1.

If radius of curvature of curved surface of plano-convex lens of refractive index 1.5 is 60 cm, then its focal length is ...... cm.

Answer»

`-60`
`120`
`60`
`-120`

SOLUTION :`1/f=(n-1)[(1)/(R_1)-(1)/(R_2)]`
`=(1.5-1)[(1)/(infty)-(1)/(-60)] R_1=infty,R_2=-60` cm
`=0.5xx(1)/(60)`
`therefore f=(60)/(0.5)`
= 120 cm and fis positive for convex lens.


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