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Homework Help: A Haunting question on diffraction grating!

  1. Apr 8, 2012 #1
    1. The problem statement, all variables and given/known data
    This question is pretty interesting, but a bit long, so bear with me

    http://www.xtremepapers.com/CIE/International A And AS Level/9702 - Physics/9702_w07_qp_2.pdf
    question 5 part c

    For (i) The width of each slit is increased but the separation remains constant.
    Firstly what exactly does it mean to increase the width? Is d increasing? but it says separation remains constant
    the answer says the bright fringes will become brighter and there will be fewer fringes on the screen. HOW?
    I thought the fringes would become less bright, because increasing the slit mean diffraction decreases (considering the small wavelength of the light) so there should be less superposition, and hence the intensity of the resultant wave should be lower!! ANd how in the world is "will be fewer fringes on the screen" derived??

    And for the second one (ii) The separation of the slits is increased.
    Why is increase in the nos. of maximas(more fringes seen) wrong?? following the equation dsin∅=nλ, if d increases then n will increase won't it??

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 8, 2012 #2

    http://www.xtremepapers.com/CIE/International A And AS Level/9702 - Physics/9702_w07_ms_2.pdf → answer sheet if anyone wants to view
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