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Refraction of Light
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What is Refraction?
What is Optical Density?
The Laws of Refraction
The Refractive index
Angle of Incidence / Refraction
Total Internal Reflection
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Refraction is the change in
direction of light when it passes
from one medium to another.
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If light ray enters another medium perpendicular to
boundary, the ray does not bend.
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Incident ray
normal
i
air
When the light ray travels
from air to water, the
refracted ray bends
towards the normal.
water
r
Refracted ray
i angle of
incidence
r angle of
refraction
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Refracted ray
normal
air
When the light ray travels
from water to air, the
refracted ray bends away
from the normal.
water
i
Incident ray
i angle of
incidence
r angle of
refraction
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During refraction, light
bends first on passing
from air to glass and
again on passing from
the glass to the air.
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Incident ray
During refraction, light
bends first on passing
from air to glass and
again on passing from
the glass to the air.
Reflected ray
air
Refracted ray
glass
air
Emergent ray
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Light slows down when it enters an optically denser
medium. The refracted ray bends towards the normal
when the second medium is optically more dense than
the first.
Incident ray
normal
air
water
r
Refracted ray
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Light speeds up when it enters an optically less dense
medium. The refracted ray bends away from the normal
when the second medium is optically less dense than
the first.
Incident ray
normal
water
air
r
Refracted ray
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Among the 3 transparent mediums (air, water and
glass), glass has the highest optical density.
Incident ray
air
air
Refracted
r ray
i1
r1
water
r1
i2
Refracted ray
i2
water
Refracted ray
glass
r2
Refracted ray
i1
Incident ray
glass
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Complete these ray diagrams.
air
water
glass
glass
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Complete these ray diagrams.
water
air
air
glass
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The incident ray, the refracted ray and the normal
at the point of incidence all lie in the same plane.
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Incident ray
normal
i
air
For two given media, the ratio
sin i sin r is a constant,
where i is the angle of
incidence and r is the angle
of refraction
water
r
Refracted ray
Refractive
Index, n
sin i
sin r
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The higher the optical density, the greater the
refractive index. The greater the refractive index, the
greater the bending of light towards the normal.
Incident ray
air
air
Refracted
r ray
i1
r1
water
r1
i2
Refracted ray
i2
water
Refracted ray
glass
r2
Refracted ray
i1
Incident ray
glass
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If light is incident upon a piece of glass (refractive
index 1.52) at an angle of 45o, what is the angle of
refraction?
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Given that the refractive index of water is 1.33,
calculate the angle of refraction when the incident
ray comes in at 60o to the normal.
Solutio
n
n=
60
o
air
r
wate
r
sin i
sin r
sin 60o
1.33 =
sin r
o
sin
60
sin r
1.33
=
r =40.6
o
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When light travels from a
less dense medium to a
denser medium
sin r
n=
sin i
sin i
n=
sin r
i
air
When light travels from a
denser medium to a less
dense medium
wate
r
wate
r
r
air
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The figure shows light travelling from water into the air. The
ray is incident upon the boundary at 30o. What is the angle of
refraction if the refractive index of water is 1.33?
Solutio
n
30
wate
r
air
sin r
n=
sin i
sin r
1.3 =
sin
3
30o
sin =
r 1.3sin
o
3
o30
r 41.9
=
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Other ways of calculating the refractive index
Refractive
index, n =
Speed of light in
vacuum / air
Speed of light in
medium
c
=
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Take a look at this...
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The critical angle is the angle of incidence in the
optically denser medium for which the angle of
refraction is 90o.
When i = critical angle,c
r = 90o.
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When i > critical angle, the ray gets reflected internally.
This is called TOTAL INTERNAL REFLECTION.
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For TOTAL INTERNAL REFLECTION to take place:
The light ray must travel from an optically denser
medium towards a less dense one.
Di
no
o
i
t
rec
th
pa
t
lig h
The angle of incidence must be
greater than the critical angle.
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How do we calculate the critical angle?
We know that r = 90o
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We know that when
light travels from a less
dense medium to a
denser medium
We know that when
light travels from a
denser medium to a
less dense medium
Refractive
Index, n
Refractive
Index, n
sin i
sin r
sin r
sin i
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How do we calculate the critical angle?
We know that r = 90o
Refractive
Index, n
sin r
=
sin i
sin 90o
=
sin c
1
=
sin c
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How do we calculate the critical angle?
sin c =
c = sin
1
n
-1
1
n
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Mediu Glass
m:
Refractive 1.50
Index:
Critical
Angle:
c = sin-1
= sin-1
= 41.8o
1
n
1
1.50
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Mediu Water
m:
Refractive 1.33
Index:
Critical
Angle:
c = sin-1
= sin-1
= 48.8o
1
n
1
1.33
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Mediu Diamo
m:
nd
Refractive 2.42
Index:
Critical
Angle:
c = sin-1
= sin-1
= 24.4o
1
n
1
2.42
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Total Internal Reflection in Prisms
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Total Internal Reflection in Prisms
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Fibre Optics
