# Why do fringes appear, as opposed to a simple spot, in a Michelson interferometer?

A simple Michelson interferometer is generally presented like the one bellow

If we observes through the telescope, we see fringes like these

Is it the lens of the telescope that form theses fringes? Because if we use a monochromatic light source, say a laser, the result we are supposed to see is simply a spot that would be bright or dim depending on the kind of interference, not fringes. So is there some other optical equipment between the final beam and the telescope? Or is it just the lens of the telescope that causes these fringes, as opposed to a spot, to appear?

• I am honestly still confused about why there should be any fringes at all if mirror 2, for example, is $\lambda/2$ away from the beam splitter than mirror 1 is, then the resulting optical path difference applies to all the rays of light that hit the screen, thus all the rays should either constructively interfere or destructively interfere, and so we should have either a bright spot, or a dark spot, regardless of the beam's "thickness". Feb 6, 2020 at 22:27