1. Physics 102: Lecture 18
Snell’s Law, Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses 1

2. Snell’s Law: A Quick Review
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(q1)= n2 sin(q2)
ACT: n1 q1
q1 < q2
sinq1 < sinq2
n1 > n2
Demo 281: Snell’s Law
1) n1 > n2
2) n1 = n2
3) n1 < n2 q2 n2
Compare n1 to n2. 44

3. Total Internal Reflection
Recall Snell’s Law: n1 sin(q1)= n2 sin(q2)
(n1 > n2  q2 > q1 )
q1 = sin-1(n2/n1) then q2 = 90
“critical angle” q2
Light incident at a larger angle will only have reflection (qi = qr) n2 n1 qr
Demo 281: Snell’s Law
Only possible if n1>n2 qi qc q1
normal 06

4. Snell’s Law Practice Example
Usually, there is both reflection and refraction!
A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60o. The other part of the beam is refracted. What is q2? 1 r n1 n2
normal 48

5. Snell’s Law Practice Example
Usually, there is both reflection and refraction!
A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60o. The other part of the beam is refracted. What is q2?
q1 =qr =60o 1 r
sin(60o) = 1.33 sin(q2) n1
q2 = 40.6o
Question/ACT: is total internal reflection possible? n2
normal 48

6. Preflight 18.1
Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ?
“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.”
1) Yes 2) No
51% % 10 8

7. Green Flash
See for all you could ever want to know about green flashes
For students looking for an honors project, this would be an interesting topic
see me if you are interested
Tell Hawaii story

8. ACT: Refraction
As we pour more water into bucket, what will happen to the number of people who can see the ball?
1) Increase 2) Same 3) Decrease
ACT, then demo 1129 (ball in tub of water) 11

9. ACT: Refraction
As we pour more water into bucket, what will happen to the number of people who can see the ball?
1) Increase 2) Same 3) Decrease 11

10. Fiber Optics
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.
noutside
ninside
Telecommunications
Arthoscopy
Laser surgery
Total Internal Reflection only works if noutside < ninside 13

11. Fiber Optics
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.
noutside
ncladding
Fiber optics are used for probing and micro-surgery. Cladding lets optics be used to probe tissues with variable indices of refraction.
Demo 289
ninside
Add “cladding” so outside material doesn’t matter!
We can be certain that ncladding < ninside 13

12. horiz. and vert. polarized
Brewster’s angle
Reflected light is usually unpolarized (mixture of horizontally and vertically polarized). But…
horiz. and vert. polarized qB
90º-qB
90º
horiz. polarized only! n1 n2
When angle between reflected beam and refracted beam is exactly 90 degrees, reflected beam is 100% horizontally polarized !
n1 sin qB = n2 sin (90-qB)
n1 sin qB = n2 cos (qB) 15

13. 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.
ACT, then demo 664 19

14. Preflight 18.3, 18.4
Polarizing sunglasses are often considered to be better than tinted glasses because they…
27%
62% 9% 1%
block more light
block more glare
are safer for your eyes
are cheaper
When glare is around qB, it’s mostly horiz. polarized!
Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction?
horizontal
vertical
60%
40% 21

15. Dispersion The index of refraction n depends on color!
In glass: nblue = nred = 1.52
nblue > nred
prism
White light
Blue light gets deflected more 23

16. Wow look at the variation in index of refraction!
Rainbow:
Preflight 18.5
Wow look at the variation in index of refraction!
Which is red?
Which is blue?
refraction, reflection, refraction
Skier sees blue coming up from the bottom (1), and red coming down from the top (2) of the rainbow.
Blue light is deflected more! 25

17. In second rainbow pattern is reversed
LIKE SO!
In second rainbow pattern is reversed 25

18. Flat Lens (Window)
Incident ray is displaced, but its direction is not changed. n2 n1 q1 q1
Start this by :35 d
If q1 is not large, and if t is small, the displacement, d, will be quite small.
t 27

19. Converging Lens Principal Rays
Example F
Image
P.A.
Object F
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.
Image is: real, inverted and enlarged (in this case).
Assumptions:
• monochromatic light incident on a thin lens.
• rays are all “near” the principal axis. 35

20. Converging Lens Preflight 18.6
All rays parallel to principal axis pass through focal point F.
Double Convex
Preflight 18.6
A 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?
P.A. F F
nlens > noutside
P.A. F
53%
20%
27%
At F
Inside F
Outside F 30

21. 3 Cases for Converging Lenses
Object
Image
Past 2F
Inverted
Reduced
Real
This could be used in a camera. Big object on small film
Between
F & 2F
Image
Object
Inverted
Enlarged
Real
This could be used as a projector. Small slide on big screen
Demo 71
Image
Object
Inside F
Upright
Enlarged
Virtual
This is a magnifying glass 40

22. ACT: Converging Lens
Which 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. F
Object
P.A.
Demo 71 40

23. Diverging Lens Principal Rays
Example F
P.A.
Image
Object F
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.
Only 1 case dir dirverging lens:
Image is always virtual, upright, and reduced. 45

24. ACT: Diverging Lenses
Which way should you move object so image is real?
Closer to lens
Further from lens
Diverging lens can’t create real image. F
Object
P.A.
Demo 50

25. See You Wednesday