Wave Phase - Simplified Explantion

WAVE PHASE

This is used to describe a location within a specific wave. 

Consider the following wave. For one wavelength, say from A to E, all the points are doing different things i,e, they all are moving in different directions and don't represent the same place... obviously. Basically, all these points are different and represent different things. So, we say that all the points from A to E are not in phase.

Now, lets consider two cycles of the wave. From A to I. By cycles I mean wavelengths. Here, we use wave phase to talk about all the points that are kind of the same. You will understand better by an example.

Look at points A and E. Both of them are doing the same thing, i,e, they are moving upwards. Now the most important both of the points represent the same location for one wave cycle. So, A represents the beginning of wave cycle AE just as E represents the beginning of wave cycle EI.

Similarly, B, F, J, N are in phase. 

Similarly, C, G, K, O are in phase.

Similarly, D, H, L are in phase.

Hence we can say A and E are kind of the same thing. Hence, we use wave phase to talk about all the points in multiple cycles of the wave that are located at the same place within 1 cycle.

Lets talk about points A and C. Both of them are located at the same level but they are doing the same thing. A is moving upwards whereas C is moving downwards. So points A and C are out of phase.

Using Wave Phase To Define Wavelength

Wavelength is distance between two consecutive points which are in the same phase. 

So now you don't have to define wavelength as the distance between two consecutive crests or troughs (even though its right). Defining wavelength in terms of phase sounds more precise. 

Just remember, any two consecutive crests are always in the same phase as are two consecutive troughs.