13.4 Double slit interference

From one source and two gaps

1st bright fringe

1st bright fringe

central fringe

From one source and two gaps

1st bright fringe

1st bright fringe

central fringe

- the closer the slits become the wider the fringe separation.

- waves of short wavelength give narrow fringe separation because blue light diffracts less.

Path difference

Phase difference

Path difference

Phase difference

W

First bright fringe

Central fringe

screen

Double Slit interference - theory

W

First bright fringe

Central fringe

screen

Double Slit interference - theory

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D

= wavelength = 600nmD = slit to screen distance = 2mS = slit separation = 0.5 mmW = fringe width = 1mm

For constructive interference at the screen:( ie a bright fringe )

Wave fronts from S1 and S2 must arrive at the screen in phase with a path difference of a whole number of wavelengths

For constructive interference at the screen:( ie a bright fringe )

Wave fronts from S1 and S2 must arrive at the screen in phase with a path difference of a whole number of wavelengths

For destructive interference at the screen:( ie a dark fringe )

Wave fronts from S1 and S2 must arrive out of phase with a path difference of half a wavelength

soh cah toa

soh cah toa

soh cah toa

soh cah toa