1.

Two identical and coherent sources of light are used in Young's double-slit experiment and resulatant intensity at the centre of the screen is found to be I_1. When two identical sources of intensity same as before but incoherent are used for the experiment, then resultant intensity at the centre of the screen is found to be I_2. What is the value of I_1//I_2?

Answer»

1
2
4
0.5

Solution :LET I be the intensity of each source in all cases. When COHERENT sources are used, then there is constructive INTERFERENCE at the CENTRE of screen and resultant intensity is four times the intensity of sources.
`I_1 = 4I`
But when sources are incoherent, then phase difference at all points changes randomly and very fast so that intensity keeps changing randomly between 0 and 4I. Change in intensity is so frequent that resultant becomes 2I every where.
`I_2 = 2I`
Hence, `I_1//I_2 = 2`.


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