Lens is kept nearer to eye to observe a small thin net of wire at 8 cm distance by magnifying glass of 10 cm focal length, then magnification = …....

Options

A. 5
B. 8
C. 10
D. 20

Answer

A

Explanation

u=-8 cm , f= +10 cm
`1/v-1/u=1/f`
`therefore 1/v=1/f+1/u`
`=(1)/(10)+(1)/(-8)=(1)/(10)-1/8=(4-5)/(40)`
`1/v=(-1)/(40)`
`therefore v=-40` cm
Now `m=v/u=(-40)/(-8)=5`
Second method :
`m=(f)/(f+u)=(10)/(10-8)=(10)/(2)=5`