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JAMB Physics 2011 Paper

Question : 43

Total: 44

Solution:

Concept:

For a convex lens, an upright and magnified image means the image is virtual, so the image distance

v

is taken as negative.

Explanation:

Let object distance be

u

and image distance be

v

Magnification is

m = \frac{v}{u} = -3

because the image is virtual and erect.

v = -3u

Focal length

f = 15 \, \text{cm}

Using the lens formula:

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

Substitute

v = -3u

\frac{1}{15} = \frac{1}{u} + \frac{1}{-3u}

Simplify:

\frac{1}{15} = \frac{1}{u} - \frac{1}{3u} = \frac{2}{3u}

Multiply through:

3u = 2 \times 15 = 30

Thus

u = \frac{30}{3} = 10 \, \text{cm}

Answer:

A. 10 cm

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