Question

In displacement method we use a lens of focal length f and distance between object and screen is 60 cm. Possible value for focal length is

• A. $-15cm$
• B. $30cm$
• C. $12cm$
• D. $20cm$

Answer 1

The correct option is C $12cm$

Given $D=60cm$

The formula for focal length is displacement method is given as

$f=\frac{D^2-x^2}{4D}$

$\Rightarrow f=\frac{D}{4}-\frac{x^2}{4D}$

For $x<<<D$ we may write

$f=\frac{D}{4}=\frac{60}{4}=15cm$

Hence the focal length should be less than $15cm$ and positive since convex lens form real images.

Therefore the correct answer is option (C) $12cm$.