130_notes.dvi

d sinθ

d

laser θ

EM waves of wavelengthλare emitted from a single light-source, like a laser. They travel to two
narrow slits, (for simplicity) equidistant from the source, and a distancedapart. Light travels from
the slits to a detection screen. A diffraction pattern can be seen onthe detection screen, like the
one shown in the picture below.

The center of the diffraction pattern occurs at the location on thescreen equidistant from each
slit where thewaves from the two slits are in phase (because they have traveled exactly
the same distance) and the fields add, so the wavesinterfere constructivelyand there is an
intensity maximum. Some distance off this center of the diffraction pattern, there will be destructive
interference between waves from the two slits and the intensity willbe zero. This will occur when