A second ray of red light strikes the prism at the point P as shown below. Continue the path of the second ray, incident at P, through and out of the glass prism.

Hayward’s prisms were based on total internal reflection which occurs when light tries to exit the glass above a critical angle. Eventually, prism glass was also applied to vertical windows with ...

is called the critical angle. The diagram below shows the light refracting from glass into air. For light travelling from glass into air the angle of refraction is greater than the angle of incidence.