1, the absolute error is certain, the greater the n, the smaller the relative error and the more accurate the measurement. 2. When M2 is strictly perpendicular to M 1, that is, M2ˊ is strictly parallel to M 1, the interference obtained is isoclinic interference. Interference fringes are concentric rings of light and dark located at infinity or on the focal plane of a lens. The characteristics of interference ring are: sparse inside and dense outside According to the theory of equal inclination interference, when the distance d between M 1 and M2' decreases, the radius of any designated K-level fringe will decrease and gradually shrink and disappear at the center, that is, the fringe will "sink"; When d increases, that is, the stripes are "exposed", the greater the thickness of M 1 and m2', the smaller the distance between adjacent bright (or dark) stripes, that is, the denser the stripes, the more difficult it is to identify. Whenever a ring "sinks" or "pops up", D correspondingly increases or decreases the distance of λ/2. If the number of "trap" rings or "exit" rings is n and the change of d is δD, then δD = N *λ/2.

Then: λ = 2 δ d/n

If δ d and n are known, λ can be calculated. 3. According to the theory of isoclinic interference, when the distance d between M 1 and M2' decreases, the radius of any designated K-level fringe will decrease, and gradually shrink and disappear at the center, that is, the fringe will "sink in" (is this what you call "grey annihilation"? ); When d increases, the stripes are "used up". When M2 moves from left to right, "null return error" will appear. Therefore, after stopping at a certain position, there may be stripes or annihilation due to "empty return error".