Refractive Index; Definition - Mettler Toledo RA-510M Bedienungsanleitung

3.

Refractive index

3.1

Definition

The refractive index n of a substance is the ratio of the velocity of a ray of light in vacuum to
its velocity in the medium (dimension = 1).
α
Air
β
α
Water
α
Critical angle
As the refraction depends on the wavelength of the incident light, the refractive index is
measured at the wavelength of the D line of sodium (589.3 nm) as standard and symbolized
by n .
D
The refractive index depends not only on the wavelength used to measure it, but also on the
temperature of the solution being measured. The standard temperature is 20 °C. If measure-
ments are performed at a different temperature, e.g. 25 °C, this must be specified: n
RA-510M
METTLER TOLEDO
n e.g.
1
of air
β
n e.g.
2
of water
β
α α
Total reflexion
If a ray of light at a particular angle passes
from, e.g. air to water (optically less dense
to optically more dense medium), it chan-
ges its direction except when the incident
light is vertical. According to Snell's law of
refraction, the ratio of the refractive indices
of the two media is proportional to the ratio
of the angle of refraction and angle of inci-
dence of the ray of light:
n
1
=
n
2
If a ray of light passes into an optically less
dense medium from an optically more den-
se medium, it also changes its direction. If
the angle of incidence α is increased, it
reaches a critical value at which the ray of
light no longer passes into the optically less
dense medium: the angle of refraction β =
90 °C. If this "critical angle" is exceeded,
total reflection occurs. The critical angle α is
used to calculate the refractive index:
β = 90° → sin β = 1
n
=
1
Refractive index
sin β
sin α
n
2
sin α
25
.
D
5