A Parallel beam of monochromatic light is incident on a plane reflecting surface. If width of the plane incident and reflected plane wavefront is represented by A B and C D respectively then

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Let the width of the beam on the plane reflecting surface be $l$ .

Let the angle of incidence be $i$ and the angle of reflection be $r$ .

The width of the incident wavefront is given by $A B = l cos i$ .

The width of the reflected wavefront is given by $C D = l cos r$ .

According to the law of reflection, the angle of incidence is equal to the angle of reflection, so $i = r$ .

Therefore, $cos i = cos r$ , which implies $A B = C D$ .

Answer: $A B = C D$

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