24. Interference


General Description

This program will prove invaluable to physics teachers trying to explain the thorny problems of interference to A-level classes. The program produces two sets of overlapping semi-circular waves as a simulation of two-slit diffraction. The simulation produces clearly identifiable interference patterns which can be used in parallel with verbal explanation. The user instructions go to some lengths to point students in the right direction, and ask the user to supply sample values for slit operation and wave-length.

Detailed Description

   Lines 60-380 Main Program.
   80-160 Instructions.
   170-230 Draw slits at required separation and display data supplied. Incident beam and diffracted waves are drawn. If requested, interference pattern is emphasised.
   240-380 Run program again? Line 320, INKEY waits 10 secs before returning null string.
   420-680 PROChelp: This explains the program's facilities.
   700-820 PROC data: This requests user data. D% = slit separation. L% = wavelength. The scaling factor of 2 is introduced so that the values of wavelength may be more realistic for visible light.
   840-990 PROCslits: Slits are drawn using command PLOT85.
   1010-1130 PROCbeam: The incident beam is assumed to be a parallel one. The spacing between waves is L% and the position of the waves is calculated to fit with the first diffracted wavelet. 1030 calculates position of first wave, 1040 the number of waves that will fit in available space.
   1150-1370 PROCwaves: Waves from left-hand slit drawn by 1200-1280. Right-hand slit waves are drawn 1290-1360.
   1390-1530 PROCrerun: Another set of waves to be super-imposed on the first?
   1540-1690 PROCmaxima: This procedure draws lines in the direction of constructive interference.

Educational Notes

In the classroom, this simulation could effectively replace existing audio-visual demonstrations. If used with A-Level groups (with a well-adjusted TV monitor), measurements of angle can be made directly from the screen to verify (or otherwise) the students' predictions.
   For the best results, I would suggest a slit separation of less than 400mm and wavelengths between 100 and 350mm. For example D%=300, L%=230mm.