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JAMB Physics 2018 Paper

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Question : 79

Total: 87

If the focal length of a camera is 20cm, the distance from the film at which the lens must be set to produce a sharp image of 100cm away is

17cm
20cm
25cm
100cm

Solution:

Concept:

Use the lens formula relating object distance, image distance, and focal length.

Explanation:

For a sharp image,

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

, where

u

is the object distance and

v

is the image distance.

Given

f = 20\,\mathrm{cm}

and

u = 100\,\mathrm{cm}

:

\frac{1}{20} = \frac{1}{100} + \frac{1}{v}

\frac{1}{v} = \frac{1}{20} - \frac{1}{100} = \frac{4}{100} = \frac{1}{25}

So

v = 25\,\mathrm{cm}

.

Answer:

The lens must be set

25\,\mathrm{cm}

from the film.

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