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JAMB Physics 2022 Paper

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Question : 4

Total: 40

An object 40 cm high is 30cm from the pin hole camera. If the height of the image formed is 20 cm. What is the distance of the image from the pin height?

15 cm
70 cm
40 cm
40 cm

Solution:

Concept:

In a pinhole camera, the magnification is the same for height and distance.

Explanation:

Magnification,

M = \frac{\text{image height}}{\text{object height}} = \frac{\text{image distance}}{\text{object distance}}

.

So,

\frac{H_i}{H_o} = \frac{D_i}{D_o}

.

Given:

H_i = 20\,cm

,

H_o = 40\,cm

,

D_o = 30\,cm

.

Substitute:

\frac{20}{40} = \frac{D_i}{30}

.

Cross-multiply:

D_i = \frac{20 \times 30}{40}

.

D_i = \frac{600}{40} = 15\,cm

.

Answer:

The image distance is

15\,cm

.

Correct option: A. 15 cm.

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