Consider a set of plane waves moving towards a reflecting surface, indicated by the line x-x’. At time t = 0, point A on the wave-front reaches the reflecting surface.

We will try to find the position of the wave-front at time t, the instant when point B reaches the reflecting surface.

Draw an arc of radius equal to the distance B C. The "secondary wavelets" from point A will have travelled this far by the time point B meets point C. The new wave-front is the tangent to this arc which passes through point C.

Using the observed fact that the direction of propagation of a wave is always at 90° to the wave front, we can predict the direction of motion of the waves after reflection (shown by the arrow).

The "angle of incidence" is the angle between the direction of propagation of the waves and a normal to the reflecting surface before reflection.

The "angle of reflection" is the angle between the direction of propagation of the waves and a normal to the reflecting surface after reflection.

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