Question

The graph between object distance u and image distance v for a lens is given. The focal length of the lens is

• A. 5$\pm$ 0.1 cm
• B. 5$\pm$ 0.05 cm
• C. 0.5$\pm$ 0.1 cm
• D. 0.5 $\pm$0.05 cm

Answer 1

Answer: (B)

5$\pm$ 0.05 cm

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

($v=10cm,u=-10cm$) lies on curve

$\frac{1}{f}=\frac{1}{10}-\frac{1}{-10}$

$f=5cm$

Now, calculating error
$|\frac{df}{f^2}|$ $=$$|\frac{dv}{v^2}|$ + $|\frac{du}{u^2}|$

$dv=du$ $=leastcount=0.1cm$

therefore, $|df|=$25*[$\frac{0.1}{100}$+$\frac{0.1}{100}$ ]

$df=$ $\pm 0.05cm$

Hence the answer : $f=$ $5\pm 0.05cm$