First slide

The lens

Question

An object placed 10 cm in front of a lens has an image 20 cm behind the lens. What is the power of the lens (in dioptres)?

Moderate

A

1.5
B

3.0
C

– 15.0
D

+ 15.0

Solution

$u = - 10 cm, v = 20 cm \frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{20} - (- \frac{1}{10}) = \frac{3}{20} \Rightarrow f = \frac{20}{3} cm Now P = \frac{100}{f} = \frac{100}{20 / 3} = + 15 D$

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