Question

the correct answer is C and i dont get how? why is it not B?
between X and Y one and a half waves are formed and 1λ represents one complete wave and this corresponds to a phase of 2π. so n should be 2.5, how can it be 3?

Answer 1

here's the question

Answer 2

So, take cosine for example:
x (in radians) cos
0 1
1/2 0
1 -1
3/2 0
2 1

So if you went 2.5, or 2 and a 1/2 phases around you would first do a full wave and end up in the same spot, and then go forward for a 1/2 and end up in 0.

But where you need to end up is at the opposite of your start and do a full wave in between. So a full wave is 2pi and then to get to the opposite value you do a pi again. Remember the full wave length is done every 2pi, not pi.