When one arm gets longer the laser takes a little longer to travel through it. That changes the interference pattern.<p><a href="http:&#x2F;&#x2F;www.phdcomics.com&#x2F;comics.php?f=1853" rel="nofollow">http:&#x2F;&#x2F;www.phdcomics.com&#x2F;comics.php?f=1853</a>