JAMB Physics 2011 Objective — Question 26

Question 26 of 49 from the Joint Admissions and Matriculation Board (JAMB) Physics 2011 Objective paper, with the correct answer and a full explanation.

A certain far-sighted person cannot see objects that are closer to the eye than 50 cm clearly. Determine the power of the converging lens which will enable him to see at 25cm.

A. A. 0.03 D
B. B. 0.04 D
C. C. 0.06 D
D. D. 0.02 DCorrect

Explanation

For the person to see clearly at 25cm, the image must appear at 50cm (virtual, so v=-50cm). u=25cm. Using 1/f=1/u+1/v = 1/25-1/50 = 1/50, so f=50cm=0.5m. Power = 1/f = 1/0.5 = 2D. (Note: option D as given in the source, 0.02D, appears to be a decimal-placement transcription issue in the original scan; the correctly computed power is 2D.)

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