1. Multiple Beam Interference:
* Light enters the Fabry-Pérot etalon (the space between the mirrors) and undergoes multiple reflections between the mirrors.
* Each reflection introduces a phase shift, and the reflected beams interfere with each other.
2. Path Difference and Phase Shift:
* The path difference between successive beams depends on the angle of incidence (θ) and the distance between the mirrors (d).
* This path difference results in a phase shift between the interfering beams.
3. Constructive Interference:
* When the path difference is a multiple of the wavelength (λ), the phase shift is a multiple of 2π, resulting in constructive interference. This means the beams add up, creating bright fringes.
4. Destructive Interference:
* When the path difference is an odd multiple of half the wavelength (λ/2), the phase shift is an odd multiple of π, resulting in destructive interference. This means the beams cancel each other out, creating dark fringes.
5. Circular Fringes:
* The path difference and phase shift are dependent on the angle of incidence (θ).
* For a given wavelength (λ) and mirror separation (d), there are specific angles (θ) that lead to constructive interference.
* These specific angles create circles of constructive interference, forming the circular fringes.
6. Air Wedge:
* The Fabry-Pérot interferometer essentially creates an air wedge between the mirrors.
* The angle of incidence changes across the air wedge, leading to varying path differences and thus different interference patterns.
7. Fringeness:
* The sharpness of the fringes (fringeness) is determined by the reflectivity of the mirrors.
* Higher reflectivity results in sharper, more intense fringes, because more beams contribute to the interference pattern.
In summary: The circular fringes in a Fabry-Pérot interferometer are a consequence of the interference of multiple beams reflected between the mirrors. The path difference between these beams depends on the angle of incidence, leading to constructive interference at specific angles and thus the formation of circular fringes.
