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Homework Help: Diffraction Grating Questions

  1. Aug 14, 2006 #1
    I just want to make sure i got these questions answered correctly about multiple slit diffraction gratings: specifically a grating spectroscope.

    1. Why is the 2nd order spectrum more "spread out" than the first order spectrum?

    Is it because one light wave has to travel 2 wavelengths farther than the other wave of light? Also because the angle increases as you move from the first order to the second order spectrum?

    2. Explain why it is important to illuminate as much of the grating as possible.

    Is it because you need as much light as possible to produce mth order maximums. If there is not enough light then some maximums wont show up.

    Please help with anything i left out, should add, or did wrong. thanks.
  2. jcsd
  3. Aug 14, 2006 #2


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    Are you asking or stating an answer?

    Take a point some distance 'a' from a plane. A beam of light is pointing normal to the plane. Then rotate the beam of light some angle. What happens to the distance between the point when the light was shining normal (directly at) the plane, and the point when the light shines at angle. Remember the beam of light is on the hypotenuse of a triangle formed by the normal between point and plane, and the distance from normal to the light as it strikes the plane at an angle, and the beam itself.

    Hopefully that's not too confusing.

    The diffraction pattern will certainly be brighter with a more intense light, but "as much of the grating as possible" seems to indicate illuminating as many slits as possible. What would be the consequence of that? In other words, if one illuminates 2, 3, 4, 5, 6, . . . slits, what happens?
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