In-Phase and Out of Phase Points / Waves

What are In-Phase Points?
Inphase points are points of a wave that are located at the same position in the wave cycle. Waves that are inphase are separated by an even multiple of half wavelength (π).Such waves will undergo constructive interference.

Inphase

The x’s in the diagram above marks points that are in-phase in the wave cycle.

What are Out of Phase Points?
Out of phase points are points that are not located in the same position in the wave cycle. Waves that are out of phase are usually separated by an odd multiple of half wavelengths (π). These waves undergo destructive interference.

Looking at the diagram above we are going to determine which are in phase against those that are out of phase.

1. π to 3π would be inphase because the difference is an even number 2π.
2. 2π to 4π would be inphase because the difference is also an even number 2π.
3. 2π to 6π would be inphase because the difference is an even number 4π.
4. 1π to 4π would be out of phase because it is separated by an odd number 3π.