Monday, 28 June 2010

Issue #2 - Eye Wonder

Where does light go after entering the eye?
Asked by: Adam Ellison

After going for an eye test, in which a photo of my retina was taken, I got thinking about where light goes after entering the eye. Does it bounce off the retina and back out again? Or does it bounce around the eye? Which would surely result in a confusing display of multiple images. The answer to both of these questions is a simple no. Now my question may have been a silly one, as the blackness of the pupil highlights the fact that no light is escaping, but this only makes me ask, why? What stops the light from leaving the eye?

The main reason light does not leave the eye is due to the layer that sits both behind the retina and the front of the eye. This layer is made up of the choroid, a vascular connective tissue, which supplies oxygen and nourishment to the outer layers of the retina, ciliary body and the iris. The choroid forms the uveal tract, which contains melanin, a reddish-blue, almost black pigment, that improves the contrast of images on the retina by absorbing excess light and blocking light that may get through the sclera (the exterior layer of the eye or ‘white of the eye’). However in Albino humans melanin is absent, resulting in poor vision as light reflects around the eye, creating a blurred image. In many animals the low levels of melanin (and tapatum layer) contributes to their ability to see well in the dark.

There is one exception to the rule, where the eye is unable to control excess light. It is known as the red-eye effect, which can ruin many a good photo. A quick flash of light, when used in a low light condition, will enter a wide-open pupil and, because the eye is too slow to react, it reflects off the retina. This is how the Optometrist was able to take a photo of my retina. After administering tropicamide eye drops, my pupil dilated, allowing the flash to bounce of my retina and into the eye of the camera.

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