Light and lasers
Light rays and infrared rays change speed when they pass from glass to air, or Perspex to air. This is because air and glass - or Perspex - have different densities. The rays change direction if they hit the boundary of the material at an angle other than 90°. Beyond a certain angle, called the critical angle, all the rays reflect back into the glass or Perspex. This is called total internal reflection, TIR.
Have a go at the animation to check your understanding of this.
All light rays that hit the surface beyond the critical angle are effectively trapped. The critical angle for glass is about 42°.