Interference and the Two-slit Experiment

This lecture is based on Sections 27.1 to 27.2.

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, ... 
```
where d is the separation of the two slits, and lambda is the wavelength of the light.