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Question
A lens of focal length 5 cm is being used by Debashree in the laboratory as a magnifying glass. Her least distance of distinct vision is 25 cm.
- What is the magnification obtained by using the glass?
- She keeps a book at a distance 10 cm from her eyes and tries to read. She is unable to read. What is the reason for this?
Solution
Given, image distance = v = -25 cm, focal length = f = 5 cm, magnification = m?
From lens formula, `1/"f" = 1/"v" - 1/"u"`
`therefore 1/"u" = 1/"v" - 1/"f"`
`therefore 1/"u" = 1/(-25) - 1/5`
= `(- 1 - 5)/25`
`= (-6)/25`
Object distance = u = `(-25)/6`cm
We know that, m = `"v"/"u"`
= `(-25 xx 6)/-25`
= 6.
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