Physics WAEC

The distance between an object and its real image in a convex lens is 40cm. If the magnification of the image is 3, calculate the focal length of the lens.

A. 6.5 cm
B. 7.5 cm
C. 8.5 cm
D. 4.5 cm

Correct Answer: Option B

Explanation

u + v = 40

v/u = 3

v = 3u

u + 3u = 40

4u = 40

u = 10cm

v = 3u = 30cm

f = uv/(u+v)=10(30)/(10+30)=300/40

= 7.5 cm

SEE ALSO A constantan wire has a cross-sectional area of 4 x 10-8m2 and a resistivity of 1.1 x 10-6Ωm. If a resistor of resistance 11Ω is to be made from this wire, calculate the length of the wire required
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