Help with destructive interference question (as level 2023 paper 1)

I looked at the mark scheme for question 2.1 from this paper: https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2023/june/AQA-74071-QP-JUN23.PDF, but I am still confused as to why the path difference is relevant to the answer. I am also confused at the other alternative explanations like the different distances or the time based alternative. Can someone explain the easiest one?

Here's the mark scheme: https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2023/june/AQA-74071-MS-JUN23.PDF

Also, I am confused at the different path difference formula in the markscheme:
lambda * (n+0.5) & n * lambda
How would you know which one to use?

Reply 1

Joinedup

Imo path difference and wavelength is the easiest way. When the path difference is 1/2 a wavelengths you get destructive interferance and a minima.

A layer thickness of 1/4 wavelenght is significant as the light is making a two way trip through the layer, it's travelling an extra 1/4 +1/4 = 1/2 wavelength

Probably you should revise your notes about path difference and interference

Reply 2

rm3

Original post by Joinedup

Imo path difference and wavelength is the easiest way. When the path difference is 1/2 a wavelengths you get destructive interferance and a minima.
A layer thickness of 1/4 wavelenght is significant as the light is making a two way trip through the layer, it's travelling an extra 1/4 +1/4 = 1/2 wavelength
Probably you should revise your notes about path difference and interference