Question

A student focused the image of a candle flame on a white screen using a convex lens. He noted down the position of the candle, screen and lens from a reference point as:
Position of candle = 12 cm
Position of lens = 50 cm
Position of screen = 88 cm.
The focal length of the lens is:

• A. 10.56 cm
• B. 15 cm
• C. 17 cm
• D. 19 cm

Answer 1

The correct option is D 19 cm

Given:
Distance between lens and candle, object distance $\left(u\right)=50-12=38cm$
Distance between screen and lens, image distance $\left(v\right)=88-50=38cm$
Let $f$ be the focal length.

Using lens formula
$\frac{1}{f}$ = $\frac{1}{v}$ - $\frac{1}{u}$
$\frac{1}{f}$ = $\frac{1}{38}$ - $\frac{1}{-38}$
$\frac{1}{f}$ = $\frac{2}{38}$
or focal length , $f=\frac{38}{2}=19cm$