First slide

Ray optics

Question

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,

Moderate

A

diverging
B

converging
C

parallel
D

initially converging and then diverging

Solution

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.
$\begin{array}{l} ∴ \frac{1}{v} - \frac{1}{(+ 25)} = \frac{1}{(- 25)} \\ \Rightarrow V = ∞ \end{array}$
So the final emergent rays are parallel.

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