Question

OR
A person suffering from myopia (near-sightedness) was advise
wear corrective lens of power - 2.5 D. A 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 forms an image at
a distance of 10 cm from the lens ?
Draw a ray diagram to show the position and nature of the image
formed in the above case.​

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

Hence the image distance is u = - 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 = 100 / f

f = 100 / P

f = 100 / -2.5 = - 40 cm

We need to calculate the object distance

Using lens formula

1/f = 1/v - 1/u

1/ -40 = 1/ -10 - 1/-40

1/u = 1/-10 - 1/-40

u = - 13.33 cm

Hence the image distance is u = - 13.33 cm