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Please help!

Please keep the level at an introductory class level. Basic idea of coherence etc. Can't understand Quantum physics etc.

User1233
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1 Answers1

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When the film is many wavelengths in thickness, a small change in wavelength can make a big change in the amplitude. So, maybe 740 nm is suppressed but 750 nm is enhanced, but they both look red, so you don't perceive a color change. Your eyes don't have the spectral resolution.

Suppose the film is 1/2 wavelength in thickness, leading to constructive interference. You need to change the wavelength by 50% to reach fully destructive interference at 1/4 or 3/4 of a wavelength. Your eyes can easily perceive this difference in wavelength. But if the film is 25 wavelengths thick, the wavelength only has to change by 1% to reach 24+3/4 or 25+1/4 waves per film thickness. Your eyes cannot perceive such a slight change. Change the wavelength by 2%, and you get back constructive interference.

John Doty
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  • Sorry I did not understand. How does a change in wavelength relate to a change in amplitude? Also, my specific question would be, why does all light constructively interfere and we get white light when the film is thick? – User1233 Oct 10 '22 at 11:50
  • @User1233 When you see red from thin-film interference, that means that the interference has enhanced red wavelenghts and suppressed other wavelengths. – John Doty Oct 10 '22 at 11:53
  • @User1233 It doesn't all constructively interfere, but your eyes' spectral resolution isn't good enough to distinguish between wavelengths where it constructively interferes and wavelengths where it destructively interferes. – John Doty Oct 10 '22 at 11:55
  • So we get smeared white light? Fringes aren't easily distinguishable? My doubt is what does increasing thickness do exactly to path difference or coherence that leads to a smeared interference pattern? – User1233 Oct 10 '22 at 12:09
  • @User1233 Not smeared. If you look at it with a spectrograph you see rapidly varying interference with wavelength. – John Doty Oct 10 '22 at 12:24