A beam of parallel rays is brought to focus by a planoconvex lens. A thin Concave lens of the same focal length is joined to the first lens. The effect of this is

Options

A. the focus shifts to infinity
B. the focal point shifts towards the lens by a small distance
C. the focal point shifts away from the lens by a small distance
D. the focus remains undisturbed

Answer

A

Explanation

The combined focal length of plano-convex lens
`(1)/(F) = (1)/(f_(1)) + (1)/(f_(2))`
Hence, `f_(1) = oo` (for plane surface), `f_(2) = f` (say)
`:. (1)/(F) = (1)/(oo) + (1)/(f)`
`rArr F = f`
Now, when concave lens of same focal length is joined to first lens, then combined focal length
`(1)/(F') = (1)/(F_(1)) + (1)/(F_(2))`
`= (1)/(f) - (1)/(f) ( :' F_(1) - f, F_(2) = - f)`
`F' = oo`
Thus, the image can be focussed on infinity `(oo)` or focus shifts to infinity.