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What is the definition of phase lag and phase difference in waves and how are these two related? Waves have always confused me,since it is very difficult to visualize them. I have tried learning it from wikipedia but language is very difficult to understand as it contains difficult equations. Please explain in simple words and with real life examples.

Ajay Sabarish
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1 Answers1

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When you meet this the first time it can be a difficult concept to understand.

To get fit two friends decide to walk up and down a flight of stairs.
Let the steps be labelled $0$ to $10$.
So to go up and come down they have to ascend 10 steps and then descend 10 steps a total of 20 steps. Call this one repetition or cycle of the exercise.

After a time it is noted that friend $A$ is on step 3 and going up and friend $B$ is on step 6 and going up.
So friend $B$ is 3 steps ahead of friend $A$ which is $\frac {3}{20}$ of a complete repetition (cycle).

Another way of describing this is to assume that a complete repetition (cycle) is equivalent to $360^\circ$ and so the fraction $\frac {3}{20}$ of a repetition (cycle) is equivalent to $\frac {3}{20}\times 360^\circ = 54^\circ$.

So we could say that friend $B$ leads friend $A$ by $54^\circ$ or we could say that friend $A$ lags behind friend $B$ by $54^\circ$.

So phase difference is a method of comparing the motions of two things which are oscillating cyclically.

Farcher
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  • thank you very much sir,it was great,but i generally encounter waves which have same frequency and wavelength,so the friends would be moving with same speed,so why is there a phase difference?please help sir,your example is excellent. – Ajay Sabarish Jan 30 '16 at 09:45
  • OK so just imagine the friends move at the same speed but one friend started 3 steps ahead of the other. – Farcher Jan 30 '16 at 16:47
  • oh ok,so it is the initial difference that causes phase difference for waves of same omega. – Ajay Sabarish Jan 31 '16 at 03:48
  • but sir,can you also please tell me what are coherent waves? – Ajay Sabarish Jan 31 '16 at 03:49