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When
light travels from one medium to another, there is in general an
increase or decrease in velocity, which results in the phenomenon of
refraction, i.e. a change in the direction of propagation. In the case
of transparent gems, the reflected light is not so important as the
refracted light. When a ray of light passes into water obliquely, the
path of the ray is not straight, but bent. When a ray of light travels
from a rarer to a denser medium, as in the above case, the ray is bent
towards the normal to the plane of contact between the two media.
Naturally, the reverse will be the case when a ray of light travels
from a denser to a rarer medium. It is then bent away from the normal.
In fig. 27 the angle of incidence CXO is called i and the angle of refraction DXN as r. The law of.
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refraction
is: The incident and refracted rays lie in the same plane, and the
ratio of the velocities in the two media, and the ratio between the
sines of the angles of incidence and refraction, i and r, are constant for the media concerned.
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The constant is the index of refraction, the velocity of light in air being l.
For water R.I. = 1-333 and for diamond 2-42. It is evident that gem-stones could be identified by the deter-
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