If an object is placed at a distance of 5 cm from a convex lens, a real image of the object is formed at a distance of 20 cm from the lens . If the lens is used as a magnifying glass what maximum magnification can be obtained from it ? Least distance of distinct vision is 24 cm .

Explanation

Here u=-5 cm and v=20 cm .
If f be the focal length of the lens we have,
`1/v-1/u=1/f or, 1/(20)-1/(-5)=1/f or, f=4 cm `
so, focal length of the lens is 4 cm .
Maximum magnification of the magnifying glass,
m=1+`D/f=1+(24)/4=7`