Page 45 - Physics 10_Float

GEOMETRICAL OPTICS

12.6 APPLICATIONS OF TOTAL INTERNAL REFLECTION

Totally Internal Reflecting Prism
Many optical instruments use right-angled prisms to reflect a 45 o
o
o
beam of light through 90 or 180 (by total internal reflection) B
45 o
such as cameras, binoculars, periscope and telescope. One of A
o
the angles of a right-angled prism is 90 . When a ray of light 90 o 45 o
strikes a face of prism perpendicularly, it enters the prim
without deviation and strikes the hypotenuse at an angle of
o
o
45 (Fig.12.10). Since the angle of incidence 45 is greater than B’ A’
o
critical angle of the glass which is 42 , the light is totally
o
reflected by the prism through an angle of 90 . Two such Fig.12.10: Total internal
reflection through right angled
prisms are used in periscope (Fig.12.11). In Fig.12.12, the prism
o
light is totally reflected by the prism by an angle of 180 . Two
such prisms are used in binoculars (Fig.12.13).
Optical Fibre
Total internal reflection is used in fibre optics which has
number of advantages in telecommunication field. Fibre
optics consists of hair size threads of glass or plastic through
which light can be travelled (Fig. 12.14). The inner part of the Fig. 12.11: Prism periscope
fibre optics is called core that carries the light and an outer
concentric shell is called cladding. The core is made from
glass or plastic of relatively high index of refraction. The B 45 o
cladding is made of glass or plastic, but of relatively low
refractive index. Light entering from one end of the core A o 90
strikes the core-cladding boundary at an angle of incidence A’
greater than critical angle and is reflected back into the core
(Fig. 12.14). In this way light travels many kilometres with B’ 45 o
small loss of energy. Fig. 12.12
In Pakistan, optical fibre is being used in telephone and
advanced telecommunication systems. Now we can listen
thousands of phone calls without any disturbance.
Air
n = 1.00
cladding n = 1.39
r i >c core
i n = 1.53
cladding n = 1.39

Fig.12.14: Passage of light through optical fibre Fig. 12.13: Binoculars

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