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    What is path difference?
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    A diffraction grating is simply a plate with many, incredibly small and very closely spaced parallel slits ruled into it.
    Based upon the close proximity of said slits a very sharp interference pattern is produced (much better and clearer than with Young's Double slits) when a source of monochromatic and coherent light is passed through it.
    This is due to the fact that there's such a large proportion of diffraction gratings interfering (experiencing superposition).
    Waves that meet 180 degrees out of phase experience destructive interference and so produce a sharp minimum (dark point), and light rays that meet in phase (or 360 degrees out of phase) experience constructive interference and so produce a sharp maxima (bright point).

    The diffraction grating equation is (for my spec - AQA):

    n = d sin( )

    where:

    n = the order of the beam produced
    d = distance between two slits in the diffraction grating (often extremely small)
    = the wavelength of the incident light being diffracted by the grating.
    = the angle between the order beam in question (a maxima) and the zero order (normal).
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    What do you mean by the order bit for the angle?
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    (Original post by Jennifer50)
    What do you mean by the order bit for the angle?
    When we put monochromatic and coherent light through a diffraction grating we'll have a number of maximum points produced. These, we refer to as maximum orders (you get a first order maximum produced on either side of the zero order) - the angle between an order beam and the zero order (the normal line - the most intense maxima) is dependent on the wavelength of light, the distance from the screen to the grating and the slit separation within the grating, s.

    n / d = sin( )

    We can see this when we rearrange the diffraction grating equation.
    We can therefore say as we increase diffraction increases (the waves spread out more) and so is greater - all just from looking at the grating equation
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    What does coherent light mean?
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    (Original post by Jennifer50)
    What does coherent light mean?
    It means that the 2 waves produced have: a fixed phase difference, same frequency and same wavelength. Having coherent waves makes the interference pattern a lot clearer.
 
 
 
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