explain the difference between point source and extended sources of light
Answers
A condition for steady interference is that the superposing waves or the sources must be coherent. Now we know from Huygen’s principle that all the points on a wave front are coherent secondary sources. So, we can cut some how two very small portions from a single wave front and use those portions as coherent sources. For this purpose a single wave front should reach the given arrangement to cut the wave front. If the source is a point source, the spherical wave fronts are obtained and single wave front will reach the cutting arrangement and two coherent sources will be obtained. If there is extended one then many wave fronts from different points of the source will reach the cutting device and phases for different wave fronts are different and we shall not get coherent sources.
In case of interference by thin films we have division of amplitude. Here if source is point like then rays from a small portion of the film will enter our eye. But, with extended source we shall have rays from almost entire film entering our eye and we will be able to observe interference due to entire film.
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Answer:
Point source of light
It is that source of light which forms only the umbra region on the screen of the showed object.
Example: A pinhole through which sunlight is entering is a point source of light.
Extended source of light
It is that source of light that forms umbra as well as the penumbra region on the screen of the shadowed object.
Example: Sun is an extended source of light.
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