Biology Paper on Why Animal Eyes Glow Green When Illuminated At Night

Why Animal Eyes Glow Green When Illuminated At Night

Many animals that are active at night usually glow whenever they are exposed to light. These animals include cows, cats, horses, dogs, and deer. Human beings, pigs, squirrels, and kangaroos do not fall into this category because they do not have the layer that reflects the light at night. This is because night-active animals have a reflective exterior part behind their retina, which blocks the escaped light and send it back to the retina. Thus, the main aim of the reflective exterior is to reflect the light back to enhance animals’ vision.

To enhance animals’ vision at night, especially when the light intensity is quite low, a layer located within their retina reflects light back to allow the light receptors above it an additional opportunity to identify possible images (Williams 86). This layer is referred to as the tapetum lucidum, and is quite essential in facilitating animals’ sight in the dark. The retina has the capacity to allow light into the eye, but some light pass through it. The tapetum lucidum blocks this light and send it back to the retina, giving the animals another chance to see it. If a flashlight is directed towards night-active animals’ eyes at night, they are likely to shine back, emitting a white or green light.

The colors usually appear more visible during the night than in daytime because the animals’ pupils are enlarged at night, thus, allowing higher visibility of the tapetum lucidum. However, not all animals shine the same color. For instance, cats glow in bright green light while dogs glow a bit darker than cats. This is normally due to animals’ tapetum having diverse substances, such as riboflavin, or zinc. The unstable amounts of pigment that can be located in the retina also affect the color. Human beings and other primates do not have tapetum lucidum since they do not require maximizing the light interception. 

What causes “red – eye” in flash pictures?

Human eyes are capable of adjusting to varied light conditions.  The eyes can regulate the quantity of light entering them through contraction or expansion of the pupils. In photography, a “red-eye” occurs in the pupils of human beings, as light flashes directly inside their eyes. When individuals are in dark places, their pupils grow larger to allow more light, and they cannot shrink fast enough to reimburse the camera’s flash, hence, when the flashlight is reflected off the back of their eyes, it results to “red-eye” (Carlson 206). The camera’s flash is too bright to create a reflection off the retina.

When the camera’s flash enters through the human’s eye when it is already dilated, the pupil cannot contract fast to restrict the light from reflecting off the red blood vessels in the choroid, a layer at the back of the eye, which is responsible for nourishing the retina (Yang). This results in the camera picking up a red-eyed reflection. Children have higher possibility of portraying “red-eye” effect than adults because of their high adaptation to dark areas. In addition, albinos usually depict higher frequencies of “red-eye” effect than the rest of the people due to low levels of melanin in their eyes. Melanin is a coloring in the eye, which is responsible for absorbing light. A “red-eye” effect can be eliminated by placing the camera’s flask away from its lens. Besides, modern cameras have “red-eye” reduction feature, which eliminates the effect.

Works Cited

Carlson, Jeff. Photoshop Elements 10: For Windows and Mac Os X. Berkeley, CA: Peachpit Press, 2012. Print.

Williams, Ernest H. The Nature Handbook: A Guide to Observing the Great Outdoors. Oxford: Oxford University Press, 2005. Internet resource.

Yang, Bryan. “What Causes the Red Eye Effect?” Yale Scientific, May 12, 2011. Web. 18 June 2014