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UNIT-VI : OPTICS
CHAPTER- 9 : RAY OPTICS & OPTICAL INSTRUMENTS
1. REFLECTION OF LIGHT :
Reflection :
When light travelling in a medium strikes a reflecting surface, it goes back into the same medium
obeying certain laws. This phenomenon is known as reflection of light.
Laws of reflection of light N
(i) The angle of incidence (i.e.i ) is equal to the angle of A B
reflection (i.e. r).
Normal
(ii) The incident ray, the normal to the mirror at the point of
incidence and the reflected ray lie in the same plane.
i r
®
Spherical Mirror :
(A) Concave Mirror (Converging Mirror) M M
(B) Convex Mirror (Diverging Mirror) O
C F P
P F C
(A) (B)
Sign Convention :
Object on left Mirror
Heights Incident light
upwards
positive
Distances against
incident light
negative
Heights Distances along
downwards incident light
negative positive
Relation between f and R : f = R/2
1 1 1
Mirror Formula :
f u v
(Where, u Object Distance; v Image Distance; f Focal Length)
I v f f v
Linear Magnification : m
O u f u f
E 1
,
2. REFRACTION OF LIGHT :
The phenomenon of bending or change in speed of light, as it goes from one medium to another,
is called refraction of light.
2.1 Laws of Refraction (at any refracting surface) :
(i) Incident ray, refracted ray and normal always lie in the same plane.
n
i n1
n2
r
®
(ii) The product of refractive index and sine of angle of incidence at a point in a medium is
constant.
n1 sin i = n2 sin r (Snell's law)
sin i n v
Snell's law : 1 n2 2 1 1
sin r n1 v 2 2
1n2 = Refractive index of medium 2 with respect to 1.
Note : Frequency of light does not change during refraction .
2.2 Refraction through a parallel slab :
Emergent ray is parallel to the incident ray, if medium is same on both sides.
A
N
i AIR
B
GLASS
r
N'
t
90°
C x
i
D
t sin(i r)
Lateral shift x ; t = thickness of slab
cos r
Note : Emergent ray will not be parallel to the incident ray if the medium on both the sides are
different.
2 E
,
2.3 Apparent depth of submerged object:
(h < h) µ1 > µ2
µ2
µ1
h'
h
x
O
2
For near normal incidence h h
1
®
Note : h and h' are always measured from surface.
3. TOTAL INTERNAL REFLECTION (TIR) & CRITICAL ANGLE :
TIR : When a ray of light enters from denser to rarer medium at an incident angle more than
critical angle then the ray of light totally reflected back in the same medium. This phenomenon is
known as TIR.
N N N
RARER
I I'
i C i>C
DENSER
O N' N' N'
Conditions of TIR :
(i) Ray is going from denser to rarer medium
(ii) Angle of incidence should be greater than the critical angle (i > C) .
Critical angle : It is the angle of incidence at which ray of light refracted at 90º. When we incident
light ray greater than critical angle, light ray totallty reflected back into the same medium.
µ v
Critical angle : (C) sin 1 R sin 1 D sin 1 D
µD vR R
Some illustrations of total internal Reflection
Totally Reflecting Isosceles right angled triangular prism :
45º
B 45º
B
45º B A'
90 º
A
A
90º 45º A' A B'
B' 45º
B' A'
(a) (b) (c)
E 3
,
Optical Fibre : In it light through multiple total internal reflections is propagated along the axis
of a glass fibre of radius of few microns in which index of refraction of core is greater than that
of surroundings (cladding)
>
light pipe
4. REFRACTION AT SPHERICAL SURFACE :
n 2 n1 n 2 n1
•
v u R n1 n2
®
v, u & R are to be +ve
O P C I
kept with sign as
v = PI
u = –PO
R = PC
(Note : Radius is with sign)
5. THIN LENS FORMULA :
1 1
+ve
5.1 Lens Formula
1 1 1
v u f
5.2 Lens maker formula
1 1 1
n 1
f R1 R 2
v
Magnification (m)
u
5.3 Power of Lenses
Reciprocal of focal length in meter is known as power of a lens.
• SI unit : dioptre (D)
1 100
• Power of lens : P dioptre
f(m) f(cm)
4 E
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