PHY138 – Waves, Lecture 8
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Today’s overview:
Total Internal Reflection
Optical Fibres
Images Formed by Flat Mirrors
Laser as seen on
TV and movies
Real Laser
Real Laser
Shining Through
a Cloud
Reading Assignment
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Please read the following from Serway
and Jewett before class on Monday:
Chapter 26, Sections 26.4 - 26.6 and
Context 7 Conclusion on page 1037.
A www.masteringphysics.com assignment is
due this Friday at 5:00 PM. It is the last
Mastering Physics assignment of the
semester.
(hoot.)
air
water
A fish swims below the surface of the water.
An observer sees the fish at:
1. a greater depth than it really is.
2. its true depth.
3. a smaller depth than it really is.
Notes from last lecture
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The index of refraction, n, of air depends
on temperature; n decreases as air
Temperature increases
When a ray enters a layer of different n, it
bends
If n decreases, the ray bends away from
the normal. This is refraction.
Refraction effects cause mirages, twinkling
of starlight, and “heat currents” off hot
highways or other surfaces.
Hot air near the ground
Temperature Inversion: Hot air above
Total Internal Reflection
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Occurs when n2<n1
θc = critical angle.
When θ1 ≥ θc, no light is transmitted
through the boundary; 100% reflection
n2
sin  c 
n1
An Optical Fibre
Quiz 2
An observer O, facing
a mirror, observes a
light source S. Where
does O perceive the
mirror image of S to
be located?
(5 = none of the
points shown.)
mirror
O
1
3
S
4
2
Virtual Image in a flat mirror
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Light rays emerging from an object obey
the law of reflection for the specular
surface of a mirror
Our mind imagines that the rays emerge
from points beyond the mirror.
This thing beyond the mirror is called an
image. No light rays actually pass through
the image, so it is “virtual”.
It is convenient to describe the size and
location of the image as if it were an actual
thing.
How an image is formed