- Books Name
- CBSE Class 6 Science Book
- Param Publication
- CBSE Class 6
* A Shadow has two Regions :
For a non-point source of light (i.e., an extended source of light), the shadow of an object has two parts, namely, the umbra and the penumbra. The umbra is the darkest inner part of the shadow where no light falls at all. The penumbra is the part which surrounds the umbra. Some rays of light fall on this region and partially illuminate it. Thus, the penumbra is called the region of partial darkness.
(i) Umbra : The inner region of total darkness is called umbra.It is the evenly dark part of a shadow. Umbra means ‘shade’ in latin.
(ii) Penumbra : The outer region of partial darkness is called penumbra. It is the fuzzy part between the dark and the light . Pene means ‘almost’ in Latin.
Umbra and penumbra are clearly formed only when the source of light is big and opaque body is small.
If one is in the umbra of an object, the light source is completely obscured. If one is in the penumbra, the source is only partially obscured, to a greater or lesser degree as one moves through the penumbra.
With a wider light source,the shadow becomes fuzzier Also, closer the object to the light, the larger and fuzzier is the shadow. In other words, the closer the object to the screen, the smaller and well defined is the shadow. Thus, the size of the umbra becomes smaller and smaller if the screen is moved farther and farther from the light source and the object.
Shadows are often fuzzy, particularly when the surface on which the shadow lies is far from the object casting the shadow. This fuzziness is because of only point light source in space. All sources have some geometrical size.Thus, light from one edge of the source is not quite parallel to light from the other edge
The result is a larger and fainter penumbra that is hardly visible. This is the reason why we cannot see the shadow of a bird flying high up in the air .
Shadow of same object is different for same source, because of the change in the postion of an object and light falling on different surface of an object for same source.
Behaviour of light at the interface of two media :
When light travelling in one medium falls on the surface of a second medium, the following three effects may occur :
(i) A part of the incident light is turned back into the first medium. This phenomenon is called reflection of light.
(ii) A part of the incident light is transmitted into the second medium along a changed direction. This phenomenon is called refraction of light.
(iii) The remaining third part of light energy is absorbed by the second medium. This phenomenon is called absorption of light.