# 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.

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