Refractive index and total internal reflection
QUESTION:
1. Is it possible for the refractive index of a material to be less than one? Please explain.
2. What is total internal reflection?
Answer:
1. No. At least no when the light is passing from a medium of low refractive index to that of a
higher one – e.g. air to glass, air to water or water to glass. The angle of refraction is always
less than the angle of incidence.
When people talk about the refractive index of a material they usually mean that relative a
vacuum (called the absolute refractive index) – that is light going into the material from a
vacuum. In this case the refractive index is always greater than one.
2. Sometimes when light travels from glass or water to air a strange thing happens, none of it
emerges, it is all reflected back inside the glass or water.
You can see this very easily if you float a weighted test tube in a beaker of water and then
look down at the side of the test tube through the top of the water - you can’t see into the
tube and the side of it looks shiny. The same effect can be seen if you open your eyes under
water and look up at the water surface from below.
This reflection of light inside a transparent material is called TOTAL INTERNAL
REFLECTION.
When light hits the boundary between glass and air or water and air it is usually refracted out
but as the angle of incidence is increased an angle is reached where the light emerges along
the boundary - the angle of refraction is 90o.
As the angle of incidence is increased the angle of refraction also increases until the light
emerges along the boundary between the block and the air (diagram (b)).
The angle of incidence in the transparent material when this happens is called the CRITICAL
ANGLE. The critical angle for glass is 42o.
For angles of incidence greater than the critical angle ALL the light is reflected back —this is
known as TOTAL INTERNAL REFLECTION. Total internal reflection only happens when the
light is travelling from the more dense material to the less dense i.e. glass to air, glass to
water or water to air.