Question

A stamp collector uses a lens with 7.2-cm focal length as a simple magnifier. The virtual image is produced at the normal near point (25 cm).

(a) How far from the lens should the stamp be placed?

____cm

(b) What is the expected angular magnification?

Answer 1

a. Imagedistance v = -25 cm ( -ve as image is to be formed to the left of lens, sign convention)

Objectdistance = u

focallength f = 7.2cm

1/f = 1/v - 1/u

1/7.2 = 1/(-25) - 1/u

1/u = -1/25 - 1/7.2

u = (25) * ( 7.2) / ( -25 - 7.2)

= -5.59 cm (-ve sign means that the object must be to the left of lens)



b. Angularmagnification m =v/u or m = f/ (f - u) =

= 7.2/ {7.2 - 5.9}

= 4.472