Abstract
There are many analogies between Interference patterns and Moire patterns. Let a laser beam be represented by a transparent tape covered with a set of black stripes, that are perpendicular to the length of the tape. If the stripes are straight, parallel and have a constant separation, the tape represents a light beam that is both spatially and temporally coherent. If two such tapes intersect, moire fringes are formed that bisect the angle between them. These fringes remain stationary even if the two tapes move in a direction parallel to their length. This is true for any velocity, as long as it is the same for the two tapes. The situation is the same for interference fringes, they bisect the angle between the two beams and the separation of the two types of fringes also obey the same equations. The interference fringes are stationary, even in spite of the fact that the two beams both move with the impressive velocity of 300 000 km/sec. If you move one of the tapes seven stripes more than the other, then seven moire fringes will pass a stationary point. It is the same with interferometry, if you move one mirror seven half waves then seven interference fringes will pass a stationary point. That is the reason why we can measure velocity by use of the Doppler shift in spite of the fact that light has the impressive frequency of more than 100 000 000 Mc.