Question

Consider an equiconvex lens of radius of curvature R and focal length f. If `fgtR` , the refractive index `mu` of the material of the lens
A. is greater than zero but less than 1.5
B. is greater than 1.5 but less then 2.0
C. is greater than 1.0 but less than 1.5
D. None of these

Answer 1

Correct Answer - c.
`(1)/(f)=(mu-1)((2)/(R))`
or `f=(R)/(2(mu-1))`
Now, `f gt R`
`:. (R)/(2(mu-1)) gt R`
or ` (1)/(2(mu-1)) gt 1` or `2(mu-1) lt 1`
or `mu-1 lt (1)/(2)` or `mu lt (1+(1)/(2))` or `mu lt 1.5`