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?

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