The bending of light as it passes from one transparent medium to another medium is called refraction of light.
In the given figure, when a ray of light AO in the air is incident on the surface PQ of a glass slab, the ray beds along OB in the glass slab.
MN is normal to the glass surface at O.
The ray OB then emerges along BC in air.
Here, AO is an incident ray and OB is a refracted ray.
The incident ray (AO) makes an angle AOM with the normal MN draws at point O, that is the angle of incidence (i).
The refracted ray (OB) makes an angle NOB with the normal MN, that is the angle of refraction (r).
The point O is the point of incidence.
OB’ is the original direction of the incident ray AO.
Cause of refraction of light
The speed of light is generally different in different media.
For example, the speed of light in air or in a vacuum, water and glass is 3×10^8 m/s, 2.2×10^8 m/s, and 2×10^8 m/s respectively.
When a light ray passes from the air (rarer medium) to glass (denser medium) its speed decreases and light ray bends towards the normal.
Rarer and denser mediums
The medium through which the light passes is an optical medium.
The optical medium in which the velocity of light is more is a rarer medium.
And the medium in which the velocity of light is comparatively less is a denser medium.
If the pair of media are air and water, the air is a rarer medium and water is a denser medium.
But in the case of water and glass, water is rarer medium and glass is the denser one.
Thus, rarer and denser media are relative terms.
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