Transcript Lecture8

Refraction
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When there is a change of medium, a
part of the light wave gets transmitted.
However, the transmitted part travels
with different speed (v) and in a
different direction. This phenomena is
called refraction.
Reflection
Incoming
Θi
Θrefraction
Refraction
Applet:
http://www.walter-fendt.de/ph11e/refraction.htm
Index of refraction
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(medium specific)
Index of the refraction, n, is defined as
n = c/v : v: velocity of light in medium
Examples
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Vacuum, n = 1
Air,
n = 1.0003, very close to vacuum!
Water,
n=1.33
Glass,
n = 1.5
Diamond, n = 2.4
Law of refraction
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When light goes from a medium with
smaller index of refraction to a medium
with larger index of refraction, it bends
toward the normal.
When light goes from a medium with
larger index of refraction to a medium
with smaller index of refraction, it
bends away from normal.
Air
Θr
Water
Water
Θr
Air
Light is closer to the normal in a dense medium
Snell’s Law
ni Sini = nf Sinf
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From air to water, air = 30o then
water= 22o
From water to air, if water= 22o , then
air = 30o , on the other hand, water =
30o then air = 42o
Straw illusion:
A straight straw
appears to have
bent in water
due to refraction!
We always get cheated
in a bar, because the
refraction in a thick
walled beer glass makes
the walls look thin!
How to spear a fish?
We think we see exactly
where the fish is.
Actually, the fish is
lower than we perceive.
It appears closer to the
surface than it actually
is. If you put the tip
of your spear in the
water, it will seem bent.
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Remember that when the light goes
from a dense (large n) to a thin
medium (small n), the angle of
transmission is larger….
From water to air, if the incident
angle is ~50o, the transmitted angle
~ 90o. If the incident angle is larger,
there is NO transmission at all!
Reflection
Incoming
Θr
Refraction
http://www.phy.ntnu.edu.tw/java/light/flashLight.html
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The incident angle at which the total
internal reflection just occurs is called
the critical angle.
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From glass to air, 42°
From water to air, 50°
From diamond to air, 24. 5°
Examples of total internal reflection:
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To change the direction of a beam by 90°or
a 180° without energy loss.
View from the underwater.
Fiber Optics
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Use thin, flexible glass or plastic fibers
as light pipes, which have a larger index
of refraction than the surroundings.
If the light hits the surface at an angle
larger than the critical angle, it is totally
internally reflected: No loss of light
from escaping! Thus the light can be
used to transmit information through
the optical fibers.
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Why is the fiber optical transmission is
better?
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Light has high frequency of oscillation, thus
can have store much more information!
Other uses:
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See and photograph the inside of vital
organs.
What is a mirage?
The index of refraction of the air is close, but
not equal to 1, and it changes with temperature
too.
 A continuous change of the index of refraction
makes a light bend from its original direction.
 Thus we see the sun even after it has set.
 A hot, dark asphalt road can cause the light to
bend, producing a mirage.
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