Understanding Interference Fringes in a Michelson Interferometer

[rspandher]

how can you tell whether the interference fringes are moving in or out as you change the distance between the mirrors by rotating the knob clockwise or counterclockwise. say if the circular fringes are moving toward the center of the screen than in what direction the knob is being rotated. my teacher couldn't even explain this to me

 

[damgo]

When the distance difference d between the two legs of the interferometer is 0, there will be no fringes. Look at the equation for the radius of the smallest fringe -- it's proportional to 1/d, right? So as you increase the distance difference, the fringes get smaller, ie, move inwards.