silvered or unsilvered surfaces

With similar plates, r = r ^$\prime$, t = t ^$\prime$, and the phase difference $\delta$ is for light passing through the gap at an angle $\theta$ to the normal is

 

$$\delta {2 \pi \over \lambda} \theta \phi$$ (1)

where $\phi$ accounts for any phase change on reflection - if the mirrors are coated so as to increase the reflection coefficients $\phi$ may be between 0 and $\pi$ , rather than being exactly $\pi$ .

As the phase shift depends on the angle $\theta$ , we see circular fringes, but because of the form of the Airy function they will be sharper than those in the Michelson interferometer.