Question

The focal length of a camera lens is 20 cm. How far
the film must be taken in order to photograph an ob-
ject located at a distance 100 cm from the lens.​

Answer 1

Answer:

Is this a class assignment? People no longer measure focal lengths in centimetres in the Real World.

The formula you are looking for is described at . f is 2, S₂ is 3, so you can work out S₁ with no difficulty. And of course the question has nothing to do with cameras or film.

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Answer 2

Given:

Focal length of a camera lens is 20 cm. Object is placed at a distance of 100 cm from the lens.

To find:

Distance the field must be placed in order to obtain an image of the object.

Calculation:

Standard cameras used biconvex lens in order to form real and inverted image of the object on the film.

Applying Lens Formula:

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

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

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

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

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

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

$= > v = 25 \: cm$

So , image will be formed at 25 cm from the lens. Hence film must be placed at that position.

Image characteristic:

• Real Image

• Inverted

• Diminished