Although light travels in straight lines in a transparent material, such as air, if it passes into a different material, such as water, it changes direction at the boundary between the two, i.e. it is bent.
The bending of light when it passes from one material (called a medium) to another is called the refraction of light.
Define refraction | What is the refraction of light?
When a ray of light strikes the separating surface between two media obliquely (at an angle other than a right angle) it bends as it passes from the first medium to the second and this bending is called the refraction of light. This happens because the speed of light changes when it passes from one medium to another having a different optical density.
Facts about refraction
(1) A ray of light is bent towards the normal when it enters an optically denser medium at an angle, for example from air to glass as in Figure (a) below and the first refraction in it.
In this case, the angle of refraction r is less than the angle of incidence i.
(2) A ray of light is bent away from the normal when it enters an optically less dense medium, for example from glass to air. See the figure (a) above and the second refraction in it when the light passes from glass to air. Here the angle of refraction r > angle of incidence i.
(3) A ray emerging from a parallel-sided block is parallel to the ray entering but is displaced sideways, like the ray CD in figure a is parallel to ray AB but displaced sideways. Read in detail about Lateral displacement in a separate post on this site.
(4) A ray traveling along the normal direction at a boundary is not refracted (Figure b above). In this case, even if the optical density of medium 2 is different from that of medium 1, the ray of light doesn’t suffer any deviation.
(5) A ray of light that encounters a change in medium is partially reflected as if the boundary were a mirror. The rest of the light ray gets refracted as it goes through the second medium. This means refraction is accompanied by partial reflection as well.
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