Movable mirror of a Michelson interferometer

[oaly]

i know the question may be a bit stupid to some people, but i seriously don't know how to do it...

Homework Statement

How far must the movable mirror of a Michelson interferometer, illuminated by a 589-nm source, be displaces for 2000 fringes to move past the reference point?

Homework Equations

i have no idea

The Attempt at a Solution

i don't know how to solve it

 

[ideasrule]

What's the path length difference associated with 2000 fringes?

 

[oaly]

What's the path length difference associated with 2000 fringes?

it does not say, that was the whole question.

 

[ideasrule]

Yes, but it does tell you the wavelength, and constructive interference occurs when the path length difference is an integer multiple of the wavelength.

 

[bshweta]

can anyone answer this question
In Sagnac interferometer replace the beam splitter by a fourth mirror and calculate how many loops around the interferometer can the light make before its intensity is reduced by 99%??