Question

Two thin convex lenses of focal lengths 20cm and 5cm respectively are placed at a distance s. If a parallel beam incident in the first lens emerges as a parallel beam from the second lens then the value of d is

Answer 1

Parallel beams coming toward a convex lens will pass through focus and the beams of light which is coming from focus goes parallel to axis.

so here in first lens ,
the parallel beams comes and cross the lens and converge on the opposite side at a point a distance equal to focal length i.e 20 cm.

for second lens,this point act as the source.
So that point should be the focuz for coming lens because then only the ray after passing through lens will goes parallel to axis. So that point is the focus of second lens.

so distance between two lens are
20 + 5 = 25 cm...

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