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An object is placed 36cm from a converging lens of focal length 24cm.If a real...

Physics
WAEC 1988

An object is placed 36cm from a converging lens of focal length 24cm.If a real image which is 4cm high is formed, Calculate the height of the object.

  • A. 2.0cm
  • B. 4.0cm
  • C. 6.0cm
  • D. 8.0cm
  • E. 10.0cm
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Correct Answer: Option A
Explanation

F = 24cm , U = -36cm(behind lense)

Applying Lense formula = \(\frac {1}{f} = \frac{1}{v} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{24} - \frac{1}{36} = \frac{1}{72}\)

v = 72cm

but Magnification = \(\frac{V}{U} = \frac{h^i}{h_o}\) 

\(\frac{72}{-36} = \frac{-4}{h_o}\)                         \( h_1\) = -4( real image is negative)

by cross multiplying   \(h_1\) = 2cm


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