Question

The focal length of a camera lens is 20 cm find how far away from the film must the lens be set in order to photograph an object located at a distance 100 cm from the lens

Answer 1

Given:

The focal length of a camera lens is 20 cm.

Object distance is 100 cm.

To find:

Distance at which a film must be placed.

Calculation:

We know that a standard camera uses a biconvex lens in order to produce a real and inverted image of the object on the film.

Applying Lens Formula:

$\therefore \: \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$

$= > \: \dfrac{1}{20} = \dfrac{1}{v} - \dfrac{1}{( - 100)}$

$= > \: \dfrac{1}{20} = \dfrac{1}{v} + \dfrac{1}{100}$

$= > \: \dfrac{1}{v} = \dfrac{1}{20} - \dfrac{1}{100}$

$= > \: \dfrac{1}{v} = \dfrac{5 - 1}{100}$

$= > \: \dfrac{1}{v} = \dfrac{4}{100}$

$= > \: \dfrac{1}{v} = \dfrac{1}{25}$

$= > v = 25 \: cm$

So , image will be produced 25 cm from the lens. Hence a film has to be placed at that position.