Total Internal Reflection

So far, we have looked at situations such as light passing from air into water or glass. In those cases, light is beant toward the normal.

What happens as light passes from water into air?

Light passing through, into the air, is bent away from the normal. As the angle of refraction -- the angle of the light passing into the air -- gets larger and larger, more and more of the light is reflected back from the surface. For some critical angle, all of the light is reflected and none of it passes into the air. For light passing from water into air, this critical angle is 49o.

This is known as total internal reflection and is important in many situations. It is used in binoculars,

It is used in "light pipes" or fiber optics that are now commonplace in communications and in medicine.

It is fun to describe what a scuba diver or a fish might see while underwater and looking up at a very smooth surface between the water and air. We saw this in the Introduction to Optics movie taken at the MIT swiming pool. The light from the air above will be refracted so objects will appear to be somewhere other than where they "really" are. The light from some underwater objects will be reflected at the surface by total internal reflection--just as if the surface were a mirror--and mirror-images of them will appear above the water! Remember, we see something only by the light that actually reaches our eyes and then our eyes interpret that light assuming it has come along a straight path.

Q: If light travels from glass into ethyl alcohol, will it be bent toward or away from the normal?

A: The index of refraction for glass is about 1.55 and the index of refraction for ethyl alcohol is 1.36. In going from a material of higher index of refraction into a material of lower index of refraction, light is bent away from the normal.


Q: If light travels from olive oil into air can the light undergo total internal reflection?

A: Total internal reflection can occur if the first index of refraction is greater than the second. Olive oil has an index of refraction of 1.47 which is greater than air's index of refraction of 1.0. Therefore, total internal reflection may occur.



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(C) 2003, Doug Davis; all rights reserved