Question

Extended source is needed in
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Answer 1

Answer:

A source of angular size greater than the resolution of the instrument used to observe it; therefore, an extended source is said to be resolved. ... 2 In radio astronomy, a class of sources of large angular extent displaying extended structure such as jets and lobes, and tending to be brightest at long wavelengths.

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Answer 2

⇒The extended source is required in Young's double-slit experiment.

⇒When we are performing this experiment, a slit S is required to use a standard light source. But we don't need a slit when the light source is coherent.

⇒Coherent source of light=The source of light having wavelengths of light that have a constant phase difference.

⇒We perform Young's double-slit experiment to prove the phenomena of interference.

⇒Which finally helps in proving the wave nature of light.