Can black lines on a mirror cause diffraction of light?

[cragar]

I watched the Feynman lectures on light . And when he talks about a photon bouncing off a mirror, and that it usually bounces off at the angle it hits, But then he says I'm going to do a dirty trick and paint black lines in the mirror. Like if he paints enough lines the light will diffract.
My question is if I have a laser and I shine it in between two black lines, but the red dot that the laser makes is in between the lines, will this affect the path of the photons. And what about if i paint the lines like 1 inch apart? He makes it sound like it will because it will affect all the possible paths when we average the amplitudes.

 

[Claude Bile]

The light will diffract regardless... the purpose of the black lines is to act as an amplitude grating; (spatial) parts of the wave are reduced to zero, while other parts remain unaffected.

If you shine a laser between two black lines, they will essentially act like a slit. If you paint the lines really far apart, they will still act like a slit, but then, according to the laws of diffraction, a large slit is almost equivalent to no slit at all...

Claude.

 

[cragar]

OK thanks for you answer, I appreciate it