Page 175 - Physics_Form_2

Refraction and dispersion of light

medium to a less dense medium), it passes through it. Therefore, the refractive
is bent away from the normal. index of a material is also obtained from
sin( ) i the ratio of the speed of light in vacuum,
3. The ratio is the same (constant) c, to the speed of light in the material, v.
sin( ) r
c
for the different angles of incidence. That is, η = .
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4. The points E and F, the normal line, v
the incident ray and the refracted ray
c
lie on the same plane. This may not To be specific, the ratio η = .is known as
v
have been obvious. However, you
can observe this if you lift one edge the absolute refractive index of a material.
of the glass block above the other If the ratio is obtained from the speed of
edge of the block after setting the light v in one material, to the speed of
1
pins in a line. You will immediately v
notice that the linearity of pins A, B, light v in the other material, then η = v 1 .
2
2
C and D disappears. These important In this case, the ratio is called the relative
observations lead to two important refractive index.
conclusions that are essentially the
laws of refraction of light. These laws Normally, the speed of light in air is
state that; considered as the speed of light in
(a) At the point of incidence, the
incident ray, the normal line and vacuum, c. Therefore, if light travels
the refracted ray all lie on the from air to water and from air to glass,
same plane. it will have different speeds in water
(b) For a particular material, the ratio and glass because air, water and glass
sin( )i is constant. This law is have different optical densities. Letting
sin( )r the speed of light in water be v and its
1
also referred to as Snell’s law of speed in glass be v one can calculate the
2
refraction. absolute refractive indices of water and
glass as follows;

Refractive index of a material
In Activity 5.4, you observed that, for The absolute refractive index of water:
a given material or medium, the ratio η = v c

sin( )i is constant regardless of the size 1 1
sin( ) r
of the angle of incidence. This constant This implies that, c = η v .
1 1
is known as the refractive index of the
material (medium). It is a dimensionless The absolute refractive index of glass:
(it has no unit) constant denoted by the c
Greek letter η. The refractive index of η = v
2
a material is a measure of the influence 2
of the material on the speed of light as it This implies that, c = η v .
2 2
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Physics Form 2 Final.indd 169 25/10/2025 10:27