Total internal reflection (TIR) in prisms occurs when light enters the prism, strikes one of its internal surfaces at an angle greater than the critical angle, and is completely reflected back into the prism instead of passing through the surface.
A prism is typically made of a material like glass or acrylic, which has a higher refractive index than air. When light inside the prism hits an internal surface where the prism meets air, and the incident angle is large enough (greater than the critical angle specific to the material), TIR takes place.
In many right-angle prisms (like 45°-45°-90° prisms), light enters through one face, reflects off the hypotenuse face due to TIR, and exits through another face. No energy is lost through refraction at the hypotenuse face because TIR keeps the light inside.
TIR in prisms is used in optical instruments like binoculars, periscopes, and cameras. It ensures efficient and distortion-free light redirection without the need for reflective coatings. This makes prism-based systems more durable and optically precise compared to systems that use mirrors, which can degrade or scatter light.