Shadow: When light fall son an opaque object, light is obstructed and a dark patch on a screen kept behind is called shadow. This is because light propagates in straight line. If distance between object and screen is less, the shadow will be (umbra) dark and smaller. If the distance is increased shadow will be dim and larger.
Conditions Required for a shadow to form are:
There should be a source of light.
An opaque object.
A screen.
The opaque object is placed between the source of light and the screen.
