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mination of the value n. Before discussing the methods
of determining n (R.I.) another property has also to be
considered.
DispeRsion
When
a ray of light is passed through a glass prism, it is resolved into
component colours, in which red is refracted least and violet most,
with orange, yellow, green and blue occupying the intervening space in
the band of
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colours,
which is known as the' spectrum'. This is because the velocity of the
red ray is greatest and that of violet least. On account of this
property of dispersion, the index of refraction is best determined with
light of a definite colour or monochromatic light.
This
difference in the velocities of red and violet lights in passing
through a medium indicates the strength of the dispersion of that
medium or substance. It is usually expressed as the difference of the
refractive index of the two rays. Diamond has three times the
dispersion of glass, which produces the' fire' of this gem. The
component colours of white light in passing through diamond are widely
dispersed, and pure colours may be seen on emergence under favourable
conditions. Substances like glass which has a low dispersion have
little or no 'fire'.
Total Reflection and the Critical Angle
When light travels from a denser to a lighter or rarer medium the ray is bent away from the normal. The angle
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