The ‘refraction of light’ is defined as the change in direction of a ray of light as it passes from one medium to another, say, air to glass or vice versa. Refraction occurs because light travels at different speeds through transparent materials. For example, light moves at about 300,000 km per second through air but at a much lower speed through water. When it enters water it slows down, which makes it change direction. The ray of light can bend either towards or away from the normal. The normal is defined as the perpendicular line to the interface of the two media. When a ray of light passes from air into glass, it bends towards the normal. When it passes out into air again, it bends away from the normal. In other words, when a ray of light passes from a rarer to a denser medium, it bends towards the normal and when it is the other way round, it bends away from the normal. Also when a ray passes from one medium to another, its speed also changes. If the ray goes from a rarer to a denser medium its speed decreases and vice versa.
There are two basic laws of refraction. The first law states that the ray that hits the surface, called the incident ray and the ray that travels in the second medium called the refracted ray, and the normal – all lie in the same plane. The second law is called the Snell’s law which states that the ratio of the sine of the angle of incidence to the size of the angle of refraction is constant. The size of the constant depends on the two materials through which the light is passing. It is termed as the refractive index between the two materials.
The phenomenon of refraction explains the cause of many interesting facts. For example, when we put a pencil in a glass full of water, it appears bent near the surface of water due to refraction. A coin in a bucket of water appears risen due to this phenomenon. The mirage in the deserts is also caused by refraction and total internal reflection of light. The twinkling of stars is caused due to the refracted light beams in the different layers of the atmospheric air.
