Question

Question #1bc4e

Answer 1

Hopefully the diagram below may help

Explanation:

If we imagine a simple wave moving along a string (left to right).
In the above diagram, the blue particles are pulling on their neighbouring orange particles. If the wave moved forward slightly in time, we would see the orange particles having the same displacement as the current position shown of the blue particles.
On the left hand side, the blue particle at the crest will begin to move downwards, the orange particle will move upwards to the crest.

The phase of an oscillating particle is the fraction of a cycle it has completed since the start of a cycle. The blue particles are further through each cycle than the orange particles. Hence the orange particles lag slightly behind them in phase.