Question

Write a few differences between umbra and penumbra.

Answer 1

Hint: Umbra is complete shadow and penumbra is partial shadow. These are the words generally related to eclipse. Students must have a brief knowledge of the formation of solar and lunar eclipses.

Complete step by step solution:
(i) Umbra means ‘’shadow’’ while ‘’penumbra’’ means to hang on the shadow or almost a shadow. Also, penumbra was coined by Johannes Kepler whereas the coining of ‘’umbra’’ is not attributed to anyone in particular.

(ii) Umbra is the dark part while penumbra is the lighter part. ‘’Umbra’’ is similarly defined as shade while penumbra means partial shade.

(iii) The penumbra is situated at the perimeters while the umbra is at the center, directly behind the object. Correspondingly penumbra is synonyms with marginal.

(iv) During a total lunar eclipse, the earth’s inner shadow or umbra falls directly in the moon and at mid-eclipse, the moon may look blood red as it is entirely covered with shadow. When a partial lunar eclipse occurs, the umbra merely covers a part of the moon. The outer shadow of the earth falls on the moon during a penumbral lunar eclipse.

(v) During an eclipse, the umbra gets smaller as the distance from the sun increases. On the contrary, the penumbra gets bigger as the distance from the sun increases.

Note: Umbra decreases with an increase of the distance from the sun. Penumbra increases with an increase in the distance from the sun. Students can perform this activity by taking some examples where different size discs are placed successively and light is incident on anyone of the slit placed in front of the light source.