Question

You are provided with two lenses of focal lengths 20cm and 40cm respectively. Which lens will you use to obtain more convergent light?

Answer 1

focal length of 20 cm as it will converge more light rays

Answer 2

To determine the converging power of lens:

• The efficiency of converging or diverging a light ray is defined as the power of lens.
• It is the reciprocal of focal length of lens and is expressed in diopters.

• For selecting the lens with more focal length, we need to compute the power of lens from the following formula:

Power of lens (diopter) =  1 ÷ focal length (meter)

1. Focal length of first lens = 20 cm = 0.2 m

∴ Power of first lens = 1 ÷ 0.2 m

   Power of first lens = 5 diopter

2. Focal length of second lens = 40 cm = 0.4 m

∴ Power of second lens = 1 ÷ 0.4 m

    Power of first lens = 2.5 diopter

• The more the power of lens, more will be its efficiency to obtain convergent light.

• Hence, lens having focal length 20 cm is used to obtain more convergent light.