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The Physics PHYA2 thread! 5th June 2013 Watch

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    Why does light for double slit interference have to be coherent?
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    (Original post by mrdan)
    Why does light for double slit interference have to be coherent?
    You want all the waves to be in phase with eachother

    You can't have em in all sorts of planes, or else you will not get sharp fringes
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    I will do as much as I can. Work done = force times the distance (in the direction of the force). wd=force times velocity. Energy stored in the spring can be found by the equation e=1/2 times F times delta L. The light ray bends: 1) towards the normal if it passes into a more refractive substance, 2) away from the normal if it passes into a less refractive index. Newton's law (Have already posted above).2 good usages of cladding in a fiber, 1) Protects core from breakage. 2) More secure signals as it prevents signal loss. Transverse and Longitudinal waves are called progressive waves. Transverse (perpendicular) and longitudinal (parallel). Eg of transverse are all electromagnetic waves (radio wave) and eg of longitudinal is a sound wave. Stationary or standing waves are formed with nodes and anti-nodes when two or more progressive waves of the same FREQUENCY and same AMPLITUDE pass through each other. During refraction, the speed and wavelength both reduce but the frequency stays constant. Energy loss in the fiber means that the amplitude of the wave coming out of the fiber has been reduced. The angle of reflection is always the same as angle of incident. Longitudinal waves cannot be plane-polarised because they oscillate left and right. But, transverse can be plane-polarised because they oscillate up and down. Polarised waves have less intensity. Straight line on the graph from the origin states that hooke's law is obeyed up to the elastic limit. TOTAL INTERNAL REFLECTION CAN ONLY TAKE PLACE IF, 1) angle of incident exceeds the critical angle, 2) incident substance has a larger refractive index than the emergent substance, (note: both criteria have to be met for TIR to occur). Angle of refraction can be calculated by doing the following.... 90 - critical angle = angle of refraction. Rising the tension or shortening the length of a wire increases the pitch and frequency. And this is opposite for Lowering the tension. Diffraction: is the spreading of waves when they pass through a gap or by an edge. Waves will spread more if; 1) the slit spacing is made narrower (intensity will be reduced), 2) the wavelength is made larger. dsintheta=n lambds is the equation for diffraction grating- maximum number of orders is given by d=n=d/lambda. Remember: Convert everything into m. If you are given a grating has 300 lines per mm, find how much it is for 1 line per m. so, 1/300x10^3 will give length of 1 line per m. dsintheta=n lambds (d=slit spacing, theta=fringe angle, n=order of peak. Double slit (young's modulus) is known as coherent since they emit waves with a constant phase difference and same frequency. Light from nearby sources will not produce an interference pattern because they emit waves at random position so constructive/destructive pattern could not be formed. Red light has the longest wavelength and the shortest frequency (opposite for blue light). Longer wavelength=grater fringe separation (since w=lambda D/s). Actually it is a lot more. It will take ages and I should have done it before. Sorry..




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    You wouldn't get any diffraction pattern though if they were not coherent?
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    could someone explain this sentence:

    • When you get constructive interference, at your first order(s) [bright fringes] your path difference is nlambda where is an integer this also means that the phase difference is a multiple of 2pi .
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    Good luck everyone! I am off to the College. And please read the notes I have written for you.
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    (Original post by StalkeR47)
    Good luck everyone! I am off to the College. And please read the notes I have written for you.
    thanks so much! good luck!
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    (Original post by SAS18)
    could someone explain this sentence:

    • When you get constructive interference, at your first order(s) [bright fringes] your path difference is nlambda where is an integer this also means that the phase difference is a multiple of 2pi .
    The path difference is essentially dsin()... they've just quoted the formula
    dsin()= nlambda

    If you look up a diagram it is better explained. But the maxima occurs when waves are in phase hence why the path difference between them needs to be a multiple of lamda (lamda also equals 2pi)
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    (Original post by StalkeR47)
    Good luck everyone! I am off to the College. And please read the notes I have written for you.
    you're going early aren't you
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    Can some on run through polarisation quickly please
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    (Original post by Micheal123456)
    Can some on run through polarisation quickly please
    Polarisation can only happen with transverse waves.

    Filters are set a certain way so only waves that are in the same plane as the filter will pass through.

    If you then had another filter at 90 degrees to the first one, no light would pass through as this is perpendicular to the first one, so the waves that passed through the first one are not in the same plane as this second filter.


    Hope that makes sense!


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    (Original post by _JasmineMistry)
    2. look at the question, if in doubt use 3
    But you're using gravity, which is 9.81 (3)

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    sooo how did everyone find it?
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    (Original post by mikey2912)
    sooo how did everyone find it?
    Third time sitting this & I've sat unit 1 twice... I think it's the hardest AS module I've sat
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    Spent like half an hour counting little squares for the work done on the graph, got 1.04J or 1.0J
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    (Original post by mikey2912)
    sooo how did everyone find it?
    really hard

    I needed a B in this.... dont think I got it.... how did everyone else find it?
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    (Original post by Max28)
    Spent like half an hour counting little squares for the work done on the graph, got 1.04J or 1.0J
    I got 1.05J
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    (Original post by Davelittle)
    I got 1.05J
    Ah it's not the same to sig fig then :/ There will be a range though which is nice
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    Ok so first question was
    6100N
    9.5degrees
    Acceleration
    Ep
    Power efficiency (24 or something? Maybe 20.4)- You did P=W/t=Ep/55 (As Ek was negligible) then put that into efficiency equation.

    Question2:
    0.55s
    245m?
    S=Vt so if V is lower S is lower

    Question3:
    The ball in oil one? (6 marks)
    Then graph- straight line until it goes in then curve to decreasing gradient until gradient=0 and stays constant

    Question4:
    Hookes law
    Gradient for k (1100 N/m (I might have remember that wrong)
    Area under- ~1.05J
    Straight line parallel to first one intersecting x axis
    Work is less as area under graph is less

    Question5:
    Can't remember

    Question6: Progressive wave one
    90 and 270 (I think)
    Describe motion of B (up then down then down then up)
    Transverse so seismic S wave?
    Define frequency
    Wavelength (750m)

    Question7:
    Definition of monochromatic
    Fringe intensity (I was unsure, they would be wider but idk how intensity would change if it changes at all)
    Forgot
    White light fringes vs monochromatic fringes
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    That was such a sexy exam
 
 
 
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