2. 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  Mirror 47  43  = 47  This works for any angle In 3D, you need three mirrors

3. Concept Question 34  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 n 1 = 1 n 2 = 1.5n 3 = 2.4 n 4 = 1.33 n 5 = 1.5 n 6 = 1 66 22 22 33 33 44 44 55 55

4. Concept Question A 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  D) None of the above 30  22 22 33 n 1 = 2.4 n 3 = 2.4 n 2 = 1 This is impossible! Light never makes it into region 2! It is totally reflected inside region 1

5. Concept Question Object Image p 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 uprightB) Virtual and upright C) Real and invertedD) Virtual and inverted

6. Concept Question Light from the Andromeda Galaxy bounces off of a concave mirror with radius R = 1.00 m. Where does the image form? A) At infinityB) At the mirror C) 50 cm left of mirror D) 50 cm right of mirror Concave, R > 0

7. Concept Question A fish is swimming 24 cm underwater (n = 4/3). You are looking at the fish from the air (n = 1). You see the fish A) 24 cm below the waterB) 24 cm above the water C) 32 cm below the waterD) 32 cm above the water E) 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 you distance behind the boundary 18 cm

8. Using the Lens Maker’s Equation If you are working in air, n 1 = 1, and we normally call n 2 = n. By the book’s conventions, R 1, R 2 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 =  If the lenses at right are made of glass and are used in air, which one definitely has f < 0? ABC D Light entering on the left: We want R 1 < 0: first surface concave on left We want R 2 > 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

9. Concept Question What 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 mB) -1 mC) +3 mD) -3 m 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 Opticians give the inverse focal length, f -1, which is given in diopters (= m -1 )

10. Concept Question A 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) HorizontallyB) Vertically C) 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!

11. Concept Question If you used a little wider slit, the pattern would A) Get wider and dimmer B) Get wider and brighter C) Get narrower and dimmer D) Get narrower and brighter

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

13. Concept Question 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) 0B)  C) 2  D) 3  E) None of the above 1  m

14. Concept Question Suppose 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 add A) ConstructivelyB) DestructivelyC) 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

15. 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-direction E) The x-, y-, or z-direction

16. 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 oneB) The blue oneC) It is a tie D) Insufficient Information

17. 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/mcB) Increase by 2h/mc C) Decrease by h/mcD) Decrease by 2h/mc When it scatters forward,  = 0 When it scatters backwards,  = 