We imagine secondary sources of light at points A, B and C in the aperture. Consider light leaving these points at angle q to the normal line, as shown below. The aperture width is b.

If the path difference between rays from C and A to a point on a distant screen is l, then the path difference between rays from A and B is l/2.

There will therefore be destructive interference between light from A and light from B. From the diagram, it is clear that

Now consider the aperture to be made up of pairs of point sources, A and B, A’ and B’, etc, as shown in the next diagram. Light from all these pairs of points will also interfere destructively.

Therefore we will have a minimum intensity of the diffraction pattern at an angle the angle q.

As the angles are small, we can write

If the distance between the aperture and the screen is D, then the width of the central maximum of a single aperture diffraction pattern is given by

The above reasoning should not be considered as an explanation of the diffraction pattern. It does, however, give a method of predicting where the minima of the pattern will occur.

These predictions agree with experiment.

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