Question

A biconvex lens is formed by using a piece of plastic (n = 1.70). the radius of the front surface is 20 cm and the radius of the back surface is 30 cm. what is the focal length of the lens?

1. The focal length of the lens is mathematically given as
f = 17.15cm

### What is the focal length of the lens?

Generally, the equation for focal length is mathematically given as
1/f = (n-1)(1/R1 -1/R2)
f: focal length of the lens
n: refractive index of the lens
R1 radius of the front surface
R2 radius of the back surface
R1(front surface) = 20cm
R2(back surface) = 30cm
n = 1.70
therefore,
1/f = (1.70-1)(1/20 -1/-30)
1/f = 0.70 * 0.083
1/f = 0.0581
f = 17.15cm
In conclusion, the focal length of the lens
f = 17.15cm