Physics Help Forum Diffraction

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 Jun 2nd 2008, 06:14 PM #1 Senior Member   Join Date: Apr 2008 Location: HK Posts: 886 Diffraction A beam of red light passing through very thin slit shows alternate dark and bright fringes on screen. I don't understand why there would be a drop in brightness of fringes away from centre of fringes? (Book suggested that we should divide the slit opening in several parts and wavelets from one part arrive at particular position a half wavelength behind from the second part so these parts interfere destructively) It seems that it is a must for some parts lagging behind half wavelength. Is it necessary? In addition, would the reflected light rays from screen interfere with the incident rays?
Jun 3rd 2008, 04:24 AM   #2

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 Originally Posted by werehk A beam of red light passing through very thin slit shows alternate dark and bright fringes on screen. I don't understand why there would be a drop in brightness of fringes away from centre of fringes? (Book suggested that we should divide the slit opening in several parts and wavelets from one part arrive at particular position a half wavelength behind from the second part so these parts interfere destructively) It seems that it is a must for some parts lagging behind half wavelength. Is it necessary? In addition, would the reflected light rays from screen interfere with the incident rays?
This is the best diagram I could find. (I'm too lazy to draw one of my own.) The idea is that the single slit is "wide" and that we get interference between the light coming from either side of the slit. The simplest analysis of this is kind of like two slit interference in this regard, but you also have to remember the central max in this case, so we have constructive and destructive interference. In case you are wondering, yes we also get this with the double slit and diffraction gratings. But the slits are so much thinner in these cases I doubt you could ever observe them.

As far as the reflected waves interfering with the incoming waves, of course there will be some interference. However this interference will not be observed since it will take place in front of the screen and not on it. (I suppose you could smoke up the room and see it that way.)

-Dan
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