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.