Discuss why animal eyes glow green when illuminated at night.

The green glow eyes effect is a phenomena experienced in nocturnal animals. Surprisingly the green glow can take different colors in different animals. However, one thing the animals have in common is that they are used to environments with very low lighting. Nocturnal animals are those that are mostly active at night and or, are adapted to cave like environments where there is little or no lighting.

            Animals whose eyes glow green when illuminated at night have a reflective surface in their eyes known as tapetum lucidum. The way the tapetum lucidum works is that, typically, when light comes into contact with an animal’s eyes, the rays enter the eyes and are received by photoreceptors that transmit the image to the brain to be processed. Tapetum lucidium comes in when some rays miss the photoreceptor and are bounced back for registry. The reflective surface acts as a mirror that bounces back the rays to the photoreceptors. In this way, the environment becomes brighter as objects are easily perceived.

Examples of animals that  have the tapetum lucidum, including dogs—both wild and domesticated, deer, cats—both wild and domesticated, bulls and cows, ferrets and even horses. These are animals that in one way or another are used to nocturnal activity. Conversely, humans do not possess the tapetum lucidum; the green eyes glow should not be confused with the red eyes phenomena observed in humans. Pigs, kangaroos, and squirrels are examples of animals that do not have the tapetum lucidum.

In conclusion, it should be noted that not all eyes animals’ glow the same color. Dr. Powell, puts it that it is due to different substances like the protein riboflavin and nutrients like zinc that the colors may appear different. It is also observed that the colors may be affected by pigments in the retina. Other factors that affect color are age and DNA.

  1. What causes “red eyes” in flash pictures?

Commonly known as the red eye effect, the phenomena of red glowing eyes in flash photographs is caused by a exposure of blood cells behind the retina of the human eye. Apart from human beings, the red eye effect is experienced in animals that are observed to have a defect in their eyes. When exposed to light from flash photos; the usually defective blue eyes in animals turn red.

In flash pictures the light from the flash camera is received in the eyes and the pupil is closed to accommodate the light. However, in red eye effect, the light from the camera passes through the pupil fast enough to get to the fundus at the back of the eye. The reflective surface at the back of the pupil is covered with blood vessels. The red color from the blood vessels reflects back the light back through the pupil and is recorded by the camera as red eye photograph effect.    The camera typically records this reflected light.

As earlier mentioned, the main cause of the red color is the sufficient supply of blood that is used at the choroid of the eye at the back of the retina. Although some blood flows to and from the retina, the blood in the retinal circulation could be considered as obsolete in the red eye effect since the choroid has vaster supplies of blood.


Lee, Henry. On the Tapetum Lucidum. Med Chir Trans 69: 239–245.1886.

J. van de Kraats and D. van Norren: “Directional and nondirectional spectral reflection from the human fovea” J.Biomed. Optics, 13, 024010, 2008



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