Here's why it's needed:
* Optical Path Length: When light travels through a medium like glass, it slows down due to the refractive index. This change in speed affects the optical path length, which is the distance light travels in a vacuum multiplied by the refractive index of the medium.
* Unequal Path Lengths: In a Michelson interferometer, one beam travels through two glass plates (the beam splitter and the mirror), while the other beam only travels through one. Without the compensating plate, this would result in a difference in the optical path lengths between the two beams.
* Incoherence: This difference in path lengths would lead to a phase shift between the two beams, causing them to interfere less effectively, resulting in a less distinct and visible interference pattern.
The compensating plate acts as a "dummy" for the second glass plate, effectively equalizing the optical path lengths:
* It's made of the same type of glass as the beam splitter.
* It's oriented at the same angle as the beam splitter, ensuring the same refractive effect on the beam passing through it.
In essence, the compensating plate compensates for the extra glass traversed by one beam, ensuring that both beams experience the same optical path length, thus preserving the coherence and visibility of the interference pattern.
Without this compensating plate, the interference pattern would be blurry and difficult to observe, rendering the Michelson interferometer less effective.
