General Physics II Geometrical Optics: Reflection & Refraction of Light

General Physics II
Geometrical Optics:
Reflection &
Refraction of Light
Waves
• Wave front: The line or surface defined by adjacent points of a
wave that are in phase
•
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Ray = arrow
drawn
perpendicular
to any wave
front
Waves...continued...
Light incident on an object
Absorption
• Reflection (bounces)
– See it
– Mirrors
• Refraction (bends)
– Lenses
• Often some of each
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Reflection and Refraction
• When a light wave strikes a smooth interface of two transparent
media (such as air, glass, water etc.), the wave is in general partly
reflected and partly refracted (transmitted).
The Reflected Ray
• The angle of incidence θ i is equal to the angle of reflection θ r for
all wavelengths and for any pair of material.
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Types of Reflection
Refraction
The speed of a light wave is different in matter than it is in
vacuum (≈ air).
Light Beam
Normal Line
Fast
c
v=
n
AIR
WATER
Where n =
“index of
refraction” of
the material
Slow
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Think of a day at the beach…
What's the fastest path to the ball knowing
you can run faster than you can swim?
This is better
Not the quickest route…
Physics 212 Lecture 25,
Refraction
How is the angle of refraction related to the angle of incidence?
Unlike reflection, θ 1 cannot equal θ 2 !!
» Why??
Remember
v = fλ
» n1 ≠ n2 ⇒ v1 ≠ v2 (But the frequencies (f1, f2) must be the same
⇒ therefore, the wavelengths must be different!)
Therefore, θ 2 must be different from θ 1 !!
λ1 v1 n 2
=
=
λ 2 v 2 n1
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It works in either direction!
Incident Ray
Refracted
Ray
Refracted
Ray
Incident Ray
Which of the following ray diagrams could
represent the passage of light from air through
glass and back to air? (nair=1 and nglass=1.5)
(a)
(b)
(c)
air
air
air
glass
glass
glass
air
air
air
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Dispersion
• The index of refraction of a material depends on wavelength.
Example: The Rainbow
Raindrops refract,
reflect and disperse
sunlight.
Rainbows will always
appear opposite of the
Sun in the sky.
You cannot run from
or run to a rainbow!
Total Internal Reflection
Consider light moving from glass (n1=1.5) to air (n2=1.0)
Light is bent away from the normal as it enters the air. As θ1 gets
bigger, θ2 gets bigger, but θ2 can never get bigger than 90° !!
refracted
ray
n2
θ2
AIR
n1
reflected
ray
GLASS
θr
θ1
incident
ray
So, from Snell’s Law, if sin θ1 > (n2 / n1), we have NO refracted ray;
we have TOTAL INTERNAL REFLECTION.
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Total Internal Reflection: The Critical Angle
When
θ 2 is 90o then θ1 is called the critical angle (θ c).
Furthermore when θ1 > θ c , all the light is reflected (total internal
reflection).
In Summary…
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End of
Geometrical Optics:
Reflection &
Refraction of Light
Lecture
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