When light travels from one medium to another at the interface the light changes its direction or light bends. This effect is known as Refraction. When light travels from rarer medium to denser medium, the light bends towards the normal and this causes the object to appear nearer than they are.
The laws of refraction
● The incident ray reflected ray, refracted ray and the normal of the system lie in the same plane.
● The incident ray, coming from one medium to the boundary of another medium, is refracted by a rule derived from a physicist Willebrord Snellius. He found that there is a constant relation between the angle of the incident ray and angle of refracted ray. This constant is the refractive index of the second medium relative to the first medium.
An experiment can be done with a glass slab to understand the incident of refraction easily.
Experiment - Fix a sheet of white paper on a drawing board using drawing pins. Place a rectangular glass slab over the white sheet in the middle of the sheet. Draw the outline of the slab with a pencil. Name the outline ABCD. The points O and O’ lie on surfaces separating two transparent media. Draw a perpendicular NN’ to AB at O and another perpendicular MM’ to CD at O’. The light ray at point O has entered from a rarer medium to a denser medium, that is, from air to glass. Note that the light ray has bent towards the normal. At O’, the light ray has entered from glass to air, that is from a denser to a rarer medium. The light here has bent away from the normal.
You may observe that the emergent ray is parallel to the direction of the incident ray.
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