A point source of light is placed at a distance d in front of a screen S. A mirror M is put behind the source at distance d from it; as shown in the figure. We have to find the change in intensity at the screen S.
|
Solution: Click For PDF Version Let
be the intensity of illumination because of the point source of
light at the centre of the screen S
in the absence of the mirror M.
The distance of the point source from the screen S
is d. When the mirror
M is put at a distance d from
the point source, the distance of the virtual image of the source
from the screen will be 3d.
As the point source radiates uniformly in all directions so will its
image. As intensity of illumination from a point source varies
inversely as square of distance from it, the intensity
at the centre of the screen when mirror is placed behind the point
source will therefore be
|
.