Path Difference

http://filestore.aqa.org.uk/subjects/AQA-PHYA2-QP-JAN13.PDF

Question 7(e)

So my understand is that if you double the frequency, the wavelength will be halved right?

Also

Here is the MS
http://filestore.aqa.org.uk/subjects/AQA-PHYA2-W-MS-JAN13.PDF

I don't understand how they managed to get a path difference of 2 lambda? I know that if it is an integer then it will be constructive interference so therefore a maxima but how did they manage to get 2 here?

Also question 7B(II) is slightly confusing me. Why is the path difference equal to the wavelength?

Thanks

The MS is a bit confusing... but the MS isn't a model answer. I guess it means 2x new lambda (i.e after the frequency is doubled) making it a maximum (but not the first maximum anymore)

But surely if the frequency has been doubled then the new wavelength will be 1/2 rather than 2? Unless I am being absolutely idiotic here which I probably am!

But surely if the frequency has been doubled then the new wavelength will be 1/2 rather than 2? Unless I am being absolutely idiotic here which I probably am!