Question

The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall $4m$ away by mean of a large convex lens. The maximum possible focal length of the lens required for this purpose will be:

• A. $0.5m$
• B. $1.0m$
• C. $1.5m$
• D. $2.0m$

Answer 1

The correct option is B $1.0m$

Given that,

Object distance =$u$

Image distance = $v$

Focal length = $f$

Now, the distance between the object ad the image

$d=u+v$

$d=4m$

Now, the maximum focal length of the convex length

$f_{max}=\frac{4}{4}$

$f_{max}=1m$

Hence, the maximum possible focal length is $1m$