At times, one can see many animals out in the night with their eyes glowing green or yellow-green in color. In all kinds of light and media such as nature films, photographs, flashlights and even car headlights, the greenish glow in the eyes of many animals is clearly visible. Human eyes, on the other hand, give off a red glow when exposed to the flash of a camera.
The greenish glow in the eyes of many animals such as cats, dogs, deer, cattle and horses is mainly because of a special kind of reflective surface that lies behind the retina of their eyes. According to veterinary ophthalmologist Dr. Cynthia Powell, this light-reflecting surface is named as the tapetum lucidum and its purpose is to help animals see better in the dark. The mechanism behind it is that light entering the eye needs to hit a photoreceptor which will help to transmit the information to the brain to interpret the signals (Inglis-Arkell, 2014). However at times, such as in the dark, the light does not reach the photoreceptor. In this case, the tapetum lucidum serves as a mirror to help it bounce off a second time and enable the mechanism of sight (Inglis-Arkell, 2014).
The tapetum lucidum is more reflective than the original retina of the eye which is why light that reflects it has more photons to hit which enable better night vision. The tapetum lucidum allows light a second chance to reflect as much light from the retina as it can (Inglis-Arkell, 2014). However, the light which is not absorbed by the retina is actually reemitted from the eye, and it presents the greenish yellow glow which is seen in the eyes of animals in the night.
Moreover, different species of animals have varying color of light being reflected from their eyes. The basic color of light reflected from the tapetum lucidum is yellow, green, blue or white. However, a rabbit has a red shine; a Labrador has a green tapetal reflection and cats and deer show a greenish yellow or white glow (Inglis-Arkell, 2014).
Human beings, on the other hand, do not represent a yellowish or greenish color when the flash from a camera shines in their eyes. Humans lack the tapetum lucidum in their eyes like animals. Hence they cannot see in the dark as well as animals do. If a light is flashed in a person’s eyes in the dark or at night, there will be no eerie greenish-yellow glow emitted from their eyes.
However, it is different for the flash on a camera. It is brighter than a flashlight and can cause a phenomenon that makes the eyes appear red. In order to understand why it is important to know how the retina functions and how visibility is produced. Light enters the eye through the cornea which is the outer curved structure in the human eye (Scott, 2011). The light is then received by the retina which is the colored part of the eye. The retina reflects the light onto the optic nerve which carries the signal to the brain that interprets the message (Scott, 2011). The amount of light that enters the eye is controlled by the pupil, which is the darker circle present in the eye. It either dilates to allow more light to enter, in the case of darkness, so that a better visual is perceived by the brain. On the other hand, it constricts when too much light is in the eye so that a minimum amount can enter to prevent damage to the delicate structure of the eye (Scott, 2011). Then, between the retina and the sclera, which is the white part of the eye, there is the choroid, which is a connective tissue in the eye. This is a layer of the eye responsible for providing nutrients to the retina as well as the sclera for maintaining its health. It is highly vascular in its composition (Scott, 2011).
Therefore, when the flash from a camera shines on the eye, there is not enough time for the pupils to constrict and prevent excess light from entering. A large amount of light is reflected off the interior part of the eye. As the choroid is rich in blood vessels, the color of light reflected in the camera is red. It is more likely that the red eyes will be seen more prominently in the dark because of the lesser time provided for the pupils to constrict when the flash in on (Scott, 2011).
Chromatic aberration is a phenomenon that occurs when color fringing can be observed in pictures and photographs. It is also known as purple fringing and color fringing. It usually occurs when a camera lens is unable to bring together all the wavelengths of color to amalgamate on the same focal point and their diffusion away from the main focal point causes the appearance of the purple and yellow colors at the edges of the image formed (Van Walree, 2001). The cause of chromatic aberration is the dispersion in the lens where different colors of light are observed traveling at varying speeds when they pass through a lens. Resultantly, the image that is formed looks blurred with the edges of it colored in the colors of the prism-like red, green, yellow, purple, indigo, etc. These are especially visible when high contrast pictures are taken (Van Walree, 2001).
There is an explanation for the varying speed and the formation of chromatic aberrations. It is possible that different colors of light will travel at a different speed when passing through a medium. The refractive index of light is then dependent on the wavelength (Van Walree, 2001). This is a phenomenon commonly termed as dispersion. It can be explained with the example of the glass prism which is used to disperse an incident ray of white light into all the seven rainbow colors.
Figure 1: Light reflected by a prism
The lenses used for photography have dispersive and dielectric glasses used in their manufacture. These glasses do not refract all the colors at the same time which leads to all the colors reaching the focal point at different speeds. Basically, chromatic aberrations cause the imperfect formation of an image due to the phenomenon on dispersion (Van Walree, 2001).Chromatic aberrations are usually formed with polychromatic light. Hence different colors are visible.
Figure 2: Chromatic Aberration.
There are two kinds of chromatic aberrations. First is the Longitudinal chromatic aberration where the lights reflected off different wavelength cannot converge at the same point. Hence dispersion is observed. Lenses which experience the longitudinal chromatic aberration can show dispersion and the formation of the varying colors of light when the image is formed. It usually leads to the yellow and purple fringing shown in pictures (Mansurov, 2011).
There is also the Lateral chromatic aberration when light which is coming from different angles get focused on one point. It shows the blue and purple fringing in images. The only way to prevent chromatic aberration is to adjust the lenses of cameras and to make better lenses so that clear images are formed which have the least amount of chromatic aberrations (Mansurov, 2011).
In many lenses, there are both the lateral and longitudinal chromatic aberrations present simultaneously. There is no other way to reduce these aberrations by stopping down the lens by reducing the longitudinal chromatic aberration and then fixing the lateral chromatic aberration in processing software such as Photoshop or Lightroom. Many modern lenses carry the technology to deal with chromatic aberrations, however, there still remains the issue of chromatic aberrations while zooming the lens in and out. Nevertheless, photographers can fix chromatic aberrations with varying software.
Work Cited
Inglis-Arkell, E. (2014, July 4). The science of how eyes glow in the dark. Retrieved June 25,
2016, from http://io9.gizmodo.com/the-science-of-how-eyes-glow-in-the-dark-1600183971
Mansurov, N. (2011, November 8). What is Chromatic Aberration? Photography Life.
Retrieved from https://photographylife.com/what-is-chromatic-aberration
Scott. (2011, September 15). What causes red eyes in photographs. Retrieved June 25, 2016,
Van Walree, P. (2001). Chromatic aberrations. Retrieved June 25, 2016, from
http://toothwalker.org/optics/chromatic.html
