How Can Straight Line Fringes Be Observed in a Michelson Interferometer?

[ed2288]

Homework Statement

Explain how a pattern of straight line fringes of equal thickness can be observed in a michelson interferometer.

Homework Equations

see diagram 1/3 of the way down page at http://www.holo.com/holo//book/book6&7.html

The Attempt at a Solution

I really am stuck with this. From my understanding, light comes out of a laser and hits a beam splitter. One beam hits mirror 1, the other beam hits mirror 2 and they both bounce back to the splitter where they recombine, pass through the microscope objective and hit the screen. Now correct me if I'm wrong, but surely you will get total constructive interference if the path length difference due to the mirrors is one wavelength, and total destructive interference if its half a wavelength. I've read Hecht and looked online but I really can't seem to understand why you would even get concentric circle fringes!? Surely its got to be all dark or all light?

Thanks

 

[Ben Niehoff]

Laser light is not perfectly parallel. There is some small amount of beam spreading.

 

[Redbelly98]

Laser light is not perfectly parallel. There is some small amount of beam spreading.

That shouldn't matter, since both beams are spreading by the same amount.

It has more to do with the recombined beams not being in perfect alignment.