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.

1. At what distance from the film should the lens be adjusted to obtain a sharp image of the flower?
2. What would be the diameter of the image of the flower on the film?
3. What is the nature of camera lens?

Answer 1

a. Given: Focal length (f) = 50 mm = 5 cm

Object distance (u) = −20 cm

Image distance (v) = ?

By using the lens formula:

`1/v - 1/u = 1/f`

⇒ `1/v - 1/-20 = 1/5`

⇒ `1/v + 1/20 = 1/5`

⇒ `1/v = 1/5 - 1/20`

⇒ `1/v = (20 - 5)/100`

⇒ `1/v = 15/100`

⇒ v = `20/3`

⇒ v = 6.67 cm 

∴ The film should be adjusted at a distance of 6.67 cm behind the lens. 

b. Object diametere = 5 cm

Magnification (m) = `v/u`

⇒ m = `6.67/-20`

⇒ m = −0.3335

The negative sign indicates the image is inverted.

Image diameter = |m| × Object diameter

= 0.3335 × 5

= 1.667 cm

c. Since the focal length is positive, the lens is a convex lens.