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Please help i cant understand anything about diffraction grating

  1. Feb 14, 2014 #1
    why should we use a convex lens to focus the rays at the point on the screen.why is at an angle instead of being straight?
     

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  3. Feb 14, 2014 #2

    phinds

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    I don't see that there is anything saying it HAS to be at an angle, the diagram is just showing what happens when it IS at an angle.
     
  4. Feb 14, 2014 #3
    okay but why is it at an angle and cant we use a concave lens
     
  5. Feb 14, 2014 #4

    phinds

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    What part of my previous post did you not understand?

    Sure, you could use a concave lens, providing you want to spread the beams out instead of focusing them at a point.
     
  6. Feb 14, 2014 #5
    The lens is placed so that its optical axis is aligned with the direction of the incoming rays. That helps minimize all sorts of undesired optic aberrations. A convex lens is used in order to focus the rays to a point
     
  7. Feb 14, 2014 #6

    Drakkith

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    I see something about a diffraction grating in your picture. Could you give us a bit more info on what the picture is trying to explain?
     
  8. Feb 14, 2014 #7
    MN represents the section of a plane transmission grating. AB,
    CD, EF … are the successive slits of equal width a and BC, DE … be
    the rulings of equal width b.Let e = a + b.
    Let a plane wave front of monochromatic light of wave length λ be
    incident normally on the grating. According to Huygen’s principle, the
    points in the slit AB, CD … etc act as a source of secondary wavelets
    which spread in all directions on the other side of the grating.
    Let us consider the secondary diffracted wavelets, which makes
    an angle θ with the normal to the grating.
    The path difference between the wavelets from one pair of
    corresponding points A and C is CG = (a + b) sin θ. It will be seen that
    the path difference between waves from any pair of corresponding
    points is also (a + b) sin θ
    The point P1 will be bright, when
    (a + b) sin θ = m λ where m = 0, 1, 2, 3
    In the undiffracted position θ = 0 and hence sin θ = 0.
     
  9. Feb 14, 2014 #8

    Drakkith

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    It appears to me like the lens is focusing the light in order to show the interference effects. Without the lens the light would simply spread out.
     
  10. Feb 16, 2014 #9

    Philip Wood

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    Light from different slits is brought to a focus at P1, so it can produce interference at P. Why don't the different lengths of the sloping paths from the lens to P1 contribute path differences? The differences are compensated for by the different thicknesses of lens-glass through which the light travels. Longer paths from lens to P1 follow shorter paths through the glass, where the light travels more slowly. The upshot is that we can calculate the path difference between light from adjacent slits as d sin theta, simply using the triangles in the diagram next to the grating.
     
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