Question

Draw a ray diagram to show the formation of umbra and penumbra both. Label the parts umbra and penumbra in your diagram.

Answer 1

Umbra is the portion of shadow where no light reaches from the source of light due to the opaque object. It is completely dark.

Penumbra is the portion of shadow where a portion of light reaches from the source of light. It is not completely dark.

Ray diagram for the formation of umbra and penumbra is given below.



Umbra and penumbra are part of a shadow created by any light source when an opaque object is placed between the source of light and the screen.
To show the Umbra and penumbra both we will set up an experiment:
1. The opaque object is illuminated by an extended source of light such as a burning candle or a glowing torch.

2. We take the burning candle PQ. And will place a screen at some distance in front of it.

3. Now placing an opaque object AB in between the source of light and the screen. The size of the object AB is of the same size or bigger than that of the source PQ.

4. We will observe that the shadow formed on the screen is not well defined and it is not equally dark everywhere.

5. The darkest part in the middle is called an umbra.

6.And the partially shaded region which is less dark than umbra is called a penumbra.