I have been trying for hours to understand what is physically causing the interferometric rings to contract when the separation of the mirrors is reduced. From the equation: [itex]m\lambda = 2Lcos\theta[/itex], where [itex]m[/itex] is the number of fringes, if we consider just one fringe at a fixed wavelength, [itex]m\lambda[/itex] is constant and hence [itex]2Lcos\theta[/itex] is also constant. Hence reducing [itex]L[/itex] causes [itex]cos\theta[/itex] to increase, which is analogous to reducing [itex]\theta[/itex]. [Is this where I'm going wrong?] Question 1: When reducing d in the image above, does it matter if we are moving [itex]L_1[/itex] towards [itex]L_2[/itex] or vice versa? (Is it directionally dependent?) Question 2: The image suggests that [itex]\theta[/itex] is only linked to [itex]L_1[/itex]. If I move [itex]L_1[/itex] towards [itex]L_2[/itex], the adjacent side of the right-angled triangle is getting shorter, and hence [itex]\theta[/itex] must be increasing. But this goes against what happens when it is considered mathematically. How can we tell PHYSICALLY whether the rings are contracting or expanding? Thank you.