Formation of Shadow


 
 
Concept Explanation
 

Formation of Shadow

Shadow is the dark region formed behind an opaque object

In a dark room, throw light from a torch on a table tennis ball. Observe the shadow of the ball on the wall. The shadow consists of a dark patch surrounded by a partially dark patch. The dark part of the shadow is called umbra. The umbra does not receive any light from the source. The partially dark patch is called penumbra. The penumbra region is less dark because it receives only some amount of light.

In the figure below we have a point source and an object is placed in the path of light. we see that a shadow is formed shown as shadow region on the screen.

In the figure below we have an extended source of light and an object is placed in the path of light. we see that a shadow is formed shown as shadow region on the screen. The shadow consists of two area one is a dark area marked by point B and C and is known as Umbra. The dark area is surrounded by partially dark area marked by point by point AB and CD. and is known as Penumbra.

The penumbra is formed in the case of extended object only. In the case of extended object the rays are coming from all the points on the source. In the region between AB and CD some oblique rays are blocked as shown in the diagram. Due to this some of the light from the source gets blocked by the shadowing object, but the remaining rays are not blocked . Hence a partially dark shadow is formed on the screen i.e. penumbra.

Sample Questions
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Question : 1

Which of the following are correct :

(a) Shadow is the dark region formed behind an opaque object.

(b) The shadow consists of a dark patch surrounded by a partially dark patch.

(c) The partially dark patch is called penumbra.

Right Option : D
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Explanation
Question : 2

The partially dark patch of the shadow is called ________________.

Right Option : C
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Explanation
Question : 3

The _________________ that is the dark patch of the shadow does not receive any light from the source.

Right Option : A
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Explanation
 
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