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            Have you ever wondered what the green reflected light in your dog's eye was? Well, it is the tapetum lucidum. (ta-PEE-tum) The tapetum lucidum, known as the tapetum is a part of the eye that is only found in animals. The tapetum is a special, highly reflective membrane layer in the back of an animals' eye, located behind the retina. The tapetum makes an animal's eye appear to be illuminated at night. The tapetum simply allows animals to see clearer in less light. It does this by increasing the likelihood of dim light stimulating the rods and cones in an animal's eye.

            By comparing what we know about human vision, we can learn more about animal vision. When light first hits the eye, it goes through the cornea, past the pupil and iris and eventually onto your retina. In an animal, light does the same thing, until it reaches the retina. In an animal's eye, when light reaches the retina it is perceived a different way. For example, in a dog's eye, there are more rods than cones in their eyes so that they are able to see better in dark areas. Once light is projected onto the retina, the rods and cones in the retina absorb some of the light and the rest passes through the retina, onto the tapetum. When the rest of the light reaches the tapetum, is reflected and/or bounces off the tapetum. The tapetum reflects light back into the retina, where more of it can be used. In the retina, the light is now reflected and stronger and more easily stimulates the light-sensitive rods and cones. When more light is used, it allows the dog to be able to see even clearer in darker areas. The light that does not again, stimulate the rods and cones exits the eye the same way it came in, through the pupil, and causes the eye to "glow in the dark."

When light first reaches your eye and passes through the cornea and retina, some light is saved and perceived through the retina. However, other light not perceived by the retina, passes through to the back of the eye and hits the tapetum. Since the tapetum is a highly reflective surface, the light that reaches it reflects right back at the retina, doubling the amount of light perceived by the retina. Since there is more light for the cones and rods to perceive, it makes vision for the animals much clearer.

            However, the tapetum is not only used to improve animals' vision at night or in low light areas, but it also can shift wavelength for the sensitivity of the rod photoreceptors, thus enhancing the contrast. Because we humans have no tapetum, we do not have a great ability to see at night, under dark circumstances. There is no reflective membrane layer in our eye that would stimulate the rods and cones in our eye, therefore, we can not see well in the dark, like animals can.

            So, the next time one of your friends becomes afraid that an alien has inhabited their pet's body, just tell them that it is simply the tapetum reflecting light back at us.