1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    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!

Diffraction grating of laser light

  1. Apr 4, 2013 #1
    1. The problem statement, all variables and given/known data
    a narrow beam of laser light (i.e coherent monochromatic light) wavelength 630nm is incident on a grating having 300 lines per mm. a piece of paper is curved 180 degrees beyond the grating. calculate how many spots of red light should be seen


    2. Relevant equations
    n[itex]\lambda[/itex]=d*sin[itex]\theta[/itex]
    d = spacing between lines
    n = order of angle

    3. The attempt at a solution
    so theres 300 lines per mm and i want to find out d therefore i did [itex]\frac{1}{300000}[/itex] and got 3.33x10^-6 lines per m
    so now i can use n*630x10^-9=3.33x10^-6*sin(90)
    this makes
    n=[itex]\frac{3.33x10^-6*sin(90)}{630x10^-9}[/itex]

    this gives me 4.72 which I can round up to 5 and multiply by 2 to get the order for 180 degrees.
    so I get an answer of 10 and the books answer is 11.
    what am I doing wrong

    P.S first time using latex...
     
  2. jcsd
  3. Apr 4, 2013 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    This should be meter per line I think.

    You forgot the central spot at an angle of zero.
     
  4. Apr 4, 2013 #3
    check your calculation, I got n to be just greater than 5. As mfb says, you forgot the central max (n = 0)
    If n was 4.72 then you would only see 9 spots (including the central max)
    You cannot 'round up' the value of n !!!
     
  5. Apr 4, 2013 #4
    you are correct that should have been meter per line. I knew I must have been forgetting something.
    Thankyou
     
  6. Apr 4, 2013 #5
    technician, after seeing your reply I have noticed my calculator is still in radians from my math revision. now i get 5.29 down to 5. Thanks lol
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Diffraction grating of laser light
Loading...