Question

A thin convex lens object is placed in front of the lens. The distance of the object from the lens is 10$\pm$0.1 cm and the distance of the real image formed from the lens is 20$\pm$0.2cm. The error in the determination of the focal length of the lens is n%. The value of n is

• A. 2
• B. 3
• C. 1
• D. 4

Answer 1

Answer: (C)

1

Answer 2

The correct option is (C)
$\frac{1}{V}-\frac{1}{U}=\frac{1}{f}$
$\frac{1}{20}-\frac{1}{-10}=\frac{1}{f}$
$f=\frac{20}{3}$
$\frac{df}{f^2}=\pm[\frac{dv}{V^2}+\frac{du}{U^2}]\Rightarrow\frac{df}{f}=\pm f[\frac{0.2}{20^2}+\frac{0.1}{10^2}]$
$\frac{df}{f}\times100=\pm \frac{20}{3}[\frac{0.6}{400}]\times100$% = 1%