Refraction

• Light bends towards the normal and slows down when it moves from a fast medium (like air) to a slower medium (like water).
• Light bends away from the normal and speeds up when it moves from a slow medium (like diamond) to a faster medium (like glass).
• These phenomena are governed by Snell’s law, which describes the relationship between the angles of incidence and refraction.
• The refractive index (index of refraction) of a medium indicates how much the speed and direction of light are altered when travelling in or out of a medium.
• It is calculated by dividing the speed of light in a vacuum by the speed of light in the material.
• Snell’s law relates the angles of incidence and refraction to the refractive indices of the two media involved.
• Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the refractive indices.

Refraction in practice

• In practice, resources such as refractiveindex.info contain databases of the refractive index of most media through which light propagates.
• To determine the direction of light bending at the boundary between transparent media, we need to know:
  • Which medium is faster and less optically dense, with a smaller refractive index?
  • Which medium is slower and more optically dense, with a higher refractive index?
• Snell’s law, (also known as the law of refraction), describes the relationship between the angle of incidence and the angle of refraction of light as it passes through the boundary of two media with different refractive indices.
• Snell’s law relates the angles of incidence and refraction to the refractive indices of the two media involved.
• It can be understood based on the principle of least time, formalized by Fermat’s Principle.
• Fermat’s Principle states that light travels between two points along the path that requires the least time.
• As a consequence, when light encounters a boundary, it bends in a way that minimizes its overall travel time according to Fermat’s Principle.
• This bending behaviour is then described mathematically by Snell’s Law.