Transcript Refraction - cashmerephysics

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Angle of Incidence (°)
Angle of Refraction (°)
Angle of emerging ray(°)

Refraction of light is the bending of light as
it goes from one medium into another. This
is because the speed of light changes.
Angle of Incidence
Angle of
Refraction
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
When light waves travel from a less
dense (air) to a more dense medium
(glass), the ray is bent towards the
normal.
The speed of light depends on the material through which
the light is travelling.
When light enters a different material (e.g. from air into glass),
the speed of light changes.
This causes the light to bend or refract.
air
glass
The speed of light is affected by the density of the material
through which it is travelling.
When light enters a more dense medium (from air into glass),
its speed decreases and this is why refraction occurs

Because the frequency
of the wave is
determined by the
source, if the wave
speed slows down, its
wavelength must
decrease. (And vice
versa)
Fast Medium low refractive
index like air
Slow Medium higher
refractive
index like glass
Frequency stays the same
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Refraction causes visual effects such as
this ruler appearing to be bent.
This happens because light from the part
of the ruler in the water is refracted as it
travels from the water into the air.
How does refraction make this stone look closer to the
surface of the water than it really is?
image
actual location
Light rays from the stone are
refracted as they leave the water.
The brain assumes that these
rays have travelled in straight
lines and is fooled into forming
an image where it thinks the light
rays came from.
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2.
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What is refraction of light?
Does the light bend towards or away from
the normal when the light enters at an
angle into a more dense medium?
What happens to the speed of the light
when it enters a more dense medium?
What happens to the wavelength of the
light when it enters a more dense
medium?
What is the speed of light?
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is a measure of how much the speed is slowed
when entering a medium from a vacuum (or
air)

n = speed of light in a vacuum
speed of light in a medium
Values include:
n alcohol = 1.36 n diamond = 2.41
n glass = 1.52
n water = 1.33

Which substance does light travel the fastest?
Why will you never get a number less than 1?
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
The speed of light in chloroform is = 2 x 108 ms-1

Calculate the refractive index (n) for cholorform
Answer: n = 3 x 108 ms-1
2 x 108 ms-1
= 3/2
= 1.5

Calculate the speed of light in glass if n= 1.52
Answer: 1.52 = 3 x 108 ms-1
speed in glass
Speed in glass =
3 x 108 = 1.9 x 108 ms-1
1.52
Calculate the speed of light in water and diamond
Ans: in water = 2.26 x 108ms-1 :in diamond = 1.24 x 108ms-1

We can use the following equation:
n1v1 = n2v2
Where n1 is the refractive index in medium 1
Where n2 is the refractive index in medium 2
Where v1 is the speed of light in medium 1
Where v2 is the speed of light in medium 2

A light ray travels from a vacuum (n1=1.0)
into a container of water (n2=1.33). Calculate
the speed of light in the water.
Answer: n1v1 = n2v2
1 x 3 x 108 = 1.33 x v2
v2 = 2.26 x 108 ms-1



Dutch astronomer and
mathematician
Most famous for the law
of refraction (Snell’s Law)
Found a new method for
calculating the radius of
the earth as well as π
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i
Sin (i)
r
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10
30
18
45
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60
32
75
39
90
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Sin (r)
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
Plot a graph of sin (i) against sin (r) with sin (i)
on the vertical axis.

The gradient of the graph can be used to
obtain the refractive index of the glass?

Calculate the refractive index using this
feature.
n = sin (i)
sin (r)
A beam of light shines into a liquid. The
angle of incidence is 31o and the angle of
refraction is 22o. Calculate the refractive
index of the liquid?
Answer: n = sin 31
sin 22
=
0.51 = 1.4
0.37

n1v1 = n2v2
n1
n2

=
v2
v1
=
λ2
λ1
n1sin θ1 = n2 sin θ2

A ray of light enters plastic goggles from
water at 25o to the normal. The ray deviates
through an angle of 4.0o as it enters the
plastic. The R.I. of the plastic is 1.5. Calculate
the R.I. of the water.
Answer: θ1 = 25o θ2 = 21o (it moves towards the normal because
plastic if more optically dense)
n1sin θ1 = n2 sin θ2
n1sin 25o = 1.5 x sin 21o
n1 = 1.5 sin 21o
sin 25o
= 1.3
When a light ray hits the boundary
between a material which is more
dense (large n, slower speed) to a less
dense medium d (e.g. glass and air),
there is a critical angle θc at which
the angle of refraction is 90o.
If the angle of incidence of the light
ray is greater than a specific value,
called the critical angle, then the
light ray is actually reflected. This is
called total internal reflection.
It makes the inner surface of
glass act like a perfect mirror.
If the angle of incidence
is smaller than
the critical angle, then
the light ray is refracted.
If the angle of incidence
equals the critical angle,
then the light ray is
refracted along the
boundary.
If the angle of incidence is
greater than the critical
angle, then the light ray is
not refracted and total
internal reflection occurs.
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2)
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Trace the outline of the plastic cube onto
a piece of paper.
Shine a light into the block.
Mark down where the path of the light as
it is refracted.
Use Snell’s Law to find out the refractive
index of the plastic block.
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

When light travels from a high to low
refractive index, it bends away from the
normal.
A particular angle of incidence will cause
the light to refract at 90º, ie along the
boundary between the media.
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

This angle of incidence is called the
critical angle qc.
This can be calculated by putting qr= 90º
into Snell’s Law.
n1sin θ1 = n2 sin θ2
n1sin θc = n2 sin θr
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Find the critical angle for the light to be totally
internally reflected in the semi-circular plastic
block.
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The refractive index of glass is 1.52 and air 1.00.
Find the critical angle for the glass.
n1sin θ1 = n2 sin θ2
1.52 sin θ1 = 1.00 sin 90o
sin θ =
=
θ =
1/1.52
0.658
41.1°
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Different materials have a specific value for the critical angle:
The critical angle is the smallest angle of incidence at which total internal reflection
occurs.
Material
Critical angle
water
49°
acrylic plastic
42°
glass
41°
diamond
24°
Diamond has the lowest critical angle at 24°.
This means that diamond reflects more light than the other materials and accounts
for its characteristic sparkle.
Optical fibres are thin strands of solid glass,
about the size of a human hair.
They are widely used in communication,
medicine, lighting and as sensors.
The first transatlantic telephone cable to use
optical fibres went into operation in 1988.
Optical fibres can transmit light signals at
high speed over long distances and are used in
phone and internet connections.
Optical fibres do not have to be straight to carry light and can even carry light around
corners.
This curved Perspex rod
shows how light travels in an
optical fibre.
Light travels through the Perspex rod, and optical fibres, by a
process called total internal reflection.
1) Endoscopes (a medical device used to see inside the body):
2) Binoculars and periscopes (using “reflecting prisms”)
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A ray of light enters the optical fibre.
light enters
optical fibre
As the light enters the optical fibre, it is
refracted. This means that the direction the
light is travelling in changes.
The ray of light hits the wall of the fibre
and is totally internally reflected when the
angle of incidence is greater than the
critical angle.
The ray of light passes down the optical fibre
by repeated total internal reflection.
light leaves
optical fibre

A scuba diver working at a depth of
6.0m beneath a smooth surface stops long
enough to watch the sun set. At what angle
from the normal should she look if the
refractive index of the seawater is 1.38
n1 sin q1  n2 sin q 2



n2
sin q c 
n1
1.00
q c  sin
1.38
q c  460
1
460
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air
substance z
300
100
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n2
sin q1

n1
sin q 2
0
nz
sin 60

0
1
sin 50
nz
 1.13
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A prism splits a ray of white
light into the colours of the
rainbow.
This process is known as
dispersion.
The colours that make up
white light are called the
spectrum. The order of the
colours in the spectrum is
always the same.
Use this phrase to remember the order of colours:
Richard Of York Gave Battle In Vain

Which colour is diffracted the most through
the prism?
Violet is the most violently
diffracted wavelength. Red is
only slightly diffracted.
Violet has the highest frequency,
energy and smallest wavelength.


The atmosphere is densest (thickest) at the
bottom, near the Earth. It gradually thins out
as you go higher and higher up.
The blue colour of the sky is due to Rayleigh
scattering. As light moves through the
atmosphere, most of the longer wavelengths
pass straight through. Little of the red,
orange and yellow light is affected by the air.

However, much of the shorter wavelength
light is absorbed by the gas molecules. The
absorbed blue light is then radiated in
different directions. It gets scattered all
around the sky.

As the sun begins to set, the light must travel farther through the
atmosphere before it gets to you. More of the light is reflected and
scattered. As less reaches you directly, the sun appears less bright. The
colour of the sun itself appears to change, first to orange and then to red.
This is because even more of the short wavelength blues and greens are
now scattered. Only the longer wavelengths are left in the direct beam
that reaches your eyes.