jee main - physics - refraction of light
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REFRACTION OF LIGHTBy:- Prof. Dnyanesh Vaidya
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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.
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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
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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.”
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Laws of Refraction :
Snell’s law
1
2
sin.
sinsin
sin
iconst
ri
rC
C
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Snell
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Relative Refractive Index and Absolute Refractive Index
1 2
2
sinRelative Refractive Index
sin
sinAbsolute Refractive Index
sin
a
i
r
i
r
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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
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Refractive index in terms of wavelength
11 2
2
11 2
2
11 2
2
C
C
ButC f
f
f
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Limitation of Snell’s Law
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Practical examples of refraction of light
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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.
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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
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aw
w
8 8a
ww
cn
c
c 3 10 3 3 10c
4n 43
8wc 2.25 10 m / s
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Total Internal Reflection :-
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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.
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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
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Mirage: -
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Periscope
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Binacular
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Optical Fibre :-
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Why diamond will shine?
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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?
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Prizm
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∠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)
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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)
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Thus, i = e.
∴ δ = δm
∴ Equation (5) will become
A + δm = (i + i)
A + δm = 2i ………(6)
A m
i2
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As i = e then r1 = r2 = r (say) Substituting this value in equation of A, r + r = A 2r = A ……..(7)
Ar
2
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Substituting equations (6) + (7) in formula of refractive index,
sin i
sin r
A msin
2sin A / 2
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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
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Dispersion of light
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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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