I am studying about Michelson interferometer from Pedrotti and they transformed a Michelson interferometer into an equivalent system consisting of light rays originating from two virtual images of the sources.
When can such conversions into equivalent systems attempted and why ?
The actual interferometer in Figure 11-la possesses two optical axes at right angles to one another. A simpler but equivalent optical system, having a single
optical axis, can be displayed by working with virtual images of source 5 and mirror M1 via reflection in the BS mirror. These positions are most simply found by
regarding the assembly including S, M1, and beams 1 and 3 of Figure 11-la as rotated counterclockwise by 90° about the point of intersection of the beams with the BS mirror. The resulting geometry is shown in Figure 11-lb. The new position of the source plane is S', and the new position of the mirror Ml is MГ. Light from a point Q on the source plane S' then effectively reflects from both mirrors M 2 and Ml', shown parallel and with an optical path difference of d. The two reflected beams appear to come from the two virtual images, Q'l and Qî, of object point Q.