You are given two identical plano-convex lenses. When you palce an object 20cm to the left of a single plano-convex lens, the image appears 40cm to the right of the lens. You then arrange the two plano-convex lenses back to back to form a double convex lens. If the object is 20cm to th eleft of this new lens, what is the approximate location of the image?

Options

A. 10cm to the right of the lens
B. 20cm to the right of the lens
C. 80cm to the right of the lens
D. 80 cm to the left of the lens

Answer

a.

Explanation

`(1)/(f)=(1)/(v)-(1)/(u)`
`(1)/(f)=(1)/(40)-(1)/((-20))`
`rArrf=(40)/(3)` For length for the combination,
`f_(eq)= (20)/(3)`
`(1)/(f_(eq))=(1)/(v)-(1)/(u)`
`rArr(30)/(20)=(1)/(v)+(1)/(20)`
`(1)/(v)=(3)/(20)-(1)/(20)`
`v=10cm` (to the right)