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!!

Reply 1

Nat4695

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

Reply 2

username6691833

Did you consider the initial path difference of 3.5?
The waves do arrive in phase after given a phase difference of 180

Reply 3

anonymous294

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?

Reply 4

username6691833

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.

Reply 5

anonymous294

Original post by WordsFiddle

Ah I see but why do they normally say in textbooks that phase difference of 180 degrees is antiphase?

Reply 6

username6691833

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.

Reply 7

anonymous294

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?