DECLARE FUNCTION ASIN! (x)
DECLARE SUB pager ()
100 PRINT CHR$(12)
pi = 4! * ATN(1)
105 PRINT "FRESNEL'S FORMULAS"
110 PRINT
120 PRINT " Let a ray of light be incident from a medium of index n1 onto"
125 PRINT "a plane surface that separates it from a medium of n2. The incident"
130 PRINT "angle be 01, the reflected angle = 01' & the refracted angle is 02"
131 PRINT
135 PRINT " The incident ray may be either s or p polarization, that is,"
140 PRINT "       s - polarized has the E vector parallel to the surface"
145 PRINT " or"
150 PRINT "       p - polarized has the H vector parallel to the surface."
160 PRINT " This program is designed to evaluate the reflection & transmission"
165 PRINT " properties of nonconducting media via an evaluation of the FRESNEL"
170 PRINT " coefficient, r and t. Other programs in the OPTICS SERIES (See HELP"
175 PRINT " PHYS), evaluate these functions for many angles and provide"
180 PRINT " graphical display of the results."
190 CALL pager
200 PRINT "Input indicies of refraction n1 and n2? [ n1 <0 STOPS program]"
210 INPUT n1, n2
211 IF n1 < 0 THEN STOP
215 IF n1 >= 1! AND n2 >= 1! THEN GOTO 300
220 PRINT "n1 and n2 must both be > 1."
230 GOTO 200
300 PRINT "Input in degrees the incident angle, 01?"
305 INPUT ang1
306 theta1 = ang1 * pi / 180!
310 theta2 = ASIN((n1 / n2) * SIN(theta1))
320 ang2 = theta2 * 180 / pi
325 PRINT "The refractive angle is "; ang2; "  degrees."
350 PRINT "Input the state of polarization? (s or p)?"
360 INPUT P$
370 IF P$ = "s" THEN 400
380 IF P$ = "p" THEN 500
390 GOTO 350
400 REM   s polarization                                                      
405 r12s = SIN(theta2 - theta1) / SIN(theta2 + theta1)
406 R = r12s ^ 2
410 t12s = 2! * COS(theta1) * SIN(theta2) / SIN(theta2 + theta1)
420 PRINT "RESULTS:   r12s ="; r12s; "  t12s = "; t12s
430 PRINT " R= r**2 = the REFLECTANCE."
440 PRINT " R = "; R
441 PRINT " The TRANSMITANCE is NOT t**2!!! But is 1-R for nonconducting media."
442 PRINT " For s polarization, T (s) is given by"
446 PRINT "                      n2 cos(02)        2"
448 PRINT "        T (s) =   ---------------  t12s"
450 PRINT "                      n1 cos(01)"
460 PRINT
462 T = ((n2 * COS(theta2)) / (n1 * COS(theta1))) * t12s ^ 2
464 PRINT "T for s = "; T
466 PRINT "R+T="; R + T
470 CALL pager
472 GOTO 200
500 REM                                                                       
501 REM   p polarization                                                      
505 r12p = TAN(theta1 - theta2) / TAN(theta2 + theta1)
506 R = r12p ^ 2
507 t12p = COS(theta1) * SIN(theta2) / (SIN(theta2 + theta1) * COS(theta1 - theta2))
508 t12p = 2! * t12p
520 PRINT "RESULTS:   r12p ="; r12s; "  t12p = "; t12p
530 PRINT " R= r**2 = the REFLECTANCE."
540 PRINT " R = "; R
541 PRINT " The TRANSMITANCE is NOT t**2!!! But is 1-R for nonconducting media."
542 PRINT " For p polarization, T (p) is given by"
546 PRINT "                      n2 cos(02)        2"
548 PRINT "        T (p) =   ---------------  t12p"
550 PRINT "                      n1 cos(01)"
560 PRINT
562 T = ((n2 * COS(theta2)) / (n1 * COS(theta1))) * t12p ^ 2
564 PRINT "T for p = "; T
566 PRINT "R+T="; R + T
570 CALL pager
572 GOTO 200
610 END

FUNCTION ASIN (x)
ASIN = ATN(x / SQR(1 - x ^ 2))
END FUNCTION

600 SUB pager
602 PRINT , "Push RETURN to continue."
604 INPUT dum$
606 PRINT CHR$(12)

END SUB