Waves from a source, which is receding from us, are perceived to
have lower frequency and larger wavelength.
λ ' = λ ⋅ (1 ± u / c) f ' = f ⋅ (1 ∓u / c)
Red shift (shift toward larger wavelengths) in the spectra of
distant galaxies is an evidence of expanding universe.
Shock waves – sonic booms.
Source moving with Vsource = Vsound
( Mach 1 - breaking the sound barrier )
Source moving with Vsource
(Mach 1.4 - supersonic)
> Vsound
The shock waves are radiated by ANY object moving through a medium at a
speed about the speed of waves in the medium, e.g. speed of sound.
In other words, even object not emitting any waves at rest becomes a
source of shock waves at a supersonic speed.
Shock waves. Mach cone.
When an object moves faster than the information about it can be propagated into the
surrounding fluid, fluid near the disturbance cannot react or "get out of the way" before
the disturbance arrives. In a shock wave the properties of the fluid change abruptly and
almost instantaneously.
Shock waves. Mach cone.
A triangle in the cone:
a side a = T⋅v – the
travel distance of the
wave within one period.
v
a
θ
b
u
The hypotenuse b = T⋅u
– the travel distance of
the moving object.
a T⋅v v
sin θ = =
= <١
b T⋅u u
θ − the Mach angle
u
v
- Mach number
The sharper the Mach angle, the higher the speed of the object
Electromagnetic spectrum.
fλ = c
Frequency of visible light: 4.3×1014 to 7.5×1014 Hz
fλ = c
Why a single wave phenomenon is called so many names?
EM waves of different frequencies/lengths originate from
different sources and interact with matter essentially
differently.
US toxicoligists discover UV rays cause
mutations in frogs...... 'Now don't forget to
put on your UV sun-screen.’
Ear of bat, tail of
newt DING!!..
Somehow it’s just
not the same with
a microwave
You are right. Your dog did eat
your homework.
Electromagnetic Waves
http://www.phy.ntnu.edu.tw/java/emWave/emWave.html
http://www.amanogawa.com/archive/PlaneWave/PlaneWave-2.html
http://www.walter-fendt.de/ph14e/emwave.htm
http://www.educypedia.be/electronics/javawaves.htm
Electromagnetic Waves
• The E and B fields are
perpendicular to each other
• Both fields are perpendicular
to the direction of motion
– Therefore, EM waves are
transverse waves
• Propagate with the speed of
light, which does not depend on
either frequency or amplitude;
• Speed of light in vacuum – a
fundamental constant,
c = 3⋅108 m/s
E
B=
c
In every point in space, at each
moment of time
Polarization is a property of electromagnetic waves that
specifies the direction of electric field.
Why do we pick the electric field out of the two? Aren’t they on
an equal footing?
They are on an equal footing, but it is mostly the electric field,
which interacts with matter – atoms, electrons, nuclei…
EM waves from a TV tower are perfectly polarized – the Electric
field has a very well defined direction, which stays always the same.
In contrast, the light coming from the Sun or from a light bulb is
unpolarized.
What does it mean unpolarized? Does not the electric field have
some direction?
It certainly does at every instant. BUT this direction does not stay
constant and changes very rapidly and randomly.
So, after averaging over any reasonable time interval, you do not find
any particular polarization!
polarized
unpolarized
The frequency of light is about 5×1014 Hz, which means 5×1014 wave
crests per second. If the polarization changes once every 500 crests it
will still be 1012 times per second. Too fast for us to detect!