1 Lecture series for Conceptual Physics, 8 th Ed.

2 Reflection p486 The surface electrons of a page in the book reflect all the colors and appear to be white. The black letters absorb all the colors. Tuning forks are to sound as electrons are to photons. Principle of Least Time p487 Pierre de Fermat, 1650, said, “Out of all possible paths that light might take to get from one point to another, it takes the path that requires the shortest time. This concept underlies all the formulas that describe light paths.

3 Law of Reflection p487 How does light get from A to the mirror and to B? Two possible paths… But, how do we find the least time? Try this construction: In fact, it works in all cases.

4 More reflections The angle of incidence equals the angle of reflection. The reflected rays seem to come from a point behind the mirror. This is a virtual image.

5 Plane Mirrors p489 Does a mirror shift left and right? NO! front back Front and back are switched. If your nose is closest to the mirror in front, then its image is also closest to the mirror “in back”. Smaller image in a convex mirror. Larger image in a concave mirror.

6 axis Convex mirror Focal point object Virtual image axis Concave mirror object Virtual image

7 Diffuse Reflection p490 A radio telescope: Rough to light. Polished to radio waves.

8 Refraction p492 Light bends when it changes speed at an interface. You’re the lifeguard. Which path takes the least time…dashed or solid? Remember that you can run faster in the sand than you can swim. The dashed line takes the least time. The relative sizes of the angles depend on the difference in speed. The index of refraction, n, is used. n = speed of light in a vacuum speed of light in the material

9 From air through glass: A to B is perpendicular to the interface…no refraction. A to Q hits glass and bends. Q to C continues through glass and bends as it emerges. C to C continues…parallel to AQ but offset. The path of least time.

10 Light goes through these shapes always taking the least time. The index of refraction, n, for air is nearly 1, but…

11 Mirages:Cyclist sees blue sky where the road should be.

12 Cause of Refraction p496 Snell’s Law: N 1 sinO 1 = n 2 sinO 2

13 When light enters a denser medium (the speed of light is slower), the light bends toward the normal. The reflection is normal: The angle of incidence equals the angle of reflection. And, vice versa.

14 Dispersion p497 Higher frequency light is closer to the resonance frequency of glass electrons. So, light at the blue end of the visible spectrum refracts more than light at the red end of the visible spectrum. This is why prisms make rainbows. And, it’s why focal points of lenses get fuzzy when blue light is focused closer to the lens than red light.

15 Rainbows p498 Stand with your back to the sun and image an arc of 42 o C.

16 Total Internal Reflection p502 Note the critical angle: 43 o

17 Here’s the critical angle from under water. Total internal reflection causes 100% of the light to bend…no loss.

18 More on Total Internal Reflection p503 This concept is used in binoculars The critical angle for a diamond is…24.5 o C Fiber optics depend on total internal reflection.

19 Lenses p505 A converging lens A diverging lens Important features of a lens. Where do the red rays bend?

20 Surface ripples on a pond create familiar patterns of light on the bottom. Image Formation by a Lens p506 No image. Pin-hole camera Lenses make images

21 Lens Defects p509 Light passing through edge of lens focuses in front of light passing through middle of lens. Cures: 1. Cover the edges. 2. Combine concave and convex lenses. Blue light is refracted more than red. Combining different types of glass usually cures this.