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Re: Diffraction



Let us define consecutive wavefronts (for the purpose of making
the Huygens construction) are those in which oscillations are in
phase. The distances between such wavefrons are equal to
the wavelength. This observation should help you to answer
the initial question, I think.

The effect of slit edges, if you make the Huygens construction,
is strong in regions close to slit edges (no more than several
lambdas). Away from edges the situation is symmetrical and
wavefronts (lines tangent to Huygens wavelets) are practically
parallel to initial wavefronts. To simplify assume that the initial
wavefronts (before diffraction) are also parallel to the slit surface.

Near the edges there is no symmetry and tangent lines are no
longer parallel to the initial wavefronts. This phenomenon of
"wave bending" is named diffraction. Discuss it in the context of
water waves first (preferably with the water tank demo) and then
generalize the observation to cover all waves, including light.
Ludwik Kowalski

Gerry White wrote:

You are correct about the use of Huygens principle. The students have
grasped the main idea, but one particularly bright student asked why the
wavelength made any difference. Not having ever been asked this question I
was stumped.

At 08:34 24/09/99 -0400, you wrote:
The textbook you are using probably explains diffraction
in terms of Huygens principle. Are you satisfied with the
explanation? Why yes or why no?

Gerry White wrote:

Could someone give me an explanation that could be understood
by high school students as to what the wavelength has to do with
the amount of diffraction.

We know that the most diffraction occurs when the wavelength is
large compared to the opening, but why?

Any help would be appreciated.

Gerry White
St. Vincent's High School
Saint John, NB Canada