YOU MAY HAVE noticed that at night, when a dog’s eyes are caught by car headlights or in a flashlight beam, they seem to glow with an eerie yellow or green, making the dog look like some sort of hellhound. The reason has to do with the fact that the wild canines that dogs evolved from are “crepuscular,” meaning that they are usually active at dusk and dawn and therefore need eyes that work well in dim light. Their eyes are thus somewhat different from those of people.
Understanding eyes is a bit easier if you think of a camera. Both your eye and a camera require a hole to let light in (the shutter aperture in the camera, and the pupil in the eye), a lens to gather and focus the light, and some kind of sensitive surface to register the image (the film or photo detecting layer in the camera, or the retina in the eye). Both eyes and cameras need features that allow them to adjust to various light conditions, and both are continually making compromises between working well at low levels of light and being able to see small details. At every stage in the construction of the dog’s eye, the choice seems to have been made to sacrifice some ability to see small or fine aspects of the environment in order to be able to function better at low light levels.
When it comes to letting light into the eye, your dog’s pupils are much larger than those of most humans. In some dogs, you can’t really see much of anything except the wide pupil filling the eye, with just a hint of colored iris around the edge. Because of their larger lenses, dogs’ eyes also have more light-gathering power than human eyes have.
To gather a lot of light, a lens has to be big, which is why astronomical telescopes, such as the one at Mount Palomar in California, can have lenses as large as 200 inches (500 centimeters) across. Effectively two parts of the eye serve as lenses in humans and dogs. The first is the “cornea,” which is the transparent portion of the eye that bulges out at the front. The cornea is responsible for the actual light gathering. The second part, the “crystalline lens,” is behind the pupil and is responsible for changing the focus of the light. Animals that are active in dim light usually have large corneas. Notice how large your dog’s corneas are in comparison to those of people. This larger size permits more light to be gathered and sent into the eye for processing.
Now we get to the reason why a dog’s eyes glow like eerie yellow or green headlights when a light beam hits them at night. This color comes from the reflecting “tapetum,” which is behind the retina and acts as a sort of a mirror. The shiny surface of the tapetum bounces any light that has not been caught by the photosensitive cells back up to the retina, thus giving the photoreceptors a second chance at catching the dim light entering the eye.
More than simply reflecting the light, the tapetum actually amplifies it through a photoelectric phenomenon called “fluorescence.” Fluorescence not only adds to the light’s brightness but also slightly changes the color of the light that is reflected back. The color shift moves the wavelength of the light closer to the wavelength that the rods are most sensitive to and can best detect, which is a yellowish green.
Although the light bouncing off of the tapetum increases the sensitivity of the eye, there is a cost. The light that hits that reflective surface in the back of the eye comes from various directions and, like a pool ball hitting the bumper edge of the table, it does not return along exactly the same path it followed as it entered, but bounces off at an angle. Because the incoming direction of the light and the reflected direction are different, images on the retina are smeared and appear to be a bit blurred. Thus, the dog’s eye has clearly chosen to sacrifice its ability to see fine details so that it can function better in dim and dark conditions.
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