By what factor will the size of an object placed 10cm from a convex lens be increased if the image is seen on a screen placed 25cm from the lens?

by | Nov 14 | Physics JAMB | 0 comments

A. 15.0
B. 2.5
C. 1.5
D. 0.4

Correct Answer:

Option B – 2.5

Explanation

Form the lens formula = 1/U + 1/V = 1/f
=> 1/10 + 1/25 = 1/f;
∴ (5 + 2) / 50 = 1/f
∴ f = 50/7cm
since the image is observed on a screen, it is a real image. Thus magnification
M = f/U-f

= __50/7
10 – 50/7

 

∴ M = 50/7
20/7

= 50/7 x 7/20 = 2.5

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