This week’s question was asked by a teacher at Miller Elementary School.
QUESTION: Why can we see things in the dark by using our peripheral vision better than when looking straight at it?
ANSWER: When we see something out of the corner of our eye, it’s called peripheral vision. It is part of our vision we have that is outside the very center of our gaze.
A quick review on how the eye works. Light passes through the outer layer of the eye called the cornea, goes through a clear liquid solution termed the aqueous humor, then the pupil, or the black hole of the eye and then strikes the lens. The cornea and lens bend or refract light. The light passes through the eyeball filled with a liquid (vitreous humor) and then strikes the retina.
The image is formed on the retina in the back of the eye. The retina is like the film in a camera. This is where seeing takes place. There are two types of receptors that make up the retina: rods and cones. Cones are found near the center of the retina and detect color. An easy way to remember is to use the three Cs: center, cones, color.
The rods are found on the edges or periphery of the retina. Rods are 1,000 times more sensitive to light, but they do not respond to color. Rods are far more numerous. A typical set of eyes has six million cones and 120 million rods.
Most all of our vision makes use of the densely packed cones in that center part of the eye, called the fovea. That fovea gives us color and fine detail.
In dim light, when it’s nighttime and dark, the cones are fairly useless. The rods are present at the extremes of the eye. That’s why we can better pick up objects by averting our glance rather than viewing the object directly. As a matter of fact, hunter safety courses and military trainers say not to look directly at an object at night if you want to see it. Peer or scan around the object you are trying to see so that the light falls on the rods.
These trainers also teach how to maintain dark adaptation. It takes five to 20 minutes for the eyes to adapt from bright sunlight to complete darkness, so soldiers are trained to keep one eye closed while using a flashlight to read a map.
Some additional thoughts. We see a car in the far distance. We know it is a car but can’t quite make out the color of the car. Cars far away are dimly lit, or to put it more correctly, reflect very little light to us.
The brightness of an object is established by that familiar inverse square law. An object twice as far away is two-squared (four), so one-fourth as bright; three times farther away, (three squared is nine), so one-ninth as bright.
It’s the same with trees at a distance. We know they are green, but our eye tends to see them as grayish, and artists paint them as such.
Peripheral vision is important for functioning in everyday life. We see danger in “the corner of the eye,” such as a car approaching while crossing the street. Sports people use peripheral vision constantly to spot their players and opponents. A chess player may look at a particular piece on the board, but their peripheral vision takes in the entire board.
Many animals, including dogs, have a difficult time seeing stationary objects. They see movement. Their eyes are packed with rods, very few cones.
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Larry Scheckel is a retired Tomah High School physics teacher.