A First Course in Physics · Ch. XIX, §§ 516–527 · Chapter 19: Nature and Propagation of Light
61. What is light? Waves, interference and polarization
Goal: Your student can compare the corpuscular and wave theories, explain the interference bands between two glass plates, use the index of refraction, and explain how polarization shows light waves are transverse.
You'll need
- The book, open to these pages
- Two clean microscope slides or small glass panes and a strip of paper (or a soap film on a wire loop)
- Two pairs of polarized sunglasses (or one pair and a phone or computer screen)
- A tall glass of water and a coin
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
The corpuscular theory; Huygens's wave theory and the ether; interference bands in an air wedge between glass plates; the wave length of yellow light (about .000589 mm); sources of light waves; why light casts sharp shadows; the bottom of a glass of water looks raised by one fourth of the depth, giving the ratio of speeds; index of refraction and a table; tourmaline crystals and polarization. Five questions follow, with a portrait of Huygens.
The lesson, step by step
Hook question
SayIs light made of tiny bullets or of waves? Newton and Huygens disagreed.
Read together
Read §§ 516–527. The book's verdict:
Hence the corpuscular theory is now practically abandoned, and light is universally regarded by physicists as a form of wave motion.
Try it: interference colors
Press two clean glass slides together, with a strip of paper between them at one end, and look at the reflection of a window or lamp: fine colored bands appear (Fig. 430). A soap film or a puddle with a film of oil shows the same thing: light reflected from the two surfaces adds in some places and cancels in others.
Try it: the raised bottom
Look straight down into a tall glass of water at a coin and mark with a finger on the outside where the bottom seems to be. It looks raised by about a fourth of the depth (§ 523), because light travels about ¾ as fast in water. The index of refraction of water is 1.33 (§ 525).
Try it: polarization
Look through one pair of polarized sunglasses at a second pair and rotate one: at one angle almost no light gets through (§ 527, like the crossed tourmalines in Fig. 437). Or look at a phone or computer screen through polarized sunglasses and tilt your head. Light waves vibrate across their direction of travel (transverse), like waves on a rope through a slotted fence (Fig. 434).
What we know now
The ether does not exist. Experiments (Michelson and Morley, 1887) found no trace of it, and Einstein's relativity (1905) showed light needs no medium: light is a wave of electric and magnetic fields (Chapter XXII). Twentieth-century physics also brought back part of the particle idea: light comes in tiny packets of energy called photons, and behaves as both a wave and a particle.
Say it back
Your student explains how the interference bands show that light is a wave, using Fig. 431.
Questions
Work questions 1–5 on book page 415.
Afterwards
Look through polarized sunglasses at the sky in different directions, at reflections on water, and at a car's rear window. What changes?
Answers to the book's problems
1. About 225,600 km per second (300,000 ÷ 1.33). 2. Toward the perpendicular, since light travels more slowly in flint glass than in water. 3. Less, because the wave length of light is shorter in water, so the film need not thicken as much from one band to the next. 4. Less. 5. Farther from the surface (higher) than he really is.
Worked out for this site from the scan. If a number in your copy differs, trust the book.