Question

A concave. lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object from the lens ?

Answer 1

Heya here ....

Answer
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In this question , we have to calculate the object distance u.
Here,

Object distance , u = ?
image distance , v = -10 cm
and Focal. length , f = -15 cm

Now , putting these values in the lens formula:

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f} \\ \\ we \: \: get : \\ \\ \frac{1}{ - 10} - \frac{1}{u} = \frac{1}{ - 15} \\ \\ = > - \frac{1}{u} = - \frac{1}{15} + \frac{1}{10} \\ \\ = > - \frac{1}{u} = \frac{ - 2 + 3}{30} \\ \\ = > - \frac{1}{u} = \frac{1}{30} \\ \\ object \: \: distance \: \: u = - 30 \: \: cm$
Thus , the object is placed at a distance of 30 cm from the concave lens. The minus sigh with object distance shows. that the object is on its left side.
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Hope it's helps you.
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