Question

A magnifying glass is to be used at the fixed object distance of 1 inch. If it is ro produce an erect image 5 items magnified. its focal length should be

Answer 1

Convex lens :
u=-1 inch = -2.54cm
v=?
m=5

m=v/u
v=mxu
=-5x2.54= -12.7

By lens formula
1/f=1/v-1/u
=1-12.7+1/2.54
=-10/127 +100/254
=-20+100/254
= 80/254
f= 254/80=3.175cm
∴Focal of magnifying lens is 3.175cm.

Answer 2

Answer:

1.25"

Explanation:

m=v/-u

m=5

u=1

5=v/-1

v= -5

we know,

1/f=1/v+1/u

1/f=1/-5+1/-1

1/f=1.25