Question

At what distance should an object be placed from a convex lens of focal length 18 cm to obtain an image at 36 cm from it ? What will be the magnification produced in this case?

Answer 1

Here, `u = ?, f = 18 cm, v = +- 36 cm`,
`m = ?`
From `(1)/(v) - (1)/(u) = (1)/(f)`, `-(1)/(u) = (1)/(f) -(1)/(u) = (1)/(18) - (1)/(+- 36) = (2-(+-1))/(36)`
`:. u =-36 cm` and `u = -12 cm`.
`{:("When u"= -36 cm),(and v = 36 cm):}],m = (v)/(u) = (+36)/(-36) = -1`, Image is real and inverted.
`{:("When u" = -12 cm),( and v = -36 cm):}], m = (v)/(u) = (-36)/(-12) = +3`, Image is virtual and erect.