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.

1. What is the magnification obtained by using the glass?
2. 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?

Answer 1

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.