Question

A camera fitted with a lens of focal length 50 mm is being used to photograph a flower that is 5 cm in diameter. The flower is placed 20 cm in front of the camera lens. (a) At what distance from the film should the lens be adjusted to obtain a sharp image of the flower ? (b) What would be the diameter of the image of the flower on the film ? . (c) What is the nature of camera lens ?

Answer 1

We have,

Focal length, f = 50 mm = 5 cm

Size of object, d = 5 cm

Object distance, u = -20 cm

Image distance = v

Size of image = d'

Now,

Since, real image is formed the lens is a convex lens.

Using,

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

$=>\frac{1}{5}=\frac{1}{v}-\frac{1}{20}$

$=>v=\frac{20}{3}=6.67cm$

Now,

Magnification, $m=\frac{v}{u}=\frac{\frac{20}{3}}{-20}=-0.33$

The negative magnification implies real image.

So, size of the image is, d' = md = -0.33 $\times$ 5 = -1.67 cm

The negative sign implies the inverted image.

The diameter of the image is 1.67 cm.