## Focal length of plano convex lens

# 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?
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,
= 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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 Quote by Amith2006 # 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? 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?
Yes. For the benefit of others, you are using the lensmaker's forumula for thin lenses, not to be confused with the thin lens equation.

AM