1.

The diameter of a plano convex lens is `6 cm` and thickness at the centre is `3 mm`. If the speed of light in the material of the lens is `2 xx 10^8 m//s`, what is the focal length of the lens ?A. 15 cmB. 20 cmC. 30 cmD. 10 cm

Answer» Correct Answer - C
Here, `d = 6 cm, h = 3 mm= 0.3 cm`,
`v = 2 xx 10^8 m//s, f = ?`
If `R` is radius of curvature of the convex surface of lens, then from
`((d)/(2))^2 = (2 R - h) h = 2 R h` `(because h lt lt R)`
`R = (d^2//4)/(2 h) = (6^2//4)/(2 xx 0.3) = 15 cm`
`mu = ( c)/(v) = (3 xx 10^8)/(2 xx 10^8) = (3)/(2)`
From `(1)/(f) = (mu -1) ((1)/(R_1) - (1)/(R_2))`
=`((3)/(2) - 1) ((1)/(oo) + (1)/(15)) = (1)/(30)`
`f = 30 cm`.


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