Refraction is the bending of a light ray as it travels through the surface of a transparent material. Refraction can distort the apparent size, shape, and location of an object when light travels from one material to another. Refraction will occur only when a light ray travels through a surface at an angle other than a right angle. Different materials bend rays by different amounts.
Refraction occurs because light travels at different speeds in different materials. The speed of light in a vacuum is 186,282 miles (299,792 kilometers) per second. This value is commonly given the symbol c. The speed of light in a material is v=c/n where n is the refractive index of the material. The refractive index of a vacuum is exactly 1. The index of air at sea level is 1.000293. Water has a refractive index of 1.33. The indexes of most kinds of glass are around 1.5 to 1.6.
An equation known as Snell’s law gives the relationship between two factors involved in refraction: (1) the amount of bending that occurs when a light ray passes from one material to another and (2) the indexes of refraction of the two materials. Snell’s law is commonly given as nisin(i)=nrsin(r) The terms ni and nr represent the indexes of refraction of the first and second materials, respectively. The letter i represents the angle of incidence. This is the angle between the ray traveling through the first material and the normal, a line perpendicular to the point where the ray enters the second material. The letter r represents the angle of refraction, the angle between the normal and the ray traveling through the second material.
The expressions “sin i “ and “sin r “ are the sines of the two angles. A sine is a function in a branch of mathematics known as trigonometry. As an angle increases from 0° to 90°, its sine increases from 0 to 1.
Suppose a ray travels from air into a typical glass at a given angle and another ray travels from air into water at the same angle. Because nr for glass is larger than nr for water, sin r for glass must be smaller than sin r for water. Therefore r for glass will be smaller than r for water. A smaller r indicates a larger amount of bending toward the normal, and so the glass will bend the light more than the water will bend it. Snell’s law is named for the Dutch mathematician Willebrord Snell van Roijen, who formulated it around 1621.
