Download Chapter 33. Electromagnetic Waves

Chapter 33. Electromagnetic Waves
33.1. What is Physics?
33.2. Maxwell's Rainbow
33.3. The Traveling Electromagnetic Wave,
Qualitatively
33.4. The Traveling Electromagnetic Wave,
Quantitatively
33.5. Energy Transport and the Poynting
Vector
33.7. Polarization
33.8. Reflection and Refraction
33.9. Total Internal Reflection
33.10. Polarization by Reflection
What is Physics?
Maxwell's Rainbow
Sensitivity of the average human eye
Electromagnetic Waves
• It consists of mutually perpendicular and oscillating electric and
magnetic fields. The fields always vary sinusoidally. Moreover, the
fields vary with the same frequency and in phase (in step) with
each other.
• The wave is a transverse wave, both electric and magnetic fields
are oscillating perpendicular to the direction in which the wave
travels. The cross product E  B always gives the direction in
which the wave travels.
• Electromagnetic waves can travel through a vacuum or a material
substance.
• All electromagnetic waves move through a vacuum at the same
speed, and the symbol c is used to denote its value. This speed is
called the speed of light in a vacuum and is:
• The magnitudes of the fields at every instant and at any point are
related by
Properties of the Wave
• Wavelength λ is the horizontal distance between any two
successive equivalent points on the wave.
• Amplitude A is the highest point on the wave pattern.
• Period T is the time required for the wave to travel a distance
of one wavelength. Unit is second.
• Frequency f : f=1/T. The frequency is measured in cycles per
second or hertz (Hz).
• Speed of wave is v=λ/T= λf
The Speed of Light
• All electromagnetic waves travel through a vacuum at the same
speed, which is known as the speed of light c=3.00×108 m/s.
• All electromagnetic waves travel through a material substance with
the speeds less than the speed of light in vacuum c=3.00×108 m/s.
The waves with different wave lengths may have different speeds in
a material substance.
• In 1865, Maxwell determined theoretically that electromagnetic
waves propagate through a vacuum at a speed given by
c
1
 0 0
 3.00 108
(m/s)
The Energy Carried by Electromagnetic
Waves
Poynting Vector
• The rate of energy transport per unit area in EM wave is
described by a vector, called the Poynting vector
• The direction of the Poynting vector of an electromagnetic wave
at any point gives the wave's direction of travel and the direction
of energy transport at that point.
• The magnitude of S is
Intensity of EM Wave
• The time-averaged value of S is called the intensity I of
the wave
1 Em2

c 0 2
The root-mean-square value of the electric field, as
The energy associated with the electric field exactly equals
to the energy associated with the magnetic field.
•
Variation of Intensity with Distance
Polarization
• A linearly polarized electromagnetic wave is
one in which the oscillation of the electric
field occurs only along one direction, which is
taken to be the direction of polarization.
• Polarized randomly, or unpolarized wave is one
in which the direction of polarization does not
remain fixed, but fluctuates randomly in time.
• Partially polarized wave
Polarizing Sheet
•
An electric field component parallel to the polarizing direction is passed
(transmitted) by a polarizing sheet; a component perpendicular to it is
absorbed.
• one-half rule: an unpolarized light pass through a polarizing sheet,
the intensity I of the emerging polarized light is
SP 
1
S0
2
MALUS’ LAW
Example
Example
Example
What value of θ should be used in Figure, so the average intensity
of the polarized light reaching the photocell is one-tenth the
average intensity of the unpolarized light?
Geometrical Optics
•Wave fronts: the surfaces through all
points of the wave that are in the same
phase of motion are called wave fronts.
•Rays: the radial lines pointing outward
from the source and perpendicular to the
wave fronts are called rays. The rays
point in the direction of the velocity of the
wave.
Although a light wave spreads as it moves
away from its source, we can often
approximate its travel as being in a
straight line. The study of the properties
of light waves under that approximation is
called geometrical optics
Reflection and Refraction
The Reflection of Light
Why are we able to see
ourselves from mirror?
LAW OF REFLECTION
The incident ray, the reflected ray, and the
normal to the surface all lie in the same plane,
and the angle of reflection θr equals the angle
of incidence θi:
 
r
i
Example
Two plane mirrors are separated by 120°, as the
drawing illustrates. If a ray strikes mirror M1, at a
65° angle of incidence, at what angle θ does it leave
mirror M2?
Law of refraction
A refracted ray lies in the plane
of incidence and has an angle θ2 of
refraction that is related to the
angle of incidence θ1 by:
the symbols n1 and n2 are dimensionless
constant, called the index of refraction
c
ni 
vi
Dispersion
The index of refraction n
encountered by light in any
medium except vacuum
depends on the wavelength of
the light. The dependence of n
on wavelength implies that
when a light beam consists of
rays of different wavelengths,
the rays will be refracted at
different angles by a surface;
that is, the light will be spread
out by the refraction. This
spreading of light is called
chromatic dispersion,
•The index of refraction n in the
different materials is different
for the same wave length of
lights.
•The index of refraction n in the
same materials is different for
different wave length of lights.
Dispersion
Total Internal Reflection
Polarization by Reflection
• A ray of unpolarized light incident
on a glass surface. The electric
field vectors of the light has two
components. The perpendicular
components are perpendicular to
the plane of incidence The parallel
components are parallel to the
plane of incidence. Because the
light is unpolarized, these two
components are of equal magnitude.
• The reflected light also has both
components but with unequal
magnitudes.
• When the light is incident at a
particular incident angle, called the
Brewster angle , the reflected light
has only perpendicular components,