The Peters brother has a magnifying glass has a focal length

The Peter\'s brother has a magnifying glass has a focal length of 11.7 cm.

(a)To obtain maximum magnification, where must the object be placed when it is used by the elder brother with normal eyesight?

______cm from the lens
What will be the magnification for him?
______
(b) When used by the younger brother with a near point at 64 cm, where must the object be placed for him to enjoy the maximum magnification?
______cm from the lens
What will be the magnification for him?
______

Solution

m = di/do

di distance to the image

do distance to the object

the magnification can be changed only by changing the dista d_o.So the magnification increases as an object is brought closer to the eye.The maximum magnfication occurs when the object is brought to the near point.The near-point is about 25 cm,

with relaxed eyes(normal eyesight)

m = 25/f

In order to be in sharp focus, the object must be placed at a distance of

d0 = f

object should be 11.7 cm from the lense

maginification m=25/11.7= 2.136

b)

m = 64/f + 1 = 64/11.7 + 1

m= 6.47

d0 = 64*f/(64+f)

=> d0 = 64*11.7)/(64+11.7)

d0= 9.891 cm