lecture 35: wed 15 apr ch:33.8-10
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Physics 2102 Jonathan Dowling. Lecture 35: WED 15 APR Ch:33.8-10. Reflection and Refraction. When light finds a surface separating two media (air and water, for example), a beam gets reflected (bounces) and another gets refracted (bends). - PowerPoint PPT PresentationTRANSCRIPT
Lecture 35: WED 15 APR Lecture 35: WED 15 APR
Ch:33.8-10Ch:33.8-10
Physics 2102Jonathan Dowling
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Reflection and Reflection and RefractionRefraction
Law of reflection (Light Bounces): the angle of incidence 1 equals the angle of reflection ’1. 1 = ’1
Law of reflection (Light Bounces): the angle of incidence 1 equals the angle of reflection ’1. 1 = ’1Law of Refraction (Light Bends):
n2 sin2 =n1 sin1 Snell's Law
Law of Refraction (Light Bends):
n2 sin2 =n1 sin1 Snell's Law
When light finds a surface separating two media (air and water, for example), a beam gets reflected (bounces) and another gets refracted (bends).
n is the index of refraction of the medium. In vacuum, n = 1. In air, n ~ 1. In all other media, n > 1.
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Plastic Glass Speed of Light is Slowed n>1
v ≡c / n & n≥1 ⇒ v≤c
n=1
n>1
Hits Sand
Road vL
vR
Turns Left
ExampleExampleWater has n=1.33. How much does a beam incident at 45o refracts?
n2 sin 2= n1 sin 1
sin 2= (n1 /n2) sin 1
=(1/1.33) sin 45o
=0.00982= 32o
n2 sin 2= n1 sin 1
sin 2= (n1 /n2) sin 1
=(1/1.33) sin 45o
=0.00982= 32o
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Water on Desert Road Water on Desert Road IllusionIllusion
The index of refraction decreases with temperature: the light gets refracted and ends up bending upwards.
We seem to see water on the road, but in fact we are looking at the sky!
The index of refraction decreases with temperature: the light gets refracted and ends up bending upwards.
We seem to see water on the road, but in fact we are looking at the sky!
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Water on the Desert Road Mirage
Chromatic DispersionChromatic DispersionThe index of refraction depends on the wavelength (color) of the light.
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The Single Rainbow
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The Double Rainbow
TotalInternalReflection!
Total Internal Total Internal ReflectionReflection
From glass to air, the law of refraction uses n2<n1, so 2> 1: it may reach 90o or more: the ray is “reflected” instead of “refracted”.
For glass (fused quartz) n=1.46, and the critical angle is 43o: optical fibers!
Condition for TIR:n2 sin2= n1 sin1
&2 ≥ n1>1
n2~1
1
2
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The cut of the diamond favors total internal reflection. Most rays entering the top of the diamond will internally reflect until they reach the top face of the diamond where they exit. This gives diamonds their bright sparkle. A fiber optic is a glass "hair"
which is so thin that once light enters one end, it can never strike the inside walls at less than the critical angle. The light undergoes total internal reflection each time it strikes the wall.
Fiber optic cables are used to carry telephone and computer communications.
Fiber Optic Cables
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FIBER OPTIC GUIDED MISSILE
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Polarization By Polarization By ReflectionReflection
Different polarization of light get reflected and refracted with different amplitudes (“birefringence”).
At one particular angle, the parallel polarization is NOT reflected at all! This is the “Brewster angle” B, and B + r = 90o.
ΒΒΒ =−= cos)90sin(sin 221 nnn o
1
2tannn=Β
1
2tannn=Β
Polarizing Sunglasses
Polarized Sunglasses
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