Question

A double concave lens of glass of RI 1.6 has radii of curvature of 40cm, 60cm. write the sign
convention used. Calculate its focal length. Also find focal length of lens if it is immersed in
a liquid of RI 1.3.

Answer 1

The focal length of the concave lens is 105.26 cm

Explanation:

Use the lens makers formula:

P(lens) = [ n(lens) - no ] / no ( 1/R1 - 1/R2 )

• R1 = 40 cm
• R2 = -60 cm
• n (lens) = 1.6
• no = 1.3

Now calculate P(lens)

P(lens) = [ n(lens) - no ] / no ( 1/R1 - 1/R2 )

P(lens) = [ 1.6 - 1.3 ] / 1.3 ( 1/40 + 1/60)

P(lens) = [ 0.3 / 1.3 ] ( 3 + 2 / 120)

P(lens) = (0.2307 )(0.041) = 0.0095

Focal length = 1 / P (lens)

Focal length = 1/ 0.0095

Focal length =  105.26 cm

Thus the focal length of the concave lens is 105.26 cm