A beam of parallel rays is brought to a focus by a plano-convex 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

When two lenses are joined together, the combined focal length, F is given by`(1)/(f_(1))+(1)/(f_(2))`As f is convex, siis positive, `f_(2)` is negative as it is concave.
`therefore`  `(1)/(F)=(1)/(f_(1))-(1)/(f_(2)) rArrF=(f_(1)f_(2))/(f_(2)-f_(1))`
Asi `f_(1)=f_(2)`one gets the image at, `infty` t.e. the rays proceed parallel.