handbook of modern sensors

4.1 Radiometry 125
propagation, polarization, spectral contents of light beam can all be modified, and
even the speed of light and phase of its wavelength can be changed.
4.1 Radiometry
Let us consider light traveling through a three-layer material. All layers are made of
different substances called media. Figure 4.2 shows what happens to a ray of light
which travels from the first medium into a flat plate of a second medium, and then to
a third medium. Part of the incident light is reflected from a planar boundary between
the first and second media according to the law of reflection, which historically is
attributed to Euclid:
1 = ′ 1 (4.1)
A part of light enters the plate (Medium 2) at a different angle. The new angle 2 is
governed by the refraction law, which was discovered in 1621 by Willebrord Snell
(1580–1626) and is known as Snell’s law:
n 1 sin 1 = n 2 sin 2 , (4.2)
where n 1 and n 2 are the indices of refraction of two media.
In any medium, light moves slower than in vacuum. An index of refraction is a
ratio of velocity of light in vacuum, c 0 , to that in a medium, c:
n = c 0
c , (4.3)
Fig. 4.2. Light passing through materials with different refractive indices.