We get to see diffraction patterns when light from a coherent source passes through a thin slit. But consider a mirror with a line scratch on it (as shown in the figure) and we shine a red light from a coherent laser source near the scratch, will we get a diffraction like pattern on the screen ? Here's the picture of it :-
P.S. :- I didn't have the setup so I couldn't try it myself.
I was going to say no before I set up the apparatus.
I have used scratches made on a mirror surface to illustrate diffraction from one slit and interference from two slit but this is with light after it has passed through the mirror.
With reflected light from a mirror with two scratches on it I can get two slit interference but, as might be expected, the contrast is not as good.
I arranged for the mirror to be over the edge of a table and shone the laser beam, which was almost parallel to the mirror at the two scratches.
I could then see two interference patterns with the diffraction envelope, one above the plane of the mirror (reflected light) and one below the plane of the mirror (transmitted light), on a white wall close to the arrangement.
This arrangement made it easier for me to know what I was looking for with the reflected light as the transmitted light fringes were so clear.
I then tried the same arrangement with a single scratch.
With one slit there wasw a pattern both by reflected and transmitted light.
You will in principle see the diffraction pattern of a single slit or rather of its complement, a single line. However, your 'scratch' is likely to be much wider than the typical optical wavelength scale of 500 nanometer. Hence the diffraction pattern will be very narrow. Since you have only one scratch, the contrast will be very poor.
