/*
 * dif3.java - revised 16 Oct 06 - width 481, height 241
 * @author jackord@kw.igs.net
 * the diffraction grating: 1, 3, or 5 slits of width w=2 microns
 *   with a separation between slits of w, 2w, or 3w (at a separation of w
 *   the pattern is that of a single slit of width n*w)
 * the intensity pattern is that viewed on a screen 1 m from the grating
 *   with one pixel representing 1 mm in the x direction
 * the wavelengths are red C, yellow D, green E, and blue F
 * a line plot (in magenta) of the analytic intensity expression for red light
 *   is superimposed for the case of 3 slits spaced 2w apart
 */

import java.applet.Applet;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class dif3 extends Applet 
                  implements ActionListener {
  int kk=0;                                            // Declarations
  String bs = "Plot"; Button bb = new Button(bs);
  Choice chd=new Choice();
  Choice chn=new Choice();
  Color col[]={ Color.red, Color.yellow, Color.green, Color.blue };

  public void init() {                                 // Layout
    setBackground(new Color(211, 211, 211));
    chd.setBackground(getBackground());
    chn.setBackground(getBackground());
    chd.addItem("d=w"); chd.addItem("d=2w"); chd.addItem("d=3w");
    chn.addItem("n=1"); chn.addItem("n=3"); chn.addItem("n=5");
    setLayout(new FlowLayout(FlowLayout.LEFT));
    add(chd); add(chn); add(bb);
    bb.addActionListener(this);
    chd.select(1); chn.select(1);
    setFont(new Font("Helvetica", Font.PLAIN, 12));
  }

  public void paint(Graphics g) {                      // Main Program
    double [] lambda=new double[4];
    double w, xg, z, zz, a, r0, pi, p0, p1, sy, sr, si, t;
    int n, nd, nw, x0, x1, x2;
    n=1+2*chn.getSelectedIndex();
    nd=1+chd.getSelectedIndex();
    lambda[0]=.6563/1000;                              // Red C
    lambda[1]=.589/1000;                               // Yellow D
    lambda[2]=.5269/1000;                              // Green E
    lambda[3]=.4861/1000;                              // Blue F
    pi=Math.PI; w=.002; a=1;
    g.setColor(Color.black); g.drawRect(0, 0, 480, 240);
    if (kk>0) {
      nw=5; x0=240; z=1000.; zz=z*z;
      for (int j=-n/2; j<=n/2; j=j+1) {                // Show slit pattern
        for (int i=-nw/2; i<=nw/2; i=i+1) {
          x1=x0+j*nd*nw+i;
          g.drawLine(x1, 10, x1, 30);
        }
      }
      nw=141;                                          // Use 141 point model
      sy=200./(n*n*nw*nw);
      for (int nl=0; nl<=3; nl=nl+1) {                 // Wavelength loop
        g.setColor(col[nl]); 
        g.drawLine(x0, 40, x0, 239);
        for (int nx=1; nx<=239; nx=nx+1) {             // Screen loop
          r0=Math.sqrt(zz+nx*nx); sr=0; si=0;          // Algorithm C
          for (int j=-n/2; j<=n/2; j=j+1) {            // Slit number loop
            for (int i=-nw/2; i<=nw/2; i=i+1) {        // Slit loop
              xg=(j*nd*nw+i)*w/nw;
              t=2*pi*nx*xg/r0/lambda[nl];
              a=.5*(1+z/r0)*Math.sqrt(z/r0);           // Angle and distance
              sr=sr+a*Math.cos(t); si=si+a*Math.sin(t);//   factors
            }
          }
          x2=240-(int)(sy*(sr*sr+si*si));
          if (x2<240) {
            g.drawLine(x0+nx, x2, x0+nx, 239);         // Plot intensity
            g.drawLine(x0-nx, x2, x0-nx, 239);         //   assuming symmetry
          }
        }
      }
      if ((n==3)&&(nd==2)) {                           // If d=2w and 3 slits
        g.setColor(Color.magenta); x1=40;              // Magenta curve
        for (int nx=1; nx<=239; nx=nx+1) {
          t=Math.atan(nx/z);                           // Analytic calculation
          p0=2*pi*w*Math.sin(t)/lambda[0];
          p0=Math.sin(p0/2)/(p0/2); 
          p1=2*pi*nd*w*Math.sin(t)/lambda[0];
          p1=Math.sin(n*p1/2)/Math.sin(p1/2);
          x2=240-(int)(200.*p0*p0*p1*p1/n/n);
          if ((x1<240)||(x2<240)) {
            g.drawLine(x0+nx-1, x1, x0+nx, x2);        // Analytic plot
            g.drawLine(x0-nx+1, x1, x0-nx, x2);
          }
          x1=x2;
        }
      }
    }
    kk=0;
  }

  public void actionPerformed(ActionEvent e) {         // Button
    String tst;
    tst=e.getActionCommand();
    if (bs.equals(tst)) { kk=1; }
    repaint();
  }
}