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Unformatted text preview: direction, and at near distances the
wave fronts have the shape of the aperture.
The wave eventually spreads out. Similar to a wave from a point source.
This effect is called diffraction.
Lecture 4, p 20 Single-slit Diffraction
So far in the multiple-slit interference problems we have assumed that each
slit is a point source.
Point sources radiate equally in all directions.
Real slits have a non-zero extent – - a “slit width” a.
The transmission pattern depends on the ratio of a to λ.
In general, the smaller the slit width, the more the wave will diffract.
Small slit: Diffraction
screen Large slit: Diffraction
Lecture 4, p 21 Single-Slit Diffraction
Slit of width a. Where are the minima?
Use Huygens’ principle: treat each
point across the opening of the slit as a
The first minimum is at an angle such
that the light from the top and the
middle of the slit destructively interfere. Incident Wave
(wavelength λ) a/2 a
δ = sin θ min =
δ ⇒ sin θ min = λ
a y a This works, because for every point in
the top half, there...
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