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REFRACTION OF LIGHTBy:- Prof. Dnyanesh Vaidya

Reflection of light.

When light rays fall on any opaque surface (like plane mirror), they are returned back on the same side of the surface. This phenomenon is called reflection of light.

Laws of Reflection:- 1. The angle of incidence and angle of refraction

are of equal measures.

i.e.

2. Incident ray, reflected ray and the normal at the point of incidence lie in the same plane.

3. Incident ray and reflected ray are on the opposite sides of the normal at the point of incidence.

m i m r

Refraction of light.“the bending of a ray of light when it travels

from one transparent medium to another,

having different optical density is called

refraction of light.”

Laws of Refraction :

Snell’s law

1

2

sin.

sinsin

sin

iconst

ri

rC

C

Snell

Relative Refractive Index and Absolute Refractive Index

1 2

2

sinRelative Refractive Index

sin

sinAbsolute Refractive Index

sin

a

i

r

i

r

11

22

2 2 1

1 1 2

11 2

2

21 2

1

c

C

c

C

c C C

c C C

CBut

C

Refractive index in terms of wavelength

11 2

2

11 2

2

11 2

2

C

C

ButC f

f

f

Limitation of Snell’s Law

Practical examples of refraction of light

Numericals

If the refraction indices of glass and water is 1.5

and 1.33 respectively calculate the velocity of

light in glass and water.

Ans: Ca = 3 × 108 m/s

ng = 1.5 ,

cg = ? , cw = ?

w

4n 1.33

3

ag

g

8a

gg

8g

cn

c

c 3 10c

n 1.5

c 2 10 m / s

aw

w

8 8a

ww

cn

c

c 3 10 3 3 10c

4n 43

8wc 2.25 10 m / s

Total Internal Reflection :-

Necessary conditions for total internal reflection

1. The ray of light must travel from optically denser medium to optically rarer medium.

2. The angle of incidence in the denser medium must be greater than the critical angle for that pair.

Thus, refractive index of rarer medium (air)

with respect to the denser medium (water)

is: sin

sin 90sin 90 1

sin

1 1

sin

d r

d r

r dd r

c

But

c

c

Mirage: -

Periscope

Binacular

Optical Fibre :-

Why diamond will shine?

For Home works1. If the refractive indices of diamond and glass

are 2.4 and 1.5 respectively. Compare the velocity of light in diamond and glass.

2. The R.I of water relative to air is 4/3. A ray of light passing through water into air is incident at interface at an angle of 35° with the normal. What angle does the refracted ray make with the normal?

Prizm

∠δ = EQR + ERQ .(exterior angles property) ∠ ∠

= (i – r1) + (e – r2)

∠δ = (i + e) – (r1 + r2) ………(1)

From ΔQDR,

r1 + r2 + D = 180° ………(2)

from □AQDR,

□ AQDR is a cyclic quadrilateral

∵ ∠Q = R = 90° ∠ ∴ ∠A + D = 180° ∠ ……..(3)

From (2) and (3)

r1 + r2 + D = A + D

∴ r1 + r2 = A

Substituting A in equation (1),

δ = (i + e) – A …….(4) i.e. A + δ = (i + e) ……(5)

Thus, i = e.

∴ δ = δm

∴ Equation (5) will become

A + δm = (i + i)

A + δm = 2i ………(6)

A m

i2

As i = e then r1 = r2 = r (say) Substituting this value in equation of A, r + r = A 2r = A ……..(7)

Ar

2

Substituting equations (6) + (7) in formula of refractive index,

sin i

sin r

A msin

2sin A / 2

Thin Prism

(A < 6°). sin θ = θ.

m

m

m

m

m

A msin

2sin A / 2

A2A2A

=   A

A   A  

A A 

 A 1

Dispersion of light

Dispersion of white light takes place because :

1. White light is composed of seven different

colours. 2. Velocities and refractive indices 3. After emerging out from the prism, different

colours travel in different directions. Hence, they remain separated.

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