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Question
An object of height 3cm is placed at 10cm from a concave lens of focal length 15cm. Find the size of the image.
Solution
u = -10 cm [left side of lens]
f = -15 cm [left side of lens]
h = 3 cm
m = `"v"/"u"`= `_-6/-10` = 0.6
m = `"h"/"h"` ⇒ h = m x h
h = 0.6 x 3
h = 1.8 cm
`1/"f"`= `1/"v"`-`1/"u"`
`1/"v"`= `1/"f"`+ `1/"u"`
= `1/-15`+`1/-10`
`1/"v"`=- `1/15`-`1/10`= `"(-10-15)"/15` = `(-25)/150`
`1/"v"`=-`1/6`
v = - 6 cm
Nature of image: When an object is placed at 10 cm from the left side of the lens, a virtual image is formed between the optical centre and focus of a concave lens. The size of the image is smaller than that of the object.
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