At a glass-air interface, Fresnel’s equations explain how light behaves when it passes from glass (a denser medium with a higher refractive index) into air (a rarer medium with a lower refractive index), or vice versa.
When light travels from air to glass, a portion of the light is reflected at the surface, and the rest is transmitted into the glass, bending toward the normal. Fresnel’s equations determine how much of the light is reflected and how much is transmitted based on the angle of incidence and the polarization of the light.
When light travels from glass to air, more complex behavior can occur:
- At small angles of incidence, part of the light is transmitted into the air and part is reflected back into the glass.
- As the angle increases, the amount of transmitted light decreases, and reflected light increases.
- At a specific angle called the critical angle, the transmitted light travels along the boundary.
- Beyond this angle, total internal reflection occurs — all the light is reflected back into the glass and none is transmitted into the air. Fresnel’s equations confirm that the transmission drops to zero in this case.
Thus, at a glass-air interface, Fresnel’s equations help calculate reflection and transmission ratios, predict total internal reflection, and explain how these depend on angle and polarization.