Chapter 15
Optics
Abstract Chapter 15 deals with geometric optics. Problems are solved under
internal reflection in slabs and prisms, fibre optics, matrix methods, Fraunhofer
diffraction by single slit, double slit and grating, missing orders, resolving power,
Rayleigh’s criterion, interference, colours in thin films, Newton’s rings, polarization,
Malu’s law and Brewster’s law.
15.1 Basic Concepts and Formulae
Geometrical Optics
Fermat’s principle: A ray of light traverses from one point to another by a route
which takes least time.
Momentum of photon
p = hν/c
(15.1)
Fraction ( f ) of light escaping from an isotropic point source in a medium of refractive index n through a flat surface is given by
f =
1
2
1 −
1
n
n 2 − 1
(15.2)
Intensity of light (I ) at distance r from a point source of power W is related to
pressure P by
P =
I
c
=
W
4πr 2 c
(15.3)
Mirage is a type of illusion formed by light rays coming from the low region of
the sky in front of the observer on a sunny day.
Optical path length (O.P.L.) =
N
i=1
n i s i
(15.4)
where s i is the path length of the ray in the ith medium.
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