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In a double slit interference, looks like there is a minimum value of distance (d) between the slits, below which the dark fringes are not observed. What is that value of distance in terms of wavelength.?

user31058
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  • I'm curious to know what you have observed that indicates a minimum distance between slits, beyond which fringes are not visible. If you have seen this effect, I suspect it is due to the ratio between slit width and distance between slits. If the slit width is fairly close to the slit spacing, fringe contrast gets very low. – S. McGrew Oct 07 '18 at 18:29

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The fringe spacing is between the bright fringes caused by constructive interference. This spacing is called dark fringes as you say.

The separation of the maxima is

d f = λ/ ( sin ⁡ θ)

and df is known as the fringe spacing. The fringe spacing increases with increase in wavelength, and with decreasing angle θ.

So the answer to your question is, that the spacing can decrease by decrease in wavelength, or increase in angle. If you would like to find the minimum, there is no theoretical limit. You could in theory decrease the wavelength as close to zero and increase the angle as close to 180 as you can get.

In practice, though, the wavelength of photons that we are able to create in experiments is limited to a certain minimal wavelength, and the angle is technically limited too.

Árpád Szendrei
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