Supposing in young’s double split experiment, I cover one slit with red filter and the other slit with blue filter. The light coming out from the first slit would be red and the second slit would be blue. Would there be any interference fringes? I tried googling this question but all the answers say that two different monochromatic lights cannot interfere and hence no interference pattern. But, we do know that if we did the experiment using white light, there is a pattern(for a few fringes). So what should be the right answer?
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2 Answers
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So the concept of double slit "interference" is historical dating back to Young and Fresnel (early 1800s)... and is still the basis for the typical high school/university textbook. When quantum mechanics came about in the early 1900s some of the most famous scientists (Dirac and others) pointed out that "each photon interfered with itself" which was just their way of saying each photon determines its own path. (But it was complex, subject to argument, and textbook writers were happy with what they had written.)
So photons do not cancel each other, that would be a violation of conservation of energy. When an atom/electron is excited it is already generating virtual EM forces .... when the EM forces see 2 slits they cause the pattern ... the real photon is eventually released and follows the pattern.
So when you filter the slits the red "to be" photons/electrons/atoms only see one slit ..... so no "interference".
Unfortunately there may very likely be high school textbook problems that want you to calculate an interference pattern based on the fact that the 2 colours have a different wavelength value .... it becomes more of a math problem than a physics problem!
answered Jan 28, 2023 at 20:43
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$\begingroup$ Thank you so much for answering! But could you explain it in a bit more simpler terms? I only recently learnt about the double split experiment so I am kind of confused. $\endgroup$– studentCommented Jan 29, 2023 at 0:33
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$\begingroup$ This was actually a question that came on my exam. The answer key said no interference pattern occurs since light of different wavelength and frequencies do not interfere. My teacher didn’t really explain further.. $\endgroup$– studentCommented Jan 29, 2023 at 0:35
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$\begingroup$ All the DSE formulas we learn in high school work pretty well to calculate the pattern .... it is just that so many teachers say the photons are cancelling each other which is not true. Each real photon is just following the forces the excited electron is generating in the EM field .... for forces scientists use the term "virtual" photons, there is no energy exchange at this early stage just pushing and pulling and most importantly oscillating like waves ... these virtual waves interfere and that's why the math works out. $\endgroup$ Commented Jan 29, 2023 at 3:11
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$\begingroup$ The statement "no interference pattern occurs since light of different wavelength and frequencies do not interfere" is NOT true. This is only true if we put a color filter over the slit(s). If there is no filter(s) we just get overlapping "interference" patterns as Bill Alsept mentions above. $\endgroup$ Commented Jan 29, 2023 at 3:17
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$\begingroup$ could you explain more on the statement "each photon determines it's own path?" $\endgroup$– studentCommented Jan 29, 2023 at 8:09
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You would have two single slit diffraction patterns overlapping on the screen. The two pattern have different spacings between their fringes. For example the blue spacings are smaller than the red spacings. Like any beat pattern you will find points where bright fringes from blue coincide with bright fringes from red. These points will form a double slit pattern with a color mix of magenta.
answered Jan 28, 2023 at 22:26