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JAMB Physics 2012 Paper

Question : 25

Total: 42

Solution:

Concept:

A pinhole camera forms images by similar triangles. The magnification is the same for distances and heights.

Explanation:

Using the magnification formula for a pinhole camera:

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

Let

h_o

be the height of the building.

Convert all lengths to metres:

v = 5\text{ cm} = 0.05\text{ m}

h_i = 2.5\text{ cm} = 0.025\text{ m}

u = 300\text{ m}

Substitute into the formula:

\frac{0.05}{300} = \frac{0.025}{h_o}

Cross-multiply:

0.05 \times h_o = 0.025 \times 300

h_o = \frac{0.025 \times 300}{0.05} = 150\text{ m}

Answer:

150\text{ m}

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