A point object is placed at a distance of `20 cm` from a thin plano-convex lens of focal length `15 cm(mu=1.5).` The curved surface is silvered. The image will form at

Options

A. `60 cm` left of AB
B. `30 cm` left of AB
C. `20/7 cm` left of AB
D. `60 cm` right of AB

Answer

C

Explanation

`1/1.5=(1.5-1)(1/R-1/oo)`
`:. R = 7.5 cm `
With polished surface,
`1/F =(2(mu_2//mu_1))/(R_2)-(2(mu_2//mu_1)-1)/(R_1)`
`=(2(1.5))/(-7.5)-(2(1.5-1))/oo`
`F=-2.5 cm`
Now, using mirror formula,
`1/v+1/(+20)=1/(-2.5)`
Solving, we get `v=-20/7 cm`