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Introduction to
Light
Light is an EM wave
fast
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faster
All EM waves travel at the same speed
through a vacuum: c
= 3 x 108 m/s - FASTEST!
4 Properties of Waves
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reflection
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refraction
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diffraction
interference
Let’s start with Reflection
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Law of Reflection
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• When a ray of light is
incident upon a
reflecting surface, the
angle of incidence is
equal to the angle of
reflection.
• We measure both angles
with respect to the
normal.
Types of Reflection
Specular Reflection
Occurs when surface is very
smooth. A surface is considered
polished
if the distance between
successive elevations is less
than 1/8 the  of light.
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Diffuse Reflection
Although each ray
obeys the law of
reflection, the normal
is different
everywhere so the
light scatters in many
directions.
Reflect on these questions…
• When light hits the
paper you are taking
notes on, does it
reflect specularly or
diffusely? How do
you know?
• Why is it more
difficult to see when
driving on a wet road?
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Next we’ll discuss Refraction
Refraction is
the bending of
light as it enters
a new medium
at an angle.
• What happens to light
when it enters a new
medium?
• Why does it bend?
sdsdsdsdsdsdsds
How much slower is the light
in the water?
The speed is reduced
by a factor of n, where
n is the index of
refraction of water.
c
v
n
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n is a measure of the optical density of the medium.
Snell’s Law
• Just like the lifeguard,
light wants to take the
fastest path possible.
• Light will take a more
direct path when it enters a
denser medium - it will
bend towards the normal.
• Snell’s Law tells us the the
exact angle at which light
bends:
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n1 sin1  n2 sin2
Example:
Determine how fast light
travels through this slab of
glass if the angle of
incidence is 60 and the
angle of refraction is 34.
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At what angle with respect to the normal
does the light ray emerge from the glass?
What if the light was shined closer
to the right so that the ray exited the
right end as shown? Now what
would be the exit angle?
1.94 x 108 m/s, 60, ???
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Total Internal Reflection
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• As the angle of incidence increases, the refracted angle
increases until it reaches 90. The incident angle that
corresponds with the ray skimming the surface is called the
critical angle. When the angle of incidence exceeds the
critical angle, the ray undergoes total internal reflection.
Examples of TIR
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Finding the Critical Angle
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In order to have TIR, light must
originate in the denser medium.
Use Snell’s Law and set 2 = 90
n1 sin  c  n2
n2
sin  c 
n1
What is the critical angle for a
diamond? The index of refraction
for a diamond is 2.42.
24.4º
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