Question

In an experiment for estimating the value of focal length of converging mirror, image of an object placed at $40\, cm$ from the pole of the mirror is formed at distance $120 \,cm$ from the pole of the mirror These distances are measured with a modified scale in which there are $20$ small divisions in $1 \,cm$ The value of error in measurement of focal length of the mirror is $\frac{1}{ K } cm$ The value of $K$ is ______

Answer 1

The correct answer is 32.
v1​+u1​=f1​
120−1​−401​=f1​,f=−30cm
Now,
v2−1​dv−u21​du=−f21​df
Also dv=du=201​cm
∴(120)2201​​+(40)2201​​=(30)2df​
On solving
df=321​cm
∴k=32