Unit 2 Electromagnetic Waves
Summary
Electromagnetic Waves
An electromagnetic wave consists
of oscillating electric and magnetic ﬁelds. An electromagnetic wave traveling along an x
axis has an electric ﬁeld and a magnetic ﬁeld with magnitudes
that depend on x and t:
E = E_{m} sin(kx - ωt)
and B = B_{m} sin(kx - ωt)
where E_{m} and B_{m} are the amplitudes of and E
and B.
The electric ﬁeld induces
the magnetic ﬁeld and vice versa.
The
speed of any electromagnetic
wave in vacuum is c = [E/B] where E and B are the simultaneous magnitudes of the ﬁelds.
The various possible frequencies
of electromagnetic waves form a spectrum,
a small part of
which is visible light.
Energy Flow
The intensity of the waves at distance r from a point
source of power is I = [P_{s}/4πr^{2}].
Radiation Pressure
When a surface intercepts electro
magnetic radiation, a force and a pressure are exerted on the surface. If the radiation is totally absorbed by the surface, the force is F = [^{IA}/c] for total absorption and F = [^{2IA}/c] for total reflection.
The radiation pressure (force per unit area) p_{r} = [^{I}/c] for total absorption and p_{r} = [^{2I}/c] for total reflection.
Polarization
Electromagnetic waves are polarized if their
electric ﬁeld vectors are all in a single plane, called the plane of oscillation.
Light waves from common sources are not polarized; that
is, they are unpolarized, or polarized randomly. |