The President holds office for a term of five years. However he can resign...
- 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.