Why do Michelson interferometer usually needs 2 mirrors?

[Jerrynap]

I've been wondering this for quite some time. Why do a typical set-up of Michelson interferometer usually requires 2 mirrors? Why not one for adjustment? Why not just shine the laser through the beam splitter?

Thanks.

 

[Mech_Engineer]

You need two mirrors to recombine the beam after the beamsplitter.

http://en.wikipedia.org/wiki/Michelson_interferometer

 

[Jerrynap]

I meant the mirrors used to adjust the light source before the beamsplitter... Just like the set-up in this video by PHYWE (although I'm not using their equipment, but the set-up is almost identical)