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!

Double Slit Help

  1. Jan 5, 2015 #1
    1. The problem statement, all variables and given/known data
    A light source shines light of wavelength 490nm onto a pair of slits separated by 0.44m. Calculate the angular location and the location in cm of the second order dark fringes on a screen 1.4m from the slit

    2. Relevant equations
    Xn = (n-1/2)L lambda/d
    delta x = L lambda/d
    3. The attempt at a solution
    So I understand how to find the angular location, but I don't understand why only one method for finding the location works. The method which works is using the formula containing Xn which gives the answer of 0.23cm. I don't understand why it is not possible given that this is double slit to use delta X = L lambda/d then multiply that answer by 2 to get your answer as it gives a different value.

    X2=(2-1/2)(1.4)(4.9x10-7)/0.44x10-3 = 0.23cm (the answer in the back of the textbook)

    Using ΔX = Lλ/d
    ΔX = (1.4)(490x10-9)/0.44x10-3
    = 0.0015
    as it is to the second dark fringe, shouldn't 0.0015x2 get me the same answer as my previous Xn calculation?
     
    Last edited: Jan 5, 2015
  2. jcsd
  3. Jan 5, 2015 #2

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Please show your work step by step.
     
  4. Jan 5, 2015 #3

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Is the sit separation supposed to be 0.44 mm?
    Should the first equation have a factor of L?

    Think about the meaning of "delta x" and think about the meaning of Xn. In particular, "delta x" represents the distance between what two points? Xn represents the distance between what two points?
     
  5. Jan 5, 2015 #4
    I thought it wasn't necessary as it was more an issue with the concept as I was able to reach the correct answer on my own using the Xn formula, so sorry!
    Updated
     
  6. Jan 5, 2015 #5

    NascentOxygen

    User Avatar

    Staff: Mentor

    Hi chrisahn97. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Sketch the expected pattern of fringes, and mark in those 2 distances precisely using dimensioning lines such as:

    |<--------------- Xn --------------->|
     
    Last edited by a moderator: May 7, 2017
  7. Jan 5, 2015 #6

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Is the first dark fringe a distance Δx away from the center? If not, should the second dark fringe be a distance 2Δx away?
     
  8. Jan 5, 2015 #7
    oh I see now thank you!
    I think my method would have only worked if the question was asking for the second bright fringe, as the distance from the central maximum to the first dark band would not be delta X but would be 1/2ΔX
     
  9. Jan 5, 2015 #8

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Yes. In fact, this is how you would derive your other formula, which gives the location of an arbitrary dark fringe.
     
  10. Jan 5, 2015 #9
    Careless error on my part thanks for the help. This question can be marked as solved if that's a thing on these forums
     
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: Double Slit Help
Loading...