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Consider a 'beam expander' which consist...

Consider a 'beam expander' which consists of two converging lenses of focal length 40 cm and 100 cm having a common optical axis. A laser beam of diameter 4 mm is incident on the 40 cm focal length lens. Then what is the diameter of the final beam (see figure)

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The correct Answer is:
1 cm

From similar triangles `(0.4)/(40) = (x)/(100) , x = 1` cm
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ALLEN -GEOMETRICAL OPTICS-EXERCISE-04[A]
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