Chapter 27, Problem 18A

Distinguish between an umbra and a penumbra.

To determine

To differentiate Umbra and Penumbra.

Explanation of Solution

Introduction:

Umbra and Penumbra are two distinct parts of a shadow cast by celestial bodies. And they are created by the light source after impinging on opaque objects. Umbra and Penumbra are the terms associated with eclipses.

  

Figure: Shows the Umbra and Penumbra

Umbra	Penumbra
1. It is the darkest portion of the shadow.	1. It is the less dark portion of shadow.
2. Umbra is known to be a shade	2. Penumbra is known to be partial shade.
3. Umbra is a shadow that is directly behind the object on which a light source impinges.	3. It is not a true shadow but it is a gradient of Umbra.
4. No light reaches in this region.	4. Light from some parts of the source reaches.
5. Umbra is situated at the centre directly behind the object.	5. Penumbra is situated at the perimeter.
6. During the solar eclipse, it is bigger in size as distance the from the sun increases.	6. Smaller in size as the distance from the sun increases.

Conclusion:

During an eclipse, Umbra and Penumbra are cast. Umbra gets smaller as it goes away from the sun and it is the dark centre of the eclipse shadow. The Penumbra gets larger as it goes away from the sun.