see colors that a color-blind person can see, namely, yellow, blue, and violet − no red, green and orange. Cats and dogs have poorer color vision than humans. They have two types of cones, while we humans have three types. They make up for it by having more rods, which gives them superior night vision, and they are better at tracking motion. Cats’ night vision is eight times better than humans.

Why is the pupil black? Make a light trap. Make something that is blacker than black. Find a half gallon ice cream container. Cut a small (about ¼-inch diameter) hole in the center of the lid. Close the box and look into the hole. Regardless of the color of the inside of the box, the hole is going to look extremely black. If you paint the outside of the box black, the hole will look blacker than the box.

What is going on here? When the light enters the hole, it bounces around many times. With each bounce, some light is absorbed by the inside surface. After so many bounces, hardly any light is left to come out of the hole. The pupil of our eye looks black for the same reason.

Send questions and comments to:

Larry Scheckel is a retired Tomah High School physics teacher.


Tomah Journal editor

Steve Rundio is editor of the Tomah Journal. Contact him at 608-374-7785.

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