1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Optics : Diffraction gratings.

  1. Apr 24, 2008 #1
    1. I am attempting a question from a textbook but the wording or perhaps the question itself is confusing me.
    *Light falls at perpendicular incidence on a transmission diffraction grating. The second order diffracted light leaving the grating is examined.
    The grating has 600 slits per mm, a total width of 10 cm, and is being used to examine spectral features near a wavelength of 450 nm. How close ( in nm) can the wavelength of two spectral lines be, for the two to still be seen as two, rather than blended into a single intensity peak?

    2. Relevant equations
    Ok so I have done question son diffraction gratings before, but all straightforward, and using the equation d*sin theta =m*lamda

    3. The attempt at a solution

    i have worked out theta to be 32.6 degrees, and (not sure if this is right) but used the equation for double-slit diffraction: y=m*lamda*D/d and worked out the spacing between the maximum and the first minimum, y, to be 0.05389m. Is this at all on the right track or am I totally lost?

    I fear the latter. Any help much appreciated!
  2. jcsd
  3. Apr 25, 2008 #2


    User Avatar
    Homework Helper

    I think you want to look up the formulas dealing with resolving power. There is one that gives the resolving power needed to differentiate two close wavelengths in this experiment; and there is another that gives the resolving power of a specific diffraction grating that depends on the order number.
  4. Apr 25, 2008 #3
    Thanks, i found the equation R=lamda/deltalamda
    and R=mN, where N is the number of gratings and m the order.
    So i basically worked out R from the second equation where I have both m and N, then I plugged it into the first and got 3.75x10^-12 m for delta lamda, is that on the right track?
  5. Apr 25, 2008 #4


    User Avatar
    Homework Helper

    That looks right to me.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook