The two lenses shown are illuminated by a beam of parallel rays of light from left. Lens B is then moved slowly towards lens A. The beam emerging from lens B is,

Initially the incident beam will converge at a distance of 50 cm to the right of A. So for the lens, u= +25cm, f= -25cm.∴ 1v−1+25 = 1−25⇒ V = ∞So the final emergent rays are parallel.