Total Internal Reflection And its Applications
Consider a ray travels from denser to rarer medium.
A ray of light incident obliquely to the surface of separation of air and water, it deviates away from normal.
As the angle of incidence increases, the angle of refraction also increases.
On increasing the angle of incidence for a particular value say i = ic, the angle of refraction is found to be
90o.this value of angle of incidence is called critical angle( ic)
When i > ic , the ray goes back into water i.e., the ray is reflected into the denser medium itself.
This phenomenon is called total internal reflection. The word total means that reflection in the
above case occurs with no loss of intensity
Conditions for Total Internal Reflection

Light should travel from a denser to rarer medium

i > ic in denser medium for a pair of media in contact
Relation between Refractive Index and Critical Angle
According to Snell's law
µr =sin i /sin r
When i = ic, r = 90o
Note:
Refractive index depends on wavelength and therefore 'c' for a same pair of media in contact will be
different for different colors.
Applications of Total Internal Reflection

Mirage is an optical illusion, which occurs usually in deserts on hot summer days. On such a
day, temperature of air near the earth is maximum and hence is rarer or lighter. The upper layers
of air, which are relatively cool, are denser. A ray of light from the top of a tree travels from
denser to rarer and bend away from the normal. At a particular layer, if the angle of incidence is
greater than 'c', total internal reflection occurs. To far away observer, this ray i.e., AE appears to
be coming from I i.e., mirror image of O. Thus inverted image of tree creates an optical illusion of
reflection from a pond of water.

Right angled isosceles prism can turn light through 90o or 180o. This is based
on total internal reflection. Since m for glass-air is 1.5, the value of 'C' is 42o. In such a prism, the
angle of incidence in the denser medium is 45o(>C) and hence light suffers total internal
reflection.

The brilliance of diamond is due to total internal reflection. Now m for diamond is
2.42o and c (the critical angle) is 24.4o for diamond-air interface. The faces of the diamond are so
cut that a ray of light entering the diamond fall at angle greater than 24.4o. This results in multiple,
total internal reflections at various angles and remains within the diamond. Hence diamond
sparkles.

Optical Fibers: Optical fibres consist of a very fine quality of glass or quartz fibres. They are
coated with thin layer of material of lower refractive index than that of the fiber. The thickness of
the strand is 10-4. The optical fiber works on the principle of total internal reflection.
This phenomena enables doctors to inspect many internal body sites.A Bundle of fibres transmit an
image that can be inspected visually outside the body.
Optic fibers are as thick as a human hair. If a beam of light is send down a thin glass rod, total internal
reflection traps the light inside the rod. This technique is called 'fiber optics'.
Fiber optics finds its use in the medical field too. Endoscopes use fiber optics technique. A patient can
swallow a tube containing a fine glass fiber through which a doctor can examine the internal stomach
parts and hence unnecessary surgeries can be avoided. 'Fiber optics' is used to destroy tumors. If a fiber
optic cable is passed into the organ laser light can be directed along it. The laser is directed at the tumor
cells and kills them.
The red plastic reflector on the back of a bicycle uses total internal reflection.
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