Question

A convex lens of focal length 18cm forms a real image of height 3 times of
objects height. Find object distance.
​

Answer 1

Answer:

According to the question;

Object distance = u;

Image distance (v) = 30cm;

Focal length = 20cm

By lens formula;

⇒ u = -60cm.

Therefore object is placed at 60 cm in front of lens.

Now;

Height of object h1= 6cm;

Magnification =

=

Putting values of v and u

Magnification =

⇒

⇒

Height of image is 3 cm.

Negative sign means image is real and inverted.

Answer 2

The object distance is 12 cm.

Step-by-step explanation:

Given that,

Focal length = 18 cm

Magnification = 2

We need to calculate the image distance

Using formula of magnification

$m=\dfrac{v}{u}$

Put the value into the formula

$3=\dfrac{v}{u}$

$v=3u$

We need to calculate the object distance

Using formula of lens

$\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}$

Where, v = image distance

u = object distance

f = focal length

Put the value into the formula

$\dfrac{1}{18}=\dfrac{1}{3u}-\dfrac{1}{u}$

$\dfrac{1}{18}=\dfrac{-2}{3u}$

$u=\dfrac{-2\times18}{3}$

$u=-12\ cm$

Hence, The object distance is 12 cm.

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Topic : Optics

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