# Diameter of a Plano-convex lens is 6 cm and the thickness at the center is 3 mm. If the speed of light in the material of the lens is 2 x 10^8 m/sec, what is the focal length of the lens?(adsbygoogle = window.adsbygoogle || []).push({});

I solved it in the following way:

Let R be the radius of curvature of the convex surface.

Given that AB = 3 mm = 0.3 cm, AD = CD/2 = 3 cm, refractive index(n) = 1.5

By the property of circles,

AB = (AD^2)/(2R)

Therefore, R = (AD^2)/(2AB)

= 9/(2 x 0.3)

= 15 cm

According to thin lens formula,

1/f = (n – 1)/R

Solving I get,

f = 30 cm

Is it right?

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# Homework Help: Focal length of plano convex lens

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