Question

An image formed on a screen is three times the size of the object. The object and screen are 80 cm apart when the image is sharply focussed.
(a) State which type of lens is used.
(b) Calculate focal length of the lens.

Answer 1

Focal Length is 15 cm

Explanation:

We know that,  

• When v is the distance between lens & image,  

'u' is a distance between object & lens.

• Magnification, $m = \frac {v}{u}$,

Here, m = 3 or 3 = $\frac {v}{u}$

so, v = 3u

Distance from Object to screen, u + v = 80 cm

And u = 20 cm & v = 60 cm,

solving for u and v, we get u = 20 cm, v = 60 cm.

As per Lens Formula,

$\frac {1}{v} + \frac {1}{v} = \frac {1}{f}$

or $\frac {1}{60} + \frac {1}{20} = \frac {1}{f}$

 we get focal length =15 cm