Today 2/3

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Today 2/3

• Total Internal Reflection
• No HW due Today
• Lab Refractive index with Pins and Blocks
• Start reading chapter 26.6-8 on Lenses
• HW 2/3 Refraction Due Thursday, 2/6
• Exam I Thursday, Feb 13

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This Weeks Lab
Pins
Images will not line up when the objects do
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This Weeks Lab
Pins
Objects will not line up when the Images do
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Refraction-Bending Light
Air - index of refraction, nA 1.0
Think of rows in a marching band that slow down
in a mud puddle.
Wave crests are closer together in higher index
materials because the waves travel slower.
Water - index of refraction, nW 1.33
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Refraction-Bending Light
Air - index of refraction, nA 1.0
Think of rows in a marching band that slow down
in mud puddle.
Ray direction always perpendicular to the waves.
When light enters at an angle, it bends.
Water - index of refraction, nW 1.33
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Ray diagram example
Air - index of refraction nA 1.0
Refraction makes it seem as though its closer
Image
Object
Water - index of refraction nW 1.33
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Ray diagram example
Air - index of refraction nA 1.0
Image
Object
Refraction makes it seem as though its farther
Water - index of refraction nW 1.33
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Example
n1sin ?1 n2sin ?2
Note notation light from n1 to n2
When looking into a glass of water, where (at
what angle, ??2) should I place my eye so that I
can just barely see the red dot?
??2
tan ??1 4/7 ??1 30
n 1.0
1.33 sin 30 1.0 sin?2
?2 41.7?
n 1.33
How deep does the water appear to be?
??1
tan ??2 4/d
7cm tall
d 4/tan41.7? 4.5cm
4cm
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Total internal reflection
Air - index of refraction nA 1.0
nAsin ?A nWsin ?W
As ?W increases, so does ?A
?A
until ?A becomes 90. ?C is called the critical
angle. If ?W is greater that ?C no light will
escape the water and all will be internally
reflected.
Total internal reflection only happens when light
goes from high n to low n and it depends on both
ns!
nAsin 90? nWsin ?C ornA nWsin ?C
?W
Water - index of refraction nW 1.33
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Example
n2 n1sin ?C
What is the critical angle for light traveling
from water into air?
sin ?C n2/n1 0.752
n2 1.0
?C 48.8
?C
Note that there is no critical angle for light
from low index to higher index.
n1 1.33
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Example
n2 n1sin ?C
What happens if the angle of incidence is greater
than 48.8?
n2 1.0
50
?1 50
n1 1.33
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Example
n2 n1sin ?C
n2sin ?2 n1sin ?1
What happens if I place a sheet of glass on the
water, ng 1.5?
No internal reflection possible-low to high n!
n2 1.0
1.5sin ?g 1.33sin50
?g 42.8
Now what happens?
?C Arcsin1/1.5 41.8
?g 42.8?
ng 1.5
The light reflected back into the glass.
?1 50?
n1 1.33