• A dark patch formed behind an opaque object when it obstructs the passage of light is called a shadow.
  • Shadows are formed when light either does not enter the eye from certain regions or enters in smaller amount.
  • Umbra is a shadow of total darkness. A shadow formed by a point source of light is an umbra.
  • Penumbra is a shadow of partial darkness. An extended source is general, produces umbra as well as penumbra.
  • If the screen is moved away from the object, the penumbra increases while the umbra decreases till a stage is reached when no umbra is formed. So the shadow of the birds and aeroplanes flying are not formed, as the umbra is absent.
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