Young Double and Multi Slits Java Applet Ejs Open Source Physics Computer Model by Fu-Kwun Hwang, Robert Mohr and Wolfgang Christian and now this remixed version lookang.
i got ideas and codes from several sources
http://weelookang.blogspot.sg/2013/07/young-double-and-multi-slits-java.html
http://weelookang.blogspot.sg/2013/06/ejs-open-source-young-double-slit-java.html
andrew duffy work found here http://www.compadre.org/osp/document/ServeFile.cfm?ID=9988&DocID=1622&Attachment=1
took some ideas from tat leong point P, https://dl.dropboxusercontent.com/u/44365627/lookangEJSworkspace/export/ejs_wave_double_slit_LTL.jar
Jose Ignacio Fernández Palop vcolor code from http://www.compadre.org/osp/items/detail.cfm?ID=10529

enjoy!

Full screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list

Multiple Slit Diffraction

The EJS Multiple Slit Diffraction model allows the user to simulate Fraunhofer diffraction through single or multiple slits. The user can modify the number of slits, the slit width, the slit separation and the wavelength of the incident light. The scale of the diffraction pattern can also be changed and a plot of the light intensity can be toggled on and off with a checkbox.

The minima of single slit diffraction are determined by:

D sin ? = m ? ,  where m = 1,2,3,......

where D is the single slit width.

For double slit interference, constructive interference (bright spots) are found at:

d sin ? = m ? ,  where m = 1,2,3,......

and destructive interference dark spots are located at:

d sin ? = (m+0.5) ? ,  where m = 1,2,3,......

where d is the distance between slits.

Multiple slits will sharpen (narrow) the regions of constructive interference. The overall pattern from multiple slits will be the two-slit pattern multiplied by the single slit diffraction envelope.
References:
Douglas Giancoli, Physics: Principles with Applications sixth edition, pages 664-675.
Eugene Hecht, Optics fourth edition, pages 460-464.
Credits:

The Multiple Slit Diffraction model was created by Fu-Kwun Hwang using the Easy Java Simulations (EJS) modeling tool. It was adapted to EJS version 4.1 by Robert Mohr and Wolfgang Christian at Davidson College. You can examine and modify the model for this simulation if you have EJS installed by right-clicking within the diffraction frame and selecting "Open Ejs Model" from the pop-up menu. Information about EJS is available at: and in the OSP ComPADRE collection .

this remixed model is by lookang based on the earlier EJS models by Fu-Kwun Hwang, Robert Mohr and Wolfgang Christian  and Andrew Duffy.
Enjoy!