Question

The focal length of plano-convex lens, the convex surface of which is silvered is `0.3m`, if `mu` of the lens is `7//4`, the radius of curvature of the convex surface is
A. `0.45m`
B. `1.05m`
C. `3m`
D. `0.9m`

Answer 1

(b) Here `P=2P_(L)+P_(M)=2/(f_(L))-1/(f_(m))`
`P=2/(f_(L))+2/R`
Here `1/(f_(L))=(mu-1)(1/(r_(1))-1/r_(2)))=(7/4-1)(1/(oo)+1/R)`
`1/(f_(L))=3/(4R)`
`:. P=-1/F=6/(4R)+2/R+(6+8)/(4R)=14/(4R)`
or `-10.3=14/(4R)`
`:. R=_(14xx0.3)/4=-4.2/4=-1.05`