DECLARE SUB Pager ()
 PRINT CHR$(12);
REM                                                      
pi = 4! * ATN(1!)
REM
REM ge copeland
REM dept of physics
REM old dominion university
REM norfolk va 23529
REM - revised 3/2001
REM
30 PRINT "   N  Slit  Diffraction "
40 PRINT
60 PRINT
70 PRINT "The intensity of light as a function of position, y, measured"
80 PRINT "along a screen from the center of the diffraction pattern"
90 PRINT "is given by"
100 PRINT
110 PRINT , "I(y)= I0 * sin ^2(b) * sin ^2 ( n * alpha)"
120 PRINT , "           ---------  ---------------"
130 PRINT , "             b^2       n^2 sin^2 (alpha)"
140 PRINT
150 PRINT "where:"
160 PRINT , " b = pi * a * y / ( wavelength * l)"
170 PRINT , "and   pi = 3.14159"
180 PRINT , "       a  = slit width (m)"
190 PRINT , "       y  = distance from maximum  (M)"
195 PRINT , "   f = focal length of a postive the lens."
200 PRINT , "alpha = pi*d /(wavelength *f)"
210 PRINT , "  d = separation of the center of the slits."
220 PRINT "and n = number of slits."
        CALL Pager
230 PRINT "Input the separation between slits <meters>";
240 INPUT d
dmm = d * .001

250 PRINT
260 PRINT "Input the number of slits";
270 INPUT n
280 PRINT "Select the focal length of the positive lens(m) < 1m";
290 INPUT l
300 IF l <= 0 THEN 280
310  IF l < 1! THEN 340
320 PRINT "Best to select a focal length less than one meter"
330 GOTO 280
340 PRINT "Select the slit width <mm> between 1 u and 1 mm";
350 INPUT a
amm = a
360 IF a < 1 THEN 390
370 PRINT "Best to keep slit diameter between one micron and one millameter"
380 GOTO 340
390 IF a < .0001 THEN 370
400 PRINT
410 PRINT "We will consider optical wavelengths only!"
420 PRINT "Input the wavelength in nanometers"
421 PRINT "Let's keep wavelength between 400 and 700 nm."
430 INPUT w
440 IF w < 400 THEN 400
450 IF w > 700 THEN 400
460 p1 = 4! * ATN(1!)
REM make stuff in MKS system
        a = a * .001
        w = w * 1E-09
490 PRINT
500 PRINT "You have selected a lens of focal length "; l; " meters"
510 PRINT "and a slit of width "; a; "meters at a wavelength"
520 PRINT "of "; w * 1E+09; "nm and "; n; " slits separated by "; d1; " meters."
530 PRINT
540 PRINT "Are these the input parameters you wish to use <yes or no>";
550 INPUT a1$
560 IF a1$ = "NO" OR a1$ = "no" OR a1$ = "No" THEN 230
562 IF a1$ = "yes" OR a1$ = "YES" OR a1$ = "Yes" THEN 570
564  GOTO 490
570 REM calculations                                          
REM ys= ystart is found and we evaluate yeval from -ys to +ys
REM Then write out the Values of Intensity(i) vs y(i)

680 DIM Intensity(601), y(601)
REM set ys=position of the second min of the diffraction pattern

ys = 2! * w * l / a
yinc = 2 * ys / 500
I = 0
FOR yup = -ys TO ys STEP yinc
 I = I + 1
b = pi * a * yup / (w * l)
alpha = pi * d * yup / (w * l)

top = SIN(b) ^ 2 * SIN(n * alpha) ^ 2
bot = b * b * (n * SIN(alpha)) ^ 2
Intensity(I) = top / bot
y(I) = yup
NEXT yup



960 REM calculations completed                                  



1400 REM tektronix output here write labels and the data in plttek.dat
1410 OPEN "plttek.dat" FOR OUTPUT AS #3
1412 PRINT #3, 1
1414 PRINT #3, 1
1420 PRINT #3, I
1430 PRINT #3, "  Relative Intensity"
1440 PRINT #3, "Position <"; s0; " meters>"
1450 PRINT #3, n; "slits a="; amm; "mm d ="; dmm; "mm wave = "; w * 1E+09; " nm"
1455 PRINT #3, "Fraunhofer Diffraction"
1460 FOR j = 1 TO I
1470 PRINT #3, y(j), Intensity(j)
1480 NEXT j
CLOSE #3
1485 PRINT "Now do the plot using WPLOT in windows."
CHAIN "tek2wplt.bas"
1490 STOP
1500 END

SUB Pager
PRINT " Push RETURN to Continue";
INPUT DUN$
PRINT CHR$(12);
END SUB