Light passing to a medium of smaller index of refraction bends away from the normal. In this situation, the angle of incidence reaches an upper limit beyond which the light cannot escape the first medium. This limit is known as the critical angle . As long as is less than 90°, the path of the light makes physical sense. When is greater than 90°, since the angle of refraction is measured from the normal, the path of the refracted light remains inside the first medium. The upper limit for the angle of incidence must therefore occur when the angle of refraction equals 90°:
