The refraction of light can be demonstrated experimentally by shining a ray of light through a transparent block or water
The angle of incidence is the angle between the normal and the ray of light before refraction
The angle of refraction is the angle between the normal and the ray of light after refraction
Refraction occurs on the surface between two media when light changes speed, and because the nearer side of the ray changes speed earlier that the far side, the ray bends; exiting the medium causes the opposite to happen, exiting at the same angle as entering1
Refractive index
The refractive index of a material is equal to the speed of light in a vacuum divided by the speed of light in the material
Snell’s law states that the ratio of sines of the angles of incidence and refraction is equivalent to the ratio of the speed of light in the two media, as well as the reciprocal of the ratio of the indices of refraction
The critical angle is the angle above which total internal reflection occurs2
When a ray of light hits the surface between two media, internal reflection will occur proportional to the angle incidence; above the critical angle, all light is internally reflected in a process known as total internal reflection
Total internal reflection is used in optical fibres, where light rays are sent down fibres at shallow angles without loss of strength: this is used in medicine, to allow doctors to see inside a patient’s body using a small entryway, and communications technology, as a way of transmitting information by sending pulses of light (currently, internet speeds of over 100 petabits per second have been achieved)
This is because if the sine of the angle of refraction must be greater than the angle of incidence, at some point, the angle of incidence is large enough that the sine of the angle of refraction has to be above 1, which is impossible ↩