Question

An object is held at 40cm from a concave lens of focal length 60cm .The distance of the image from the

Answer 1

Correct Question :

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An object is held at 40cm from a concave lens of focal length 60cm. The distance of image from the lens is

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AnswEr :

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We're given with the object distance and the focal length of the lens. And we've to find the image distance.

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• Object distance (u) = - 40 cm

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• Focal length (f) = - 60 cm

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Remember that, always focal length, radius of curvature and the object distance is negative in the case of concave lens as concave lens diverges the rays incident on it. So as the result image will be formed behind [backward direction] the lens.

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By using lens formula,

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$:\implies \sf \: \: \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$

$:\implies \sf \: \: \dfrac{1}{ - 60} = \dfrac{1}{v} - \dfrac{1}{ - 40}$

$:\implies \sf \: \: \dfrac{1}{ - 60} = \dfrac{1}{v} + \dfrac{1}{ 40}$

$:\implies \sf \: \: \dfrac{1}{ v} = \dfrac{1}{ - 60} - \dfrac{1}{ 40}$

$:\implies \sf \: \: \dfrac{1}{ v} = \dfrac{40 - ( - 60)}{ - 2400}$

$:\implies \sf \: \: \dfrac{1}{ v} = \dfrac{100}{ - 2400}$

$:\implies \sf \: \: { \underline{ \boxed{ \frak{ \purple{ \: v = - 24 \: }}}}} \: \bigstar$

Therefore, the image distance is 24cm.

Answer 2

Question

An object is held at 40 cm from a concave lens of focal length 60cm Find out the distance of the image ?

Answer

What is given here ?

• it is given here that the object distance is 40 cm
• the focal length is 60 CM

What to find here ?

• we how to find here the distance of the image

How to find it ?

• we can simply find it by putting the formula of lens .

Fomula to be used ?

• 1/f=1/v-1/u

now putting the value we get ,

=> 1/-60=1/v-1/40

=> 1/-60 = 1/v +1/40

=> 1/v= 1/-60-1/40

=>1/v= 40-(-60)/-2400

=> 1/v= 100/2400

=> v = 24

So, the distance of the image is 24 cm