Question

a person suffering from myopia was advised to wear corrective lens of power -2.5 D spherical lens of same focal length was taken in the laboratory at what distance should a student place an object from this lens so that it from an image at a distance of 10 cm from the lens ?​

Answer 1

Answer: The answer is -13.33

Explanation: The previous answer is wrong since in concave the focal length, image distance and object distance is always negative.

Answer 2

The object distance is -13.33 cm.

Explanation:

Given that,

Power = -2.5 D

Image distance = 10 cm

We need to calculate the focal length

Using formula of power

$P=\dfrac{100}{f}$

$f=\dfrac{100}{P}$

Put the value into the formula

$F=\dfrac{100}{-2.5}$

$F=-40\ cm$

We need to calculate the object distance

Using lens formula

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

Put the value into the formula

$\dfrac{1}{-40}=\dfrac{1}{-10}-\dfrac{1}{u}$

$\dfrac{1}{u}=\dfrac{1}{-10}-\dfrac{1}{-40}$

$\dfrac{1}{u}=\dfrac{-3}{40}$

$u=-13.33\ cm$

Hence, The object distance is -13.33 cm.

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Topic : optics

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