10 CLS:SCREEN 1
20 LINE (0,0)-(320,10),,BF
30 LINE (0,190)-(320,200),,BF
40 LINE (0,0)-(5,200),,BF
50 LINE (315,0)-(320,200),,BF
60 LINE (40,45)-(280,50),,BF
70 LOCATE 4,15:PRINT"THE  RAINBOW"
80 GOSUB 1000
90 LOCATE 8,11:PRINT"PARAMETER YOU CHOOSE"
95 LOCATE 10,5:PRINT"1. DROP RADIUS"
98 LINE (40,90)-(280,95),,BF
99 GOSUB 1000
100 LOCATE 14,6:PRINT"WATCH EACH RAY STRIKE THE DROP"
110 LOCATE 16,12:PRINT"PAY ATTENTION TO -"
112 LINE (80,130)-(240,133),,BF
115 GOSUB 1000
120 LOCATE 19,3:PRINT"1. D - THE MAXIMUM DEVIATION ANGLE"
125 GOSUB 1000
130 LOCATE 21,3:PRINT"2. I - INCIDENCE ANGLE OF RAINBOW RAY"
140 GOSUB 1000
150 SCREEN 1:CLS
160 LOCATE 5,9:PRINT"CHOOSE RAINDROP RADIUS"
162 LOCATE 10,5:PRINT"ACCEPTED SIZE RANGE 1 < R < 90"
163 GOSUB 1000
164 LOCATE 13,3:PRINT"SMALL DROPS SHOW DIVERGENCE OF RAYS"
165 GOSUB 1000
166 LOCATE 15,5:PRINT"LARGE DROPS SHOW PATHS OF RAYS"
167 GOSUB 1000
168 LOCATE 18,17:INPUT"R = ",RC
170 XC=310-RC:YC=10+RC:LL=210-RC
179 CLS
180 CIRCLE (XC,YC),RC,,,,.9
190 PI=3.141593
200 N=1.33:REM INDEX OF REFRACTION
210 DRC=PI/180
215 IF RC>30 THEN 500
220 FOR K = 0 TO 90 STEP 6
230 KR=K*DRC
240 I = PI/2-KR
250 Y=YC-.9*RC*COS(KR)
260 X=XC-RC*SIN(KR)
280 RNV=SIN(I)/N
290 R=ATN(RNV/SQR(-RNV^2+1))
300 RF1=2*R-I:RF2=4*R-I-PI:RF3=RF2-I
302 LOCATE 1,1:PRINT"                                      "
305 LOCATE 1,1:PRINT"I =";I/DRC;"    D =";RF3/DRC+PI/DRC
308 LINE (50,Y)-(X,Y)
310 LINE (X,Y)-(XC+RC*COS(RF1),YC-.9*RC*SIN(RF1))
320 LINE (XC+RC*COS(RF1),YC-.9*RC*SIN(RF1))-(XC+RC*COS(RF2),YC-.9*RC*SIN(RF2))
330 XD=XC+RC*COS(RF2):YD=YC-.9*RC*SIN(RF2)
340 LINE (XC+RC*COS(RF2),YC-.9*RC*SIN(RF2))-(XD+LL*COS(RF3),YD-.9*LL*SIN(RF3))
360 GOSUB 1000
370 NEXT K
380 END
500 FOR K = 10 TO 90 STEP 10
510 KR=K*DRC
520 I = PI/2-KR
530 Y=YC-.9*RC*COS(KR)
540 X=XC-RC*SIN(KR)
550 RNV=SIN(I)/N
560 R=ATN(RNV/SQR(-RNV^2+1))
570 RF1=2*R-I:RF2=4*R-I-PI:RF3=RF2-I
572 X1=XC+RC*COS(RF1):DX1=(X1-X)/100
573 Y1=YC-.9*RC*SIN(RF1):DY1=(Y1-Y)/100
574 X2=XC+RC*COS(RF2):DX2=(X2-X1)/100
575 Y2=YC-.9*RC*SIN(RF2):DY2=(Y2-Y1)/100
576 X3=X2+LL*COS(RF3):DX3=COS(RF3)
577 Y3=Y2-.9*LL*COS(RF3):DY3=-.9*SIN(RF3)
580 LOCATE 1,1:PRINT"                                      "
590 LOCATE 1,1:PRINT"I =";I/DRC;"    D =";RF3/DRC+PI/DRC
595 FOR J=0 TO 300
597 IF X<50+J THEN 605
600 PSET (50+J,Y)
602 NEXT J
605 FOR J = 0 TO 100
610 PSET (X+J*DX1,Y+J*DY1)
612 NEXT J
615 FOR J = 0 TO 100
620 PSET (X1+J*DX2,Y1+J*DY2)
622 NEXT J
625 FOR J = 0 TO LL
630 PSET (X2+J*DX3,Y2+J*DY3)
632 NEXT J
650 NEXT K
660 END
1000 FOR I = 1 TO 2000
1010 NEXT I
1020 RETURN
 J = 0 TO LL
630 PSET (X2+J*DX3,Y2+J*DY3)
632 NEXT J