WAEC Physics 2018 Paper

Concept:Points separated by half a wavelength, $\frac{\lambda}{2}$, are $180^\circ$ out of phase.Explanation:Two points on a wave are $180^\circ$ out of phase when one reaches its maximum positive displacement while the other reaches its maximum negative displacement.This happens when the distance between the points is $\frac{\lambda}{2}$.In the diagram, B and D are located at similar positions on opposite sides of the equilibrium position.Their separation is half a wavelength.The phase difference is:$$\Delta \phi = \frac{2\pi}{\lambda} \times \frac{\lambda}{2} = \pi = 180^\circ.$$Therefore, B and D are completely out of phase.Answer:B. B and D