Question

To compare the intensity of light from two point sources, one may use the following simple method. A thick rod D is placed in front of a transparent screen AB (Fig.). The sources are placed at distances S₁ and S₂ so that the shadows AO and OB are of the same brightness. In doing so, in which direction should the sources be moved for the shadows to remain touching one another? What pattern is observed when the sources are moved in any other direction?

Answer 1

The sources should be moved along the tangents to the rod passing through point O. If the sources are moved in any other direction, the shadows separate or become superimposed on each other.