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Refraction of light
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Objectives
Recall and use the terms for refraction, includingnormal, angle of incidence, angle of refraction
recall and apply the relationship sin i / sin r =constant to new situations or to solve relatedproblems
define refractive index of a medium in terms of theratio of speed of light in vacuum and in themedium
explain the terms critical angle and total internalreflection
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Word Splash
Use the following words and create a mindmap onRefraction!
bendingspeed of light
optical density
refractive index
vacuumair medium
apparent depth
real depth
towards
away
normal
different
r
in
sin
sin
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Refraction of light
Light source
Glass block
from: http://www.physics.brown.edu/physics/demopages/Demo/optics/demo/6a4210.htm
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What is Refraction?
Refraction is the bending of light as it travelsfrom one transparent medium into another.
This is due to a change in the speed of light
Medium: the substance that light passesthrough
E.g. air, water, glass, plastic, diamond
Optical density measure of the material toenable transmission of light
The more optical denser the medium, the slower
light travels
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When does refraction occur?
Refraction occurs when
1. there is a change in speed of the light wave,
and
2. the light wave approaches the boundary atan angle other than 90 to the boundary.
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Law of Refraction
The incident ray, the normal and the refractedray all lie in the same plane.
For 2 particular media, the ratio of the sine of
the angle of incidence to the sine of the angleof refraction is a constant i.e.
Also known as Snells Law
constantsin
sin
r
i
Willebrord Snell (1580 1626)
from: http://www.iesfelanitx.org/departaments/fisica-quimica/retrats/snell.jpg
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Refractive index
Refractive index, n, of a medium is the ratio of the
speed of light in vacuum and the speed of light in
that medium.
The bigger the value ofn, the more optically denserthe substance.
Refractive index affects how much light bends.
v
cn
mediuminlightofspeedvacuuminlightofspeed
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Refractive index
The ratio is constant, however it differs for
each medium.
For the special case oflight passing from a
vacuum (or in practice, air)into a given medium,the constant ratio is equal to the refractive index
of the medium.
r
i
sin
sin
vacuum
medium
i
rr
in
sin
sin
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When light travels from an optically less dense to a
denser medium, light bends towards the normal
When light travels from an optically denser to a less
dense medium, light bends away from the normal
normal
Incident ray
i
r
refracted ray
water
air
iis greater than r
normal
refracted ray
r
i
incident ray
water
air
iis smaller than r
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Example 1
The diagram shows a ray of light incident on thesurface of a liquid and its subsequent path. What
is the refractive index of the liquid?
air
liquid
59
40
i = 59 and r = 40,
rsinisinn
331
40sin
59sinn
.
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Example 2
air
crown glass
i
If light is incident upon crown glass (n = 1.52) atan angle of 40, what is the angle of refraction?
Also, complete the drawing of the ray diagram.
rsinisinn
rsin
40sin521
.
1.52
40sinrsin
0.25
)52.1
40sin(sin 1
r
r
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Reversibility of light
The principle of reversibility states that lightwill follow exactly the same path if its
direction of travel is reversed.
air
crown glass
40
25
air
crown glass
40
25
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Example 3
A ray of light emerges from water (n= 1.33) to air.Calculate the angle of refraction, r.
air
water
r
35
Applying reversibility of light,
use ras the angle of incidence and
35 as the angle of refraction.
i.e. solve the question as though light
is passing from air to water.
rsin
isinn
7.49
)35sin33.1(sin
35sin33.1sin35sin
sin33.1
1
r
r
r
r
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Apparent depth
Due to refraction, the depth of water looksshallower.
This perceived depth is called apparent depth.
real
depth
apparent
depth
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You can use apparent depth to calculate therefractive index of a medium.
depthapparentdepthrealn
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Distortion of objects
Due to refraction, the stick appears bentunderwater.
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Distortion of objects
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Multiple images
How manygoldfish are there?
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Investigating Total Internal
Reflection
Calculate the angle of refraction of the following lightrays. Sketch the refracted ray. The refractive index
of glass is 1.50.
30
airglass
41.81 41.82
Applet: http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=49
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A light ray enters a glass block:
When the angle of incidence is zero at theglass/air boundary, the ray emerges without
deviation.
glass
air
incident ray
undeviated ray
Semi-circular
glass block
i = 0
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A light ray exits a glass block:
When the angle of incidence is less than thecritical angle, there will be refracted ray and
also a weak internal reflection.
Strong
refracted
ray
Weak, internally
reflected ray
i
i c
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A light ray exits a glass block:
When the angle of incidence is equal to thecritical angle, the angle of refraction in the
less dense medium is 90.
90
c c
Strong
refracted
ray
Weak, internally
reflected ray
i = c
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Critical Angle :
Critical angle is defined as the angle of incidence inthe optically denser medium for which the angle of
refraction in the less dense medium is 90.
cn
c
n
rin
sin
sin
sin
sinsin
1
90
90
c
n: refractive index
c: critical angle
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When the angle of incidence is greater
than the critical angle, the ray is reflected
totally within the block.
This is known as total internal reflection.
strong totally
internally
reflected ray
i c
i
C diti f t t l i t l
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Conditions for total internal
reflection to occur:
The light ray travels from an optically densermedium to an optically less dense medium.
The angle of incidence at the point ofincidence is greater than the critical angle of
the optically denser medium.
video: http://www.youtube.com/watch?v=5Q4Cl6sDxu8
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Example:
A right angled prism is made of glass ofrefractive index 1.5. A ray of light enters the
prism.
45
(a) Calculate the critical angle of the prism
(b) Complete the path of the ray until it emerges into
the air again.
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Applications of total internal reflection
1. Periscopes
- Can be used to look over high obstacles, eg. wall
- Consists of 2 prisms positioned at an angle of 45o.
2. Camera
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Optical Fibre
consists of a central glass core of high refractiveindex glass or plastic, coated with a thin layer ofglass of lower refractive index.
a light ray entering the pipe is totally internallyreflected.
light will travel the whole length of the light pipe withrepeated total internal reflections at the surfacesand then emerges from the other end of the pipewithout loss of intensity.
can be used in endoscope to enable doctors to seeorgans inside the human body and also used intelecommunications. video: http://www.youtube.com/watch?v=UPzSuh7BPKM
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Ad t f i ti l
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Advantages of using optical
fibres:
They are lighter. They are becoming cheaper to manufacture
than wires.
They can carry more information over longdistances than copper wire.
They will experience less signal loss as
compared to wires during transmission.