# Physics question ripples

Hi please could i have help on part b? I’m not sure how the waves will be arriving in phase if the phase difference is 180 degrees?
Here is the question: https://app.gemoo.com/share/image-annotation/637761676721897472?codeId=DWlN1jox0lX6g&origin=imageurlgenerator

Thank you!!
hey,
i think the answer may be wrong or unclear, i also see no reason how they could be in phase if there is a phase difference of 180. when looking at water waves, there is not complete destructive interference so you would still see some rippling and movement on the surface but it would be a lot less compared to when the waves are in phase. Maybe it meant that the waves were coherent? even then, tt just means the phase difference is always 180, so the crest of one wave always aligns with the trough of the other wave, resulting in destructive interference and cancellation of the waves.

I think this is correct lol, maybe have another look at the markscheme or any sample answers if possible
Did you consider the initial path difference of 3.5?
The waves do arrive in phase after given a phase difference of 180
Original post by WordsFiddle
Did you consider the initial path difference of 3.5?
The waves do arrive in phase after given a phase difference of 180

Yes for part a i considered the path difference of 2.5 and therefore the waves will arrive in antiphase, which was what the mark scheme said too. But again 180 degrees phase difference should be antiphase, whereas 0 and 360 degrees is in phase?
The 180 phase difference would cater for the initial antiphase...try drawing a diagram to understand it. You could try thinking about it the following way: A 180 phase difference means that the second wave starts after the first wave has travelled a distance of half the wavelength. The difference between the two waves when the interfere at P was already half a wavelength hence constructive interference occurs at P after the 180 phase difference.
Original post by WordsFiddle
The 180 phase difference would cater for the initial antiphase...try drawing a diagram to understand it. You could try thinking about it the following way: A 180 phase difference means that the second wave starts after the first wave has travelled a distance of half the wavelength. The difference between the two waves when the interfere at P was already half a wavelength hence constructive interference occurs at P after the 180 phase difference.

Ah I see but why do they normally say in textbooks that phase difference of 180 degrees is antiphase?
That is when the path difference is an integer. I guess they don't specify this in textbooks. The questions in paper are always gonna have these challenges so its onto us to figure it out.
Original post by WordsFiddle
That is when the path difference is an integer. I guess they don't specify this in textbooks. The questions in paper are always gonna have these challenges so its onto us to figure it out.

Sorry, please could you explain that? I thought if the path difference was an integer its in phase and if its 1/2 or 3/2 etc it would be in antiphase?
Yeah what I mean is that in textbooks they don't usually talk about the collective effect of a non-integer path difference and phase differences.