Power of a convex lens of refractive index `3/2` is 2.5 D in air. If it is inerted in liquid of refractive index 2, then new power will be ......

Options

A. -1.25
B. -1.5
C. 1.25
D. 1.5

Answer

A, B

Explanation

Here,`1/f=(mu-1)((1)/(R_1)-(1)/(R_2))`
`therefore 2.5=(3/2-1)((1)/(R_1)-(1)/(R_2))` … (1) for air
Also,`(1)/(f.)=(""_lmu_g-1)((1)/(R_1)-(1)/(R_2))` [`""_lmu_g=(3/2)/(2)=3/4`
`therefore (1)/(f.)=(3/4-1)((1)/(R_1)-(1)/(R_2))` …(2)
By taking ratio of equation (1) and (2),
`2.5f.=(0.5)/(-0.25)`
`therefore f.=(-5)/(25xx0.25)` `therefore(1)/(f.)=-1.25 D`