I have a question about a particular case of light diffraction. A laser light collides into a "hairline" mirror (I mean a straight line mirror that halves the wavelength dimension), see the image below. It's placed on a black wall that could absorb the whole light. For these reasons I suppose that the mirror reflects the light because it's a mirror, but the light interferes because the mirror is a barrier. Then I ask, what is the real phenomenon that occurs?