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# AS Physics- Diffraction Grating Question watch

1. Hey, can someone help me out with these questions. I Vaguely understand it but I don't really know how to write the answer according to the mark. I feel like I'll just waffle on so can someone pint out the key points to mention?

Thanks
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2. Beacuse the phase difference between the two waves between B and C is a fraction of a wavelength, and so destructive interferrence occurs and you get a minima
3. Yeah that's what I was thinking but should I there be more since it's 3 marks? Also, what bout the other Qs?
4. (Original post by georgee913)
Yeah that's what I was thinking but should I there be more since it's 3 marks? Also, what bout the other Qs?
I think this would be enough for the 3 marks:
At B and C, the phase difference between the waves is an integer multiple of lambda and hence constructive interference. Thus between B and C the phase difference is a fraction multiple of multiple and thus destructive interference occurs and a minima is produced. Hope this helps.
Also, I'm sorry to bother you again but can you send me this paper as well? I want to use it for practice if thats okay.
5. If theres anything else you need help with let me know.
Beacuse the phase difference between the two waves between B and C is a fraction of a wavelength, and so destructive interferrence occurs and you get a minima
I'd be more specific than that for full marks. Not just 'a fraction' - it's a half wavelength (or n + 1/2 to cover all possibles) to get waves meeting in antiphase and therefore a minimum

If the 'fraction' is not 1/2, then you will get something between a maximum and a minimum.

Full nº of wavelengths = constructive = maximum
n + 1/2 wavelengths = destructive = minimum

It might sound like a pedantic correction but without 1/2 somewhere in there you're probably not going to get full marks.

(Original post by georgee913)
Hey, can someone help me out with these questions. I Vaguely understand it but I don't really know how to write the answer according to the mark. I feel like I'll just waffle on so can someone pint out the key points to mention?

Thanks
7. (Original post by phys981)
I'd be more specific than that for full marks. Not just 'a fraction' - it's a half wavelength (or n + 1/2 to cover all possibles to get waves meeting in antiphase and therefore a minimum

If the 'fraction' is not 1/2, then you will get something between a maximum and a minimum.

Full nº of wavelengths = constructive = maximum
n + 1/2 wavelengths = destructive = minimum

It might sound like a pedantic correction but without 1/2 somewhere in there you're probably not going to get full marks.
You're right I completely forgot the half thing. I need to go over my AS stuff

8. It's a perfect example of why understanding is not enough - of why you also need to memorise written answers. It's so easy in an exam to just forget a key term or word, something that might even be in your head but doesn't get on to the paper. Good students can and do come unstuck like that in exams and one point here, one point there, can easily cross a grade boundary or even two.

I saw somebody here the other day recommend understanding over memory. It'ss not enough, of course, you need both.

You're right I completely forgot the half thing. I need to go over my AS stuff
9. The need of ½ is depending on what the candidate is using.

When the candidate talks about how constructive and destructive interference giving rise to respective maximum and minimum signals based on path difference which is related to the wavelength, ½ is needed.

path difference is either λ or nλ for constructive interference giving max signal

path difference is either λ/2 or (n + ½) λ for destructive interference giving min signal.

But if the candidate is talking in terms of the phase difference, ½ may not be required.

phase difference is 360° or n × 360° or n2π rad for constructive interference giving max signal

phase difference is an odd multiple of either 180° or π rad for destructive interference giving min signal.

Note: Phase difference is related to path difference but they should not be mixed up.

Happy learning physics.
10. Yes. And while we're on the subject, "out of phase" isn't as specific as "antiphase" or "180º out of phase".

Every mark counts, leave nothing to chance.

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