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Question

Two prisms A and B have dispersive powers of 0.012 and 0.018 respectively. The two prisms are in contact with each other. The prism A produces a mean deviation of 1.2, then the mean deviation produced by B if the combination produces deviation without dispersion is

A
3.6
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B
0.8
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C
0.4
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D
1.8
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Solution

The correct option is B 0.8
We have the dispresive powers of the prisms as

wA=0.012
wB=0.018

Also, the mean deviation produced by prism A is

dA=1.2

We have to determine mean deviation produed by prism B, i.e. dB

We know that

Dispersive power=Angular DispersionMean deviation

wA=θAdA

wB=θBdB

For deviation without dispersion difference in angular dispersion for the two prisms will be zero.

θAθB=0

θA=θB

wAdA=wBdB

0.012×1.2=0.018×dB

dB=1.2×1218=0.8

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