Other uses of total internal reflection (TIR) include the reflection of light in prisms. Whilst traditional mirrors can reflect light well enough for most purposes, they rely on a metallic reflective layer. Old mirrors used silver for this, which tarnishes (oxidises) in the air, reducing its effectiveness. More modern mirrors use aluminium. By clever use of TIR, we are able to do away with the reflective layer altogether:
The prism above (called a 45-90-45° prism because of its angles) acts as a mirror, reflecting the ray through 90°. This happens because the angle of incidence is greater than the critical angle.
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