Important formulae & basic concepts
Chapter: Ray Optics & Optical instruments & Wave Optics
XII Physics
All distances are measured in meter (m)
Snell’s law states
n = sin i/ sin r
n is refractive index
i is the angle of incidence
r is the angle of refraction
Sign convention distances measured in the same direction as
incident light is positive
Distances measured in the opposite direction
is negative
All distances are measured from the pole
Mirror equation
1 1 1
v u f
+ =
u is object distance; v is image distance and
f is focal length
Sign of focal length of mirror f is negative for concave mirror;
f is positive for convex mirror
Magnification of a spherical mirror
m = -v/u
m is positive then image is erect; m is
negative then image is inverted
For prism
2 m
21
1
n sin[(A D ) / 2]
nn sin(A / 2)
+
= = n
2
is refractive index of prism material
n
1
is refractive index of surrounding medium
A is the angle of prism
D
m
is the angle of minimum deviation
For spherical interface
2 1 2 1
n n n n
v u R
−
− =
n
1
is refractive index to the left of the
refracting surface
n
2
is refractive index to the right of the
refracting surface
Thin lens formula
1 1 1
v u f
− =
u is object distance(m); v is image distance
and f is focal length
Distances measured in the same direction as
incident light is positive
Distances measured in the opposite direction
is negative
All distances are measured from the optical
centre
Lens maker’s formula
2 1
1 1 2
(n n )
1 1 1
f n R R
−
= −
R
1
and R
2
are the radii of curvature of the
lens surfaces
n
2
is the r.i. of the lens and n
1
is the r.i. of
surrounding medium
