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Question:

An object of height 6 cm is placed perpendicular to the principal axis of a concave lens of focal length 5 cm. Use lens formula to determine the position, size and nature of the image if the distance of the object from the lens is 10 cm.

Solution:

Given :u=−10cm, f=−5cm, ho=6cm
Using lens formula :
1/v − 1/u = 1/f
∴1/v − 1/(−10) = 1/−5
⇒v = −10/3 = −3.33cm (- sign shows that image is virtual)
Also, m = hi/ho = v/u
∴hi/6 = −10/3/−10
⇒hi = +2cm (+ sign shows that image is erect)