- The
**Principle of Superposition**states that when several light waves coincide, the result is equal to the sum of the individual waves. Phases are important! - If two light waves of same wavelength and phase travel different
distances to reach the same point, their interference will be
- constructive if difference in path length is integer number of wavelengths
- destructive if difference in path length is half-integer number of wavelengths

- The double-slit experiment forces light waves to interfere on a distant wall after travelling different distances
- Consider a point on the distant wall a distance
`y = L sin(theta)`away from the center point. Light will interfere- constructively (light spot) if
d sin(theta) = m lambda m = 0, 1, 2, ...

- destructively (dark spot) if
d sin(theta) = (m + 1/2) lambda m = 0, 1, 2, ...

`d`is the separation of the two slits, and`lambda`is the wavelength of the light. - constructively (light spot) if

Viewgraph 1

Viewgraph 2a

Q: What type of light source produces coherent light waves?

Viewgraph 2b

Viewgraph 3

Viewgraph 4

Viewgraph 5

Viewgraph 6

Viewgraph 7

Joe shines a blue light of wavelength450 nmthrough a pair of slits which are0.2 mmapart. Q: Find three angles at which Joe will find a bright spot on the distant wall. Q: Find three angles at which Joe will find a dark spot on the distant wall.

Viewgraph 8

Viewgraph 9

Viewgraph 8A