# 4
Reflection:
Reflection is the change in direction of a wave front
at an interface between two dissimilar media so that the wave front returns
into the medium from which it originated. Common examples include the
reflection of light, sound,
and water waves. The phenomenon of reflection is extremely valuable for our
daily lives.
For instance, the reflection of visible light allows us to
see objects that do not produce their own light. The reflection of microwaves
is useful for radar scanners. The reflection of sound waves in a
theater or concert hall enlivens an onstage production. The reflection of
seismic waves allows researchers to study the Earth's
structure and to prospect for petroleum and
other natural resources. The reflection of visible
light is also often used for aesthetic purposes.
Reflection of light may be specular (that is,
mirror-like) or diffuse (that is, not retaining the image, only the
energy) depending on the nature of the interface. Whether the interfaces
consist of dielectric-conductor or dielectric-dielectric, the phase of the
reflected wave may or may not be inverted.
When light goes from a denser medium to a less dense medium,
at a certain angle of incidence, the refracted ray goes along the boundary
between the two media. The incident angle on this occasion is called the critical
angle for the substances.
When light goes from a denser medium to a less dense medium,
as the angle of incidence exceeds the critical angle, the ray reflects back to
the denser medium. This phenomenon is called Total Internal Reflection.
Total Internal Reflection is a very efficient reflection, as
the loss of light energy is almost negligible.
1) Reflecting prisms:
In optical instruments, right-angled prisms are widely used
to divert the course of light rays. As the total internal reflection takes
place within them, the loss of light energy can be kept to a minimum. So, the
prisms are preferred to mirrors for the purpose of reflection.
2) Mirage
On hot summer days or in the deserts, patches of water
appear to us, some miles in front of us, only to find none when we approach
them. This phenomenon is caused by the total internal reflection. The air
layers on the ground become hot and less dense in these places and light, when
comes down has to pass through these less-dense layers. At a certain point, the
light exceeds the critical angles and the total internal reflection takes place
on a vast scale, creating the illusive puddles of water.
3) Sparkles in diamonds
The sparkles inside diamonds are cause by total internal
reflection. Diamond is well-known for its toughness - very dense and μ is very
high; larger refractive index means smaller critical angle. Therefore, when
light enters a diamond, the possibility of it being subjected to total internal
reflection is very high, that in turn causes sparkles.
4) Optic fibres
Optic fibers revolutionized the communication that we take
for granted today. This humble device - a thin flexible glass fiber with a
coating - carries light through a distance of miles and miles, with a very
little loss of its energy, thanks to total internal reflection. The trick is
done by keeping the outer layer known as cladding less dense relative to the
inner dense core - the first condition for total internal reflection. Since
light enters almost parallel to the fibre, the angle of incidence is high and
it easily exceeds the critical angle that triggers off the total internal
reflection. The flexibility of the fibres, light weight, low cost and the
ability to send light signals through them with very little loss of light, make
then indispensable in modern communication networks.
5) Medical uses - the endoscope
This is an instrument consisting of optic fibres. It is used by the medical professionals to see inside the body. The flexibility of optic fibres contributed to the invention of this device.
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