OPTICS - Lecture 4
CONTENTS
1. The wavelengths of visible light in the electromagnetic spectrum:
400 nm (violet) – 700 nm (red)
2. Polarization

If the electric field E of an electromagnetic wave is oscillating parallel to the certain direction,
this wave is called to be polarized. Light can be polarized by selective absorption
(e.g. polarizing filters), reflection, refraction (double refraction) and scattering.
The cosine-squared rule: I  I 0 cos 2  , where I - the intensity of the wave emerging from e.g.
polarizing sheet, I0 – the intensity of the wave incident on the sheet, θ – the angle between
polarizing directions of the wave and the sheet.
n
Polarization by reflection: total when tan  B  2 , where θB – the Brewster angle (of
n1
incidence), n1 – refraction index of the medium where the reflected ray travels, n2 – index of
refraction of the medium where the refracted ray is.
3. Law of reflection
A reflected ray lies in the plane of incidence and has
an angle of reflection equal to the angle of incidence.
n1
4. Law of refraction (Snell’s law)
A refracted ray lies in the plane of incidence and has
an angle of refraction  2 that is related to the angle
of incidence  1 by
n2 sin  2  n1 sin 1 .
Here n1 is the index of refraction of the first (upper
in Fig.6) medium, and n2 is the index of refraction
of the second (lower in Fig.6) medium.
The index of refraction of a medium is equal
n
n2
c
,
v
where v is the speed of light in that medium and c is its speed in vacuum.
Nothing has an index of refraction n below 1. For vacuum n is defined to be exactly 1, for air –
approximately 1. For water n=1.33, for glass approximately n1.5.
5. Chromatic Dispersion
The index of refraction n in any medium except vacuum (and air) depends on the wavelength
(color) of the light. The light will be spread out by the refraction. The index of refraction is
generally greater for a shorter wavelength (e.g. blue light) than for a longer wavelength (say, red
light).
6. Total Internal Reflection
These are two conditions the total internal reflection occurs:
 c  sin 1
- the incident light must be in the medium of higher index of refraction,
- the angle of incidence must be greater than the critical angle.
Applications of total internal reflection: refractometer, optical fibers, endoscopes.
1
n2
n1