A 70cmdiameter disk emits light uniformly from its surface 2

A 7.0-cm-diameter disk emits light uniformly from its surface. 20 cm from this disk, along its axis, is an 7.0-cm-diameter opaque black disk; the faces of the two disks are parallel. 20 cm beyond the black disk is a white viewing screen. The lighted disk illuminates the screen, but there’s a shadow in the center due to the black disk.What is the diameter of the completely dark part of this shadow? Express your answer to two significant figures and include the appropriate units. d =

Solution

The diameter of both the disks are same = 7cm.

Hence the shadow region will also be of same diameter, i,e 7cm unless the diameters of the two disks are different.