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Shown in the photograph at the left below is a demonstration involving a laser, a standard 20cm focal length glass lens, and a white screen.
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The laser beam hits the lens near its center and produces reflections off the front and back surfaces, which can be seen, along with the light originating at the laser, in the photograph of the screen at the right above.
If the lens is moved across the laser beam very slowly, as seen in an mpeg video by clicking your mouse on the photograph at the right above, the two reflections will move closer to each other, eventually becoming superimposed. You cannot see what will happen when the two reflections are actually superimposed, because the video stops before that happens. In fact, what happens when the two reflections are superimposed is the subject of this question.
When the lens is moved so that the reflections from the front and the back surfaces are superimposed:
Click here for Answer #293 after October 22, 2007.
