Physics help

Aoifums

My physics christmas test is tomorrow and I'm freaking. I just need someone to tell me how to do a couple of things.

In the experiment to find the frequency of a string with tension, how do you plot the graph?

What do you do when deriving the diffraction grating formula? It has come up on exams questions but I don't understand what I'm being asked.

Thanks.

KnifeWRENCH

For the experiment, you plot the frequency (y axis) against the square root of the tension (x axis). It should be a straight line through the origing, showing that they're proportional.

Deriving the diffraction grating formula requires a diagram to explain properly, but it's explained well in a few lines here:
http://www.antonine-education.co.uk/physics_a2/Module_4/Topic_6/diffraction_grating_formula.htm

Hope that helps.

Fringe

For the frequency one, the equation is f = 1/2L sqrt(T/U)
So to get a straight line, plot frequency against the square root of tension. By convention, the thing you control is put on the x axis so put the frequency on the x-axis and the sqrt of tension on the y axis.

The diffraction one is a bit odd. Don't think I've ever seen it come up before in the LC. Basically, it's just really simple trigonometry. Draw a clear diagram. If you look at the first order, it's clear that lambda/d = sin x. Rearrange to get lambda = dsinx. Of course, you have integer values of lambda so the full equation is nlambda = dsinx.

Aoifums

Thank you both. Neither of them came up and I think I aced the test.

cantona56

emm...diffraction grating derivation deffo came up last year....remember rushing through it so if your leaving out any derivations...this should be one of the...allthough its pretty str8 forward once you get your head around it!