Question

S. No.	Object distance (cm)	Image distance (cm)
1	60	15
2	48	16
3	36	21
4	24	24
5	18	36
6	16	48

A student was asked by his teacher to find the image distance for various object distances in case of a given convex lens. He performed the experiment with all precautions and noted down his observations in the following table :
After checking the observation table the teacher pointed out that there is a mistake in recording the image distance in one of the observations. Find the serial number of the observation having faulty image distance.

• A. 2
• B. 3
• C. 5
• D. 6

Answer 1

The correct option is B 3

Using lens formula : $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

For observation 2 : $u=-48$ cm $v=16$ cm

$\therefore$ $\frac{1}{16}-\frac{1}{(-48)}=\frac{1}{f}$ $⟹f=12$ cm

For observation 3 : $u=-36$ cm $v=21$ cm

$\therefore$ $\frac{1}{21}-\frac{1}{(-36)}=\frac{1}{f}$ $⟹f=13.3$ cm

For observation 5 : $u=-18$ cm $v=36$ cm

$\therefore$ $\frac{1}{36}-\frac{1}{(-18)}=\frac{1}{f}$ $⟹f=12$ cm

For observation 6 : $u=-16$ cm $v=48$ cm

$\therefore$ $\frac{1}{48}-\frac{1}{(-16)}=\frac{1}{f}$ $⟹f=12$ cm

Thus focal length of the lens calculated for observation 3 comes out to be different among all other observations, hence observation 3 has faulty image distance.