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Question

The focal point of an equi- convex lens whose refractive index is 1.5 is 10 cm in air. Its focal point inside a liquid of refractive index 1.25 is

A
20 cm
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B
25 cm
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C
30 cm
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D
50 cm
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Solution

The correct option is A 20 cm

Given that,

Focal length f=10cm

Refractive index μ1=1.5

Refractive index μ2=1.25

We know that,

1f=(n1)(1R11R2)

110=(1.51)(1R11R2)....(I)

1f=(1.251)(1R11R2)....(II)

Now, divided equation (I) by equation (II)

f10=0.50.25

f=20cm

Hence, the focal length is 20 cm


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