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Chauchau 發問於 科學及數學其他 - 科學 · 1 十年前

# interference vs diffraction

Please give detailed explanation in the difference in intensity between both cases

### 1 個解答

• ?
Lv 7
1 十年前
最愛解答

You have neglected the assumption that, in a double slit arrangement, each of the slit width should be small as compared with the wavelength of the light used. In the actual fact, this assumption is made because the slits must behave like two coherent "point sources" (a point source has no dimension, in theory).

Under the above assumption, the intensity and separation between fringes in the double slit set up are more or less constant, as what is shown in your diagram 2. However, if the assumption is not met, then the fringe distribution would appear as what is shown in your diagram 1.

In a single slit, the slit width are much wider than the width of slits in the double-slit arrangement. This will give an interference pattern with varying fring separation and intensity (your diagram 1).

The following web-page contains a very good animation of the double-slit interference experiment.

http://lectureonline.cl.msu.edu/~mmp/kap27/Gary-Tw...

Try to vary the slit width to as small as you could (down to 0.1 micrometer). When it is much smaller than the wavelength (550 micrometer), you would get the pattern similar to your diagram 2. But when you gradually increase the slit width, say to 3 micrometer, then to 8 micrometer and then to 12 micrometer, you would get the pattern of your diagram 1, from a double-slit arrangement.

The reason is apparent, the interference pattern produced by the double-slit is enveloped by the diffraction pattern. When the slit width is small, the envelop (i.e. the central bright fringe) is wide.

2011-03-27 20:08:57 補充：

In the animation, try to use a slit separation of 25 micrometer and vary the slit width from 0.1 micrometer to 10 micrometer, it would yield a better view.