Two points in a wave are considered to be in phase when they are at the same stage of their respective wave cycles. In other words, they reach their maximum or minimum values simultaneously and undergo similar changes in amplitude or phase at the same time.
Let's consider a simple example of a sine wave. A sine wave is a smooth oscillation that repeats itself over time. When two points on a sine wave have the same displacement and are separated by an integer multiple of the wavelength, they are said to be in phase.
For instance, let's say we have a sine wave where point A and point B have the same displacement and are separated by one complete wavelength. When point A reaches its maximum (crest) value, point B will also reach its maximum value simultaneously. As the wave progresses, they will both go through their minimum (trough) values at the same time and so on.
However, if two points are not separated by a whole number of wavelengths, they will be out of phase. In such cases, one point may be at a crest while the other is at a trough or at any other point in the wave cycle.
It's important to note that the concept of phase applies to all types of waves, not just sine waves. It helps describe the relationship between different points in a wave and is commonly used in the study of wave interference, diffraction, and other wave phenomena.