Refraction

Refraction is the change in direction of a beam of light. Refraction occurs when a light ray changes mediums. This is most commonly observed when a ray or beam of light passes from one medium to another, as when light traveling from air goes into water. The speed of the light beam changes when it changes mediums, and so does the direction of the light beam.

If someone half-submerges a pencil into a glass of water, he will notice that the straight pencil appears bent at the point it gets into the water. This optical effect is caused by refraction. When light passes from one transparent medium to another, it changes speed, and bends. How much it appears to bend depends on the refractive index of the mediums and the angle between the light ray and the line perpendicular (normal) to the surface separating the two mediums.

In 1621, Willebrord Snell, a Dutch physicist, derived the relationship between the different angles of light as it passes from one transperent medium to another. When light passes from one transparent medium to another, it bends according to Snell's law which states: Ni * Sin(Ai) = Nr * Sin(Ar). where: Ni is the refractive index of the medium the light is leaving; Ai is the incident angle between the light ray and the normal to the meduim to medium interface; Nr is the refractive index of the medium the light is entering; Ar is the refractive angle between the light ray and the normal to the meduim to medium interface.

Refraction