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It is known that for a diffraction pattern to be strong, the wavelength of incoming laser light should be comparable to the slit size.

The reason being if the slit is very narrow it effectly is a single point source and almost an infinitely broad central maxima is on the screen.

While if the slit is much larger, the wave is essentially travelling unobstructed through the slit gap.

What I want to know is what does "comparable" mean here exactly?

What orders of difference is not too much?

The question was prompted because of an experiment I did in the lab.

I had a 650nm laser but a 50 micron slit.

I could see the diffraction pattern on the screen (atleast the first three fringes on both sides) without any difficulty.

There is like a 100 order difference (650 nm is .65 micron) yet the pattern was clearly visible.

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    $\begingroup$ All you need is in the formula for intensity at angle $\theta$ to the normal for Fraunhofer diffraction at a single slit. It's a standard result. $\endgroup$
    – Philip Wood
    Commented Apr 4, 2022 at 12:03
  • $\begingroup$ @PhilipWood How does that address the fact that even when the slit was not comparable to the wavelength I was still getting a visible fringe pattern? $\endgroup$
    – Lost
    Commented Apr 4, 2022 at 13:28
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    $\begingroup$ @Lost You may be interested in the derivation of the individual fringes. See “Single Edge Certainty” at Billalsept.com $\endgroup$
    – Bill Alsept
    Commented Apr 4, 2022 at 21:59

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Diffraction is caused by the edges of the slit. When the "slit" is much wider than the wavelength of the light, then most of the light passing through is not close enough to an edge to be diffracted.

What I want to know is what does "comparable" mean here exactly?

If you are comparing the intensity of the diffraction pattern to the intensity of the un-diffracted light, then the wider you make the slit, the more un-diffracted light gets through, and the greater the ratio of brightness.

Possibly also complicating things: The farther apart you make the two edges, the finer the pattern of fringes will be. Trying to see that fine pattern, when it is adjacent to a much brighter spot can be a challenge.

I had a 650nm laser but a 50 micron slit. I could see the diffraction pattern on the screen (at least the first three fringes...

Sounds like the bright spot in the center of the pattern was not bright enough to completely overwhelm your optical system and prevent you seeing the fringes.

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  • $\begingroup$ I like this answer and the way it focuses on edges instead of slits. Every type of diffraction pattern (single, double or multiple slit) begins with single edge Diffraction patterns. $\endgroup$
    – Bill Alsept
    Commented Apr 4, 2022 at 15:36
  • $\begingroup$ "Sounds like the bright spot in the center of the pattern was not bright enough to completely overwhelm your optical system and prevent you from seeing the fringes."....... So essentially, if I had made the two comparable the fringes would have been better and the ones I saw were actually the bad ones (but somehow I saw the other three fringes because of the low intensity of my input laser)? $\endgroup$
    – Lost
    Commented Apr 4, 2022 at 15:42

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