Why animals’ eyes glow green at night


The characteristic eye shine produced by most animals originates from the tupetum lucidium, which a tissue layer in or behind the eye retina of some vertebrates. Animal eye retina traps all the light that goes through the eye. However, it permits some amount of light to pass through. On the other hand, the tupetum lucidium reflects the light that passed through back to the retina, hence, increasing the quantity of light existing for photoreceptors. Not all the amount of light that enters the animal eye is absorbed, some passes through as noted by StudyMode Research . Animals increase the amount of reflected light, therefore, increase the amount of light accessible for their eyes to perceive what is seen as darkness by human eyes. The tupetum lucidium is known as the transparent sphere retroreflector. This characteristic of tupetum lucidium enables it to bounce light directly along its light path. 
The retroreflector matches the reflected light, and incident light thus maintains the correct contrast and sharpness of the image in the eye retina. Tupetum lucidium play its roles using the constructive interference. Therefore, allows it to increase the quantity of the light going through the retina. In some animals, for example, the cat, tupetum lucidum increases its eye sensitivity by forty-four percent. This ability cats have, allow them to perceive the electromagnetic radiations that are invisible to human eye. Some animals that have the tupetum lucidium at the back of their retina are capable of seeing in dimmer lights that other animals cannot. 
Human eyes always reflect light. Human eyes in some of the photographs look red.  The ‘red eye’ in pictures occur when the camera flash reflect off the red tissues and red blood vessels in the back of the eye.  The existence of the ‘red eye’ happens when one uses the flash of photographs nearer to the lens of the camera in low ambient light.  Studies indicate that some animals that have the tupetum lucidium, and human beings experience ‘red eye’ effect. What exactly responsible for the occurrence of the ‘red eye’? The effect of the red eye is as a result of the fundus color, which occurs due to melanin, a pigment which located in the epithelium pigment of the retina. The light of the flash takes place very fast even before the eye pupil closes. Therefore, a significant amount of bright light that originate from the flash enters the eye through the pupil, and then mirrors off the fundus at the eyeball back, and out of the eye through the pupil. The reflected light by the tupetum lucidium is what the camera records (‘red eye’). 


The primary cause of the red-eye is the sufficient amount of blood that exists in a choroid. The choroid occurs behind the retina, and it replenishes the cells and tissues in the back of the eye.  The amount of blood moving within the choroid is more than the blood circulating within the retina. Consequently, the retinal blood does not participate in any role during the red eye effect. The eye consists of numerous photostable pigments which absorb electromagnetic radiations in the regions of short wavelengths. Hence, somewhat responsible for the effect of the red eye.  The melanin exists between the choroid and the RPE (Retinal pigment epithelium) and indicates a regular increase of light absorption towards the short wavelengths. The determinant of the red eye effect is the blood, since; blood is transparent along the electromagnetic radiation wavelengths and suddenly begins absorbing at 600nm.  

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