Question

A 20-cm-high object is placed 12 cm from a convex
lens, perpendicular to its principal axis. The lens
forms a real image, whose size is 1.5 cm. Find the
power of the lens.​

Answer 1

Explanation:

Solution:

you should know that ,

$p = \frac{1}{f} where \: f \: is \: in \: meters$

And you should also know that,

M=image height/object height

$= \frac{v}{u}$

where, v =image distance and u=object distance

In the question, we are given

image height v and object height v

so first need to find v,

$\frac{1.5cm}{2cm} = \frac{v}{2} cm$

$\frac{2cm \times 1.5cm}{2cm} = v$

$v = 1.5cm$

using mirror formula,

$\frac{1}{f} = \frac{ 1}{v } + \frac{1}{u}$

$\frac{1}{f } = \frac{1}{1.5 } + \frac{1}{2}$

$\frac{1}{f} = \frac{2}{3} + \frac{1}{2}$

$\frac{1}{f} = \frac{4 + 3}{6}$

$\frac{1}{f} = \frac{7}{6}$

$f = \frac{6}{7} cm$

convert to meters,

I. e. 6/7÷100 =6/700 =3/350 meters

so find to the power

P=350/3 D

(D IS DIOPTERS)

=167.667 D ANSWER

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