Ncert Exercises & Solutions

9.20: A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm. Determine the focal length of the lens.

Hint: Use the lens formula.

Step 1: Find the location of the image and the object.
The image of the object can be located on the screen for two positions of convex lens such that u and v are exchanged. The separation between two positions of the lens is, d= 20 cm
From the figure,
$u_{1} + v_{1} = 90 c m$
$v_{1} - u_{1} = 20 c m$
Solving,
$v_{1} = 55 c m, u_{1} = 35 c m$
Step 2: Find the focal length of the lens.
From the lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\\\frac{1}{55}-\frac{1}{-35}=\frac{1}{f}\\ \Rightarrow \frac{1}{f}=\frac{55\times35}{90}\\ \Rightarrow f=21.4 cm$

NEET Information

courses

Company

Botany Questions

Chemistry Questions

Physics Questions

Zoology Questions