DIFFRACTION OF LIGHT

Even parallel light from a pin point, such as a star, passing a shadowing object does not result in a sharp shadow but rather results in a shadow whose degree of illumination varies due to diffraction as shown below.

In this graph point O is a point source passing an opaque shadowing object A with the light falling onto surface MPN.

The line OP is extended onto the surface with a dotted line to where the light intensity is shown on a vertical scale on the right where one might expect a distinct shadow line to be formed.

However, note that not only does the light intensity not reach its full value until some point above the dotted line but that also a diminishing amount of light appears below it.

Thus, it is difficult to determine where the exact edge of the shadow.

A picture of such a shadow showing this difficulty is shown below.

Note that a vary rapidly diminishing light occurs to the left of the shadowing tip. The light strips observed on the right of the tip occur due to the effect of diffraction which amplifies and diminishes the total light intensity as adjacent light waves from the point source overlap with reinforcement and cancellation. This will not be observed in a shadow of the sun since it has width and all points to the right of the tip will be bright. However, this width will further increase the amount of diminishing light on the left of the shadow tip. (Ignore the very small dots which occur when a black and white film photo is converted to a digital format with low enough resolution to allow it to fit on this page.)