Why is the color white at the centeral bright fringe when a white(polychromatic) light passes through a small aperture
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$\begingroup$ Did you mean when a polychromatic light passes through a small aperture? $\endgroup$– John RennieCommented Mar 25, 2017 at 9:02
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$\begingroup$ @JohnRennie oh yes polychromatic. I editted the question $\endgroup$– Abed ShaarCommented Mar 25, 2017 at 9:04
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$\begingroup$ The center of the zeroth order maximum should be about as white as the incoming light. Moving to the side I would expect a red stripe, followed by white-ish or even slightly blue edge. $\endgroup$– Arturs C.Commented Apr 21, 2018 at 19:29
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$\begingroup$ Actually, the middle bit might appear slightly blue, since the central maximum for "blue" wavelengths is much narrower than for the "red" wavelengths. $\endgroup$– Arturs C.Commented Apr 21, 2018 at 19:38
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$\begingroup$ White is a mixture of wavelengths. All of them have the zero position at the same spot so the light is white there as well (approx). The higher order positions differ by wavelength, so a coluring is visible $\endgroup$– bukwyrmCommented Jul 30, 2018 at 10:48
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2 Answers
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In diffraction and interference experiments with the white light. Central fringe is white because all wavelength has same path difference on optical axis goes through the center of the system of apertures.
But as you go far from the center axis, optical path difference varies for different wavelength and different color get separated.
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$\begingroup$ It’s also interesting how the colors mix as you move further out. Noticed that the order of colors is not the same as the normal spectrum from red to blue. $\endgroup$ Commented Sep 12, 2018 at 15:03
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$\begingroup$ Fringes positions is a function of wavelength and simply when the phase related to the optical path difference is an integer cross 2pi we have a peak. So in this way after white spot in center we have blue, and so on... to red. $\endgroup$ Commented Sep 12, 2018 at 15:10
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$\begingroup$ @BillAlsept See the diagram I have added to the answer. The separation clearly demonstrated over there $\endgroup$ Commented Sep 12, 2018 at 15:16
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I have yet to see such a thing happening.
Monochromatic means light of one wavelength and so all the fringes will be of the same colour.
The only reason why the central maximum might appear to be of a different colour from the rest of the fringes is if you are looking at the interference pattern directly with the eye or a camera and the intensity of the central fringe is so great as to completely overwhelm the receptors of light on the cornea/ccd screen.
Because of the enormous contrast between the central maximum and the subsidiary maxima it is very difficult to photograph such interference patterns because if one part the exposure is correct another part will be grossly under/over exposed.
