JAMB - Physics (2008 - No. 45)
If a concave lens of focal length 12cm is used to produce a real image four times the size of the object, how far from the lens must the object be placed?
10 cm
15 cm
20 cm
25 cm
Explanation
Magnifications = V/U = 4
∴ V = 4U
1/V + 1/U = 1/f
1/4u + 1/U = 1/12
5/4U = 1/12
4U = 12 x 5 = 60
∴ U = 15 cm
∴ V = 4U
1/V + 1/U = 1/f
1/4u + 1/U = 1/12
5/4U = 1/12
4U = 12 x 5 = 60
∴ U = 15 cm
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