A First Course in Physics · Ch. XIX, §§ 506–515 · Chapter 19: Nature and Propagation of Light
60. Reflection, refraction and total reflection
Goal: Your student can state the law of reflection, explain diffuse reflection and why we see objects, describe refraction toward and away from the perpendicular, and explain total reflection and the critical angle.
You'll need
- The book, open to these pages
- A small mirror and a flashlight (or a laser pointer, used carefully)
- A clear glass or plastic tub of water with a few drops of milk stirred in (to show the beam)
- A coin, a pencil and an opaque cup
- A notebook and pencil
What's in the book
Angle of incidence equals angle of reflection; diffuse reflection from rough surfaces; why nonluminous bodies are visible and mirrors are not; light reflects at every boundary; refraction toward the perpendicular entering a slower medium and away on leaving; total reflection and the critical angle (water about 48.5°); total-reflecting prisms; paths through a prism and through plate glass. Fifteen questions follow.
The lesson, step by step
Hook question
SayWhy does a straw in a glass of water look broken at the surface?
Read together
Read §§ 506–515. The rule for refraction:
whenever light travels obliquely from one medium into another in which the speed is less, it is bent toward the perpendicular
Try it: reflection
In a dark room, shine the flashlight at the mirror at an angle and find the reflected spot. Change the angle: the reflected beam always leaves at the same angle it came in (§ 506). Shine it on white paper instead: the light scatters everywhere (diffuse reflection, § 507).
Try it: refraction
Put the coin in the cup and back away until the rim just hides it. Have someone pour in water slowly: the coin appears (question 7). Look at the pencil in a glass of water from the side (question 8). Shine the flashlight beam into the milky water at an angle and watch it bend at the surface.
Try it: total reflection
Look up from below through the side of a glass of water at the water's surface: it looks like a silver mirror. Shine the beam up toward the surface from below at a steep angle: beyond the critical angle no light escapes, and all of it reflects (§ 511). This is how optical fibers carry internet signals today.
Explain the idea
Light bends because it travels at different speeds in different materials (§ 510). Going into a slower medium it bends toward the perpendicular; coming out into a faster one it bends away. For water the critical angle is about 48.5°.
Say it back
Your student explains why a fisherman with a spear should aim below where the fish appears (question 9).
Questions
Work questions 1–15 on book pages 403–404; choose about eight if time is short.
Afterwards
Notice reflections and refraction this week: a pool that looks shallower than it is, a mirage on a hot road, a rainbow of light from a glass edge.
Tip
Never shine a laser pointer at anyone's eyes, and never look at the Sun, directly or through any lens, mirror or prism.
Answers to the book's problems
1. White walls diffuse much of the light back into the room; black walls absorb it. 2. The paint diffuses the light in all directions, so little of it goes to the spot; the clean mirror sends the beam straight to the wall. 3. We see only the part of it lit by the sun, and the lit part changes as it moves around the earth. 5. The air above us is still lit by the sun and diffuses light down to us, and the air bends the sun's rays around the curve of the earth. 6. Sunlight reflected from the earth to the moon and back to the earth (earthshine); with a three-quarter moon the bright part is far too bright to see it, and less of the lit earth faces the moon. 7. Light from the coin is bent away from the perpendicular as it leaves the water, so it reaches the eye over the rim. 8. Light from the part under water is bent at the surface, so that part looks raised. 9. Low, because the fish looks higher than it really is. 10. The air grows denser gradually toward the ground, so the ray is bent a little at a time. 11. The glass shifts the light sideways (§ 515), so the part seen through it looks displaced. 12. Upward, at about 41.5° from the horizon (48.5° from the vertical). 13. It reflects all the light (total reflection), with no tarnish and only one reflecting surface. 14. The silvering (metal) on the back of the glass. 15. The prisms bend the daylight entering the window so that it travels deep into the room instead of falling on the floor near the window.
Worked out for this site from the scan. If a number in your copy differs, trust the book.