Download - Snell’s Law, Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses Physics 102: Lecture 18
Snell’s Law, Total Internal
Reflection, Brewster’s Angle, Dispersion, Lenses
Physics 102: Lecture 18
Phys 102 recent lectures
• Lecture 14 – EM waves
• Lecture 15 – Polarization
• Lecture 20 & 21 – Interference & diffraction
• Lecture 16 – Reflection
• Lecture 17 – Spherical mirrors & refraction
• Lecture 18 – Refraction & lenses (today!)
• Lecture 19 – Lenses & your eye
Light as a wave
Light as a ray
Snell’s Law: A Quick Review
n1
n2
When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends:
n1 sin(1)= n2 sin(2)
1
2
44
Total Internal Reflection
normal
2
1
n2
n1
Recall Snell’s Law: n1 sin(1)= n2 sin(2)
(n1 > n2 2 > 1 )
1 = sin-1(n2/n1) then 2 = 90
c
Light incident at a larger angle will only have reflection (i = r)
ir
06
“critical angle”
For water/air:n1=1.33, n2=11 = sin-1(n2/n1) = 48.80
Fiber Optics
13
Telecommunications
Arthoscopy
Laser surgery
Total Internal Reflection only works if noutside < ninside
At each contact w/ the glass air interface, if the light hits at greater than the critical angle, it undergoes total internal reflection and stays in the fiber.
ninside
noutside
Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ?
1) Yes 2) No
Preflight 18.1
10 57% 43%
“There are millions of light ’rays’ coming from the light. Some of the rays will be totally reflected back into the water,but most of them will not.”
8
ACT: Refraction• As we pour more water into bucket, what
will happen to the number of people who can see the ball?
1) Increase2) Same 3) Decrease
11
ACT: Refraction
11
• As we pour more water into bucket, what will happen to the number of people who can see the ball?
1) Increase2) Same 3) Decrease
Brewster’s angle
15
When angle between reflected beam and refracted beam is exactly 90 degrees, reflected beam is 100% horizontally polarized !
Reflected light is usually unpolarized (mixture of horizontally and vertically polarized). But…
tanB
n2
n1
n1 sin B = n2 sin (90-B)
n1 sin B = n2 cos (B)
horiz. and vert.
polarized
B B
90º-B
90º
horiz. polarized only! n1
n2
ACT: Brewster’s Angle
When a polarizer is placed between the light source and the surface with transmission axis aligned as shown, the intensity of the reflected light:
(1) Increases (2) Unchanged (3) Decreases
T.A.
19
Polarizing sunglasses are often considered to be better than tinted glasses because they…
Preflight 18.3, 18.4
• block more light• are safer for your eyes• block more glare• are cheaper
When glare is around B, it’s mostly horiz. polarized!
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26%
9%
66%
0%
Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction?
• horizontal• vertical
53%
47%
Dispersion
prism
White light
Blue light gets deflected more
nblue > nred
The index of refraction n depends on color!
In glass: nblue = 1.53 nred = 1.52
23
Skier sees blue coming up from the bottom (1), and red coming down from the top (2) of the rainbow.
Rainbow: Preflight 18.5
25
Wow look at the
variation in index of
refraction!
Which is red?
Which is blue?
Blue light is deflected more!
LIKE SO! In second rainbow pattern is reversed
25
Flat Lens (Window)
n1n2
Incident ray is displaced, but its direction is not changed.
t
1
1
If 1 is not large, and if t is small, the displacement, d, will be quite small.
d
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1) Rays parallel to principal axis pass through focal point.2) Rays through center of lens are not refracted.
3) Rays through F emerge parallel to principal axis.
Assumptions: • monochromatic light incident on a thin lens.
• rays are all “near” the principal axis.
Converging Lens Principal Rays
F
F
Object
P.A.
Image is: real, inverted and enlarged (in this case).
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Image
Converging LensAll rays parallel to principal axis pass through focal point F. Double Convex
30
P.A.
F
nlens > noutside
F
• At F
• Inside F
• Outside F
P.A.
F
F62%
12%
25%
Preflight 18.6A beacon in a lighthouse produces a parallel beam of light. The beacon consists of a bulb and a converging lens. Where should the bulb be placed?
40
3 Cases for Converging Lenses
Object
Image
This could be used in a camera. Big object on small film
InvertedReducedReal
Past 2F
ImageObject
This could be used as a projector. Small slide on big screen
InvertedEnlargedReal
BetweenF & 2F
Image Object
This is a magnifying glass
UprightEnlargedVirtual
Inside F
ACT: Converging LensWhich way should you move object so image
is real and diminished?
(1) Closer to lens(2) Further from lens(3) Converging lens can’t create real
diminished image.
40
F
F
Object
P.A.
1) Rays parallel to principal axis pass through focal point.2) Rays through center of lens are not refracted.
3) Rays toward F emerge parallel to principal axis.
Diverging Lens Principal Rays
F
F
Object
P.A.
Only 1 case for diverging lens:
Image is always virtual, upright, and reduced.45
Image
Which way should you move object so image is real?
1) Closer to lens
2) Further from lens
3) Diverging lens can’t create real image.
ACT: Diverging Lenses
Demo
50
F
F
Object
P.A.
See You Wednesday