Year 12s: what will be the first way you research your future options? >>

AQA A level physics diffraction

Hi so I’m confused about diffraction and just hate the waves topic in general. The specification is very vague which is annoying because I’ve got no idea how much detail I need to go into or understand - so I feel like I’m digging myself into a hole trying to understand too much. But I’ve got a few main questions that I really don’t get:

How does the single slit in Young’s Double slit experiment produce coherent light?

Because the wavelength of light waves depends on the frequency (which is the same for coherent waves) is coherent light always monochromatic?
If this is true then how can white light -
which must not be coherent as it contains
different frequencies (different colours/
wavelengths and isn’t monochromatic)
produce the same interference pattern or
really a visible pattern at all when it diffracts
but with coloured bright fringes?

Also what does the phase difference actually depend on - is it the path difference or the frequency or speed cos I rlly don’t understand :frown:

Thanks I know that’s quite long 😊

Reply 1

wonder girl

I know you are probably hoping for someone to reply with something useful I also do this in physics (I'm on a different spec) but we never get told any of this either and the spec is very vague and doesn't specify I wish we were taught more of the background knowledge not just the question type that will make us pass the exam.

Sorry about this just wanted you to know you weren't alone. :smile:

Reply 2

nmaster06

Hey! I'm in year 13 and I do Edexcel Physics so I'm not sure how close to your spec my answers will be but I hope they help!
1- Coherent light is when the waves of light are of the same frequency, wavelength and have a fixed phase relation. The single slit acts as a 'point source' for the light and then produces a wavefront as the light gets diffracted when passing through. This means all light reaching the double-slit comes from a single point. This website probably explains it better: https://www.cyberphysics.co.uk/topics/light/A_level/YoungSlits.html
2- I think you're right so coherent waves are monochromatic but monochromatic waves aren't always coherent as the coherency (idk if that's a word) is reliant on the fixed phase relation as well which monochromatic light may not be.
3- I'm not too sure about this one so probably double-check with a teacher someone but I don't think you can use white light for the experiment because its made from multiple wavelengths as you said.
4- Phase difference is the measure of the difference of how far into their cycles the waves are. So it's more dependent on the distance from source and phase I guess?
I hope this answers your question- lemme know if I could help more :smile: