Question

(b) A student recorded the following data for the values of the object distance and corresponding image
distance while performing an experiment on real image formation by a convex lens of power 4D Two
of these observations are incorrect.
Without making any calculations identify these observations and give reasons for your choice :
C
Observation
Object Distance (in cm)
Image Distance (in cm)
30
50
60
60
50
70
30​

Answer 1

Given that,

Power = 4D

For A,

Object distance = 30 cm

Image distance = 20 cm

For B,

Object distance = 50 cm

Image distance = 60 cm

For C,

Object distance = 60 cm

Image distance = 50 cm

For D,

Object distance = 70 cm

Image distance = 30 cm

Focal length :

Focal length is the reciprocal of power of lens.

We need to calculate the focal length

$f=\dfrac{100}{P}$

Put the value into the formula

$f=\dfrac{100}{4}$

$f=25\ cm$

The focal length is 25 cm.

(I) In reading A,

The object is kept at 30 cm from the lens, it is between F and 2F. The image should be formed beyond 2F. However, it is shown to be at 20 cm, which is incorrect.

(II). In reading B,  

The object is kept at 2F. The image should be formed at 2F. However, it is also shown to be formed at 60 cm, that is beyond 2F which is incorrect.

Hence, A, and B are incorrect option.