Refraction of light
The beam changes direction (blocking) at the junction of two optical media (e.g. glass / air) with different optical thicknesses.
The blocking degree depends on the optical thickness ratio of the two media and the incident angle on the optical axis. If the light beam from a light dense medium "n" to a light sparse medium "n", there will be a larger incident angle.
However, above the critical point (the boundary angle when the propagation direction of the broken light is parallel to the boundary layer), the beam enters the medium with thickness N again, which means that the full emission is about to occur.