Refraction is the bending (change in the direction) of a wave (light or sound). When a light ray passes from one transparent medium to another, it changes speed and bends.
- Light traveling from air and going into water – half submerged straight stick or pencil into water. The stick appears bent at the point it enters the water.
- Light traveling from air and going into glass – the letters appears to be raised, when seen through a glass slab placed over it.
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- Refraction is responsible for image formation by lenses and the eye.
- Optical density is the ability of a medium to refract light.
When a light ray passes from a substance of one density into a substance of different density, the speed of light changes and bending of light occurs.
In air, light travels at 3 x 108 m/sec but in water, it travels only about 2.258 x 108 m/sec. Since light travels slower in water than in air, water is said to have a greater optical density than air.
Substance Speed of light (m/sec)
Vacuum 3 x 108
Air 3 x 108
Water 2.258 x 108
Glass 2 x 108
The change in speed causes the direction of light ray to change. When we look a pencil in a glass of water, the light from the pencil in the water is being refracted, so it appears in a different location. The result is the illusion of a broken pencil.
Consider a single ray of light strikes a boundary between two mediums and is refracted.
- The light ray which travels through the incident medium or first medium and strikes the boundary or interface is called the incident ray.
- The light ray which travels into the second medium and leaves the boundary or interface is called the refracted ray.
- An imaginary line drawn perpendicular at the point of refraction ’R’, is called a normal.
- The angle between the incident ray and the normal is called the angle of incidence or the incident angle ( i ).
- The angle between the refracted ray and the normal is called the angle of refraction or the refracted angle ( r ).
- The light beam can bend towards the normal or away from the normal.