Question

Determine the focal length of the lens from the following readings object distance =20.1 +-0.2cm image distance =50.1+-0.5cm.
Class 11 physics (units and measurement)

Answer 1

Focal length = 14.5 cm ; 28.6 cm

Explanation:

Given: Object distance u = 20.1 +- 0.2cm

Image distance v = 50.1+- 0.5cm.

Find: Focal length of the lens.

Solution:  

We can find the focal length of the lens using the lens formula.

1/f = 1/u + 1/v

(1) Let's take u = 20.1 + 0.2cm = 20.3 cm and v = 50.1 + 0.5 = 50.6 cm

  1/f = 1/20.3 + 1/50.6

   1/f = 70.9 / (20.3 * 50.6)

   1/f = 70.9 / 1027.18

   So f = 1027.18 / 70.9 = 14.5 cm

(2) Let's take u = 20.1 - 0.2cm = 19.9 cm and v = 50.1 - 0.5 = 49.6 cm

  1/f = 1/19.9 + 1/49.6

   1/f = 19.9 + 14.6 / (19.9 * 49.6)

   1/f = 34.5 / 987.04

   So f = 987.04 / 34.5 = 28.6 cm