Concept Question

Mirror

A light ray starts from a wall at an angle of 47 compared to the wall. It then strikes two mirrors at right angles compared to each other. At what angle does it hit the wall again?A) 43 B) 45 C) 47 D) 49 E) 51 M

irro

r

47

4747

4343

43

= 47

•This works for any angle•In 3D, you need three mirrors

Concept Question

34

1 1 2 2sin sinn n

A light ray in air enters a region at an angle of 34. After going through a layer of glass, diamond, water,

and glass, and back to air, what angle will it be at?A) 34 B) > 34 C) <34 D) This is too hard

n1 = 1 n2 = 1.5 n3 = 2.4

n4 = 1.33 n5 = 1.5

n6 = 1

6

2 2

3

3

4 4

5 5

1 2 2sin 34 sinn n 3 3sinn 4 4 5 5 6 6sin sin sinn n n

61 sin 34 1 sin

6 34

Concept QuestionA trick question:

A light ray in diamond enters an air gap at an angle of 30, then returns to diamond. What angle will it be going

at when it leaves out the bottom?A) 30 B) < 30 C) >30D) None of the above

30

22

3

1 2 2sin 60 sinn n

2sin 2.4 0.5 1.2

n1 = 2.4

n3 = 2.4

n2 = 1

•This is impossible!•Light never makes it into region 2!•It is totally reflected inside region 1

Concept Question

Object Imagep q

Mirror

h h’•The image distance q is how far the image is in front of the mirror

•Real image if q > 0, virtual image if q < 0

•The magnification M is how large the image is compared to the object

•Upright if positive, inverted if negative

If you place an object in front of a flat mirror, its image will be

A) Real and upright B) Virtual and uprightC) Real and inverted D) Virtual and inverted

hM

h

Concept Question

12f R 1 1 1

p q f

Light from the Andromeda Galaxy bounces off of a concave mirror with radius R = 1.00 m. Where does the image form?A) At infinity B) At the mirrorC) 50 cm left of mirror D) 50 cm right of mirror

12 50 cmf R

2 Mlyp

•Concave, R > 0 1 1 1

q f p

10

50 cm

50 cmq

Concept Question

2

1

nq p

n

A fish is swimming 24 cm underwater (n = 4/3). You are looking at the fish from the air (n = 1). You see the fishA) 24 cm below the water B) 24 cm above the waterC) 32 cm below the water D) 32 cm above the waterE) 18 cm below the water

24 cm

•R is infinity, so formula above is valid•Light comes from the fish, so the water-side is the front•Object is in front•Light starts in water•For refraction, q tells youdistance behind the boundary

24 cmp

1

2

4 3

1

n

n

1 24 cm

4 3q

18 cm

18 cm

Using the Lens Maker’s Equation•If you are working in air,n1 = 1, and we normally calln2 = n.•By the book’s conventions, R1, R2 are positive if they are convex on the front•You can do concave on the front as well, if you use negative R

•Or flat if you set R =

2

1 1 2

1 1 11

n

f n R R

If the lenses at right are made ofglass and are usedin air, which one definitely has f < 0?

A B C

DLight entering on the left:•We want R1 < 0: first surface concave on left•We want R2 > 0: second surface convex on left

•If f > 0, called a converging lens

•Thicker in middle•If f < 0, called a diverging lens

•Thicker at edge

•If you turn a lens around, its focal length stays the same

Concept QuestionWhat can you do if you don’t have perfect eyesight?•To make the eye work, just put a lens that turns the object (p) you want to see into an image at a distance (q) where you can see it

A farsighted person can’t see objects closer than 1.00 m away. What focal length lens would adjust his eyesight so he can read 0.50 m away?A) +1 m B) -1 m C) +3 m D) -3 m

1 1 1

p q f •The object will be 0.50 m in front of the lens

•p = +0.50 m•The image will be 1.00 m in front of the lens

•q = -1.00 m 1 1 1

0.5 m 1 m f

1 1 12 m 1 m

f 1 mf Opticians give the inverse

focal length, f -1, which is given in diopters (= m-1)

Concept QuestionA beam of light is to be put through a small slit as shown at right. After it goes through, which way will it spread out?A) Horizontally B) VerticallyC) An equal amount of both

•In vertical direction, waves add up to make a front going same direction•In horizontal direction, waves spread out because slit is narrow•Controlling the horizontal position causes its horizontal direction to become uncertain

•Closely related to Heisenberg’s Uncertainty Principle!

Concept Question

If you used a little wider slit, the pattern wouldA) Get wider and dimmerB) Get wider and brighterC) Get narrower and dimmerD) Get narrower and brighter

2

max

sin sin

sin

aI I

a

Concept Question

What is the approximate ratio d/a of the slit separation d over the slit size a?A) 1 B) 2 C) 4 D) 8 E) 16 F) 32

Concept Question

0

2 dn

Light of wavelength 0.5 m takes two paths, both of length 1 m, one through air, the other through glass (n = 1.5). What is the difference in phase between the two waves in the end?A) 0 B) C) 2 D) 3 E) None of the above

1

2 1 1.56

0.5

2

2 1 1.04

0.5

2

1 m

Concept QuestionSuppose we are in a glass medium, and we have a wave that goes from glass to air to glass. If the layer of air is much smaller than one wavelength, then the two reflected waves will addA) Constructively B) Destructively C) Insufficient Info

•First transition: high to low•no phase shift

•Second transition: low to high phase shift

•Compared to each other, the two waves are out of phase with each other•They will have a tendency to cancel•Very little effect from layer if much thinner than a wavelength

Concept Question

If a wave is moving in the z-direction, in which directions can the electric field point?A) The x-direction (only)B) The y-direction (only)C) The z-direction (only)D) The x- or y-direction, but not the z-directionE) The x-, y-, or z-direction

Concept Question

A red laser and a blue laser each are emitting 12 mW of power. Which one has a faster rate of photons coming out?A) The red one B) The blue one C) It is a tieD) Insufficient Information

Concept Question

What is the maximum change that can occur to the wavelength of light when it scatters from a stationary electron?A) Increase by h/mc B) Increase by 2h/mcC) Decrease by h/mc D) Decrease by 2h/mc

1 cosh

mc

•When it scatters forward, = 0

•When it scatters backwards, =

1 cosh

mc

1 1h

mc

1 cosh

mc 2h

mc

1 1h

mc