1. Chapter 35
Interference

2. To consider interference and coherent sources
Goals for Chapter 35
To consider interference and coherent sources
To study two-source interference of light
To determine intensity in interference patterns
To consider interference in thin films
To study the Michelson interferometer

3. Introduction
Rainbows in the sky: been there, seen that. A thin-film soap bubble: why should that create a rainbow effect?
It seems we need to read Chapter 35 because the man on page 1207 has a large soap bubble and this thin film is dispersing white light and revealing a r.o.y.g.b.i.v. spectrum of color.

4. Wave fronts from a disturbance
Think back to our first slide on wave motion when the father threw an object into the pool and the boy watched the ripples proceed outward from the disturbance. We can begin our discussion of interference from just such a scenario, a coherent source and the waves from it that can add (constructively or destructively).
Consider Figure 35.1.

5. A “snapshot”
The “snapshot” of sinusoidal waves spreading out from two coherent sources.
Consider Figure 35.2.

6. Two-source interference of light
Figure 35.4 shows two waves interfering constructively and destructively.
Young did a similar experiment with light. See below.

7. As the waves interfere, they produce fringes
Consider Figure 35.6 below.

8. Interference from two slits or two radio stations
Follow Example 35.1, illustrated by Figure 35.7 (lower left).
Follow Example 35.2, illustrated by Figure 35.8 (lower right).

9. Intensity distribution
Figure 35.10, below, displays the intensity distribution from two identical slits interfering.
Follow Example 35.3.

10. Thin films will interfere
The reflections of the two surfaces in close proximity will interfere as they move from the film.
Figure at right displays an explanation and a photograph of thin-film interference.

11. An air wedge between two glass plates
Just like the thin film, two waves reflect back from the air wedge in close proximity, interfering as they go.

12. Interference between mechanical and EM waves
Figure compares the interference of mechanical and EM waves.

13. Thick films and thin films behave differently
Refer to Figure in the middle of this slide.
Read Problem-Solving Strategy 35.1.
Follow Example 35.4, illustrated by Figure at the bottom of the slide.

14. Thin-film examples Consider Example 35.5.
Consider Example 35.6, illustrated by Figure shown below.

15. Newton’s rings
Figure illustrates the interference rings resulting from an air film under a glass item.

16. Using fringes to test quality control
An optical flat will only display even, concentric rings if the optic is perfectly ground.
Follow Example 35.7.

17. Michelson and Morley’s interferometer
In this amazing experiment at Case Western Reserve, Michelson and Morley suspended their interferometer on a huge slab of sandstone on a pool of mercury (very stable, easily moved). As they rotated the slab, movement of the earth could have added in one direction and subtracted in another, changing interference fringes each time the device was turned a different direction. They did not change. This was an early proof of the invariance of the speed of light.