As light travels through a given
medium, it travels in a straight line. However, when light
passes from one medium into a second medium, the light path
bends; this bending of light rays is known as refraction.
The refraction occurs only at the boundary. Refraction can
be better understood through the analogy of car moving from
concrete road onto a sandy track.
The relation between the angle of incidence and angle of
refraction is given by Snell's law:
µ= sin i / sin r. This ratio gives the refractive
index of the second media with respect to the first.
Once the light has crossed the boundary between the two
media, it continues to travel in a straight line; only now,
the direction of that line is different than it was in the
former medium. If when sighting at an object, light from
that object changes media on the way to your eye, a visual
distortion is likely to occur. This visual distortion was
witnessed in The
Bent Pencil. The ray traveling back to the medium 1
is laterally displaced. The refracted ray follows the principle
of reversibility i.e. it retraces the same path while coming
back from the medium 2 to medium 1.
If the rays traveling from a denser medium to a rarer medium
are incident at an angle greater than the critical angle
for the pair of media, the rays are reflected back into
the medium 1 from the boundary rather than being refracted
to another medium. This principle is aptly utilized in optical
If you are still unclear about the concepts, please refer