A thin lens of refractive index 1.5 has a focal length of 15cm in air. When the lens is placed in a medium of refractive index `4//3`, its focal length will becom `"______________"`cm.

Options

A. If both assertion and reason are ture and reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reson are false.

Answer

A

Explanation

`1/f = (""^(a) mu_(g) - 1) [(1)/(R_(1)) - (1)/(R_(2))] = ((mu_(g))/(mu_(w)) - 1) ((1)/(R_(1)) - (1)/(R_(2)))`
`(1)/(f) = ((mu_(g) - mu_(w))/(mu_(w))) * [(1)/(R_(1)) - (1)/(R_(2))] = ((1.5 - 1.33)/(1.33)) * [(1)/(20) + (1)/(20)]`
` (1)/(f) = (0.13 xx 2)/(20) implies f = (20)/(0.26)`
`therefore f = 76.92` cm .