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A plano- convex lens of refractive index...

A plano- convex lens of refractive index 1.5 and radius of curvature 30 cm is silvered at the curved surface. Now , this lens has been used to form the image of an object . At what distance from this lens, an object be placed in order to have a real image of the size of the object?

A

a.The refractive index of the lens is 2.5

B

b.The radius of the curvature of the convex surface is 45cm

C

c. The faint image is erect and real

D

d.The focal length of the length is 20 cm

Text Solution

Verified by Experts

The correct Answer is:
AD
For lens

`(1)/(v) - (1)/(u) = (1)/(f)`
`(1)/(60) - (1)/((-30)) = (1)/(f') implies f' = 20 cm `
Also `(1)/(f) = (n-1)((1)/(R) - (1)/(oo)) = ((n-1))/(R) …… (ii)`
For reflection from convex mirror (curved surface)

`(1)/(v) + (1)/(u) = (1)/(f') = (2)/(R)`
`(1)/(+10) + (1)/(-30) = (1)/(f') = (2)/(R) ....... (iii)`
`R = 30 cm`
from (i) , (ii) & (iii)
n = 2.5 ,
faint image erect & virtual
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