Nats 101 S04 #13
Reading: T&H 149-157
Electromagnetic radiation travels by wave motion, without the need for
Every time an electric charge is accelerated, a transverse wave is created.
Composed of two parts, electric and magnetic, 90° apart, polarization
Speed = 186,000 miles/sec = 300,000 km/sec in vacuum
Energy vs wavelength
3 ways for radiation to interact with matter: transmission, absorption,
The electromagnetic wave
So far, we have discussed the nature of waves travelling through a medium, for example,
sound waves travelling through the air, or earthquakes travelling through the Earth.
Now we will examine the nature of electromagnetic waves, i.e. light. This
is quite different from waves like sound because light can travel through
a vacuum, that is, light can travel through space without a medium.
Soon after Maxwell discovered the relationship between magnetism and electricity,
he discovered that the equations of motion for electromagnetism had the same mathematical form as
the equations for waves. The main difference with other waves is that for electromagnetic
waves, the energy is transferred by the field and not by the medium. In other words, electromagnetic waves
Every time an electric charge is accelerated then an electromagnetic
wave is created.
The anatomy of the electromagnetic wave
Let's discuss the nature of these waves.
Scientist had wondered about the nature of light for quite some time. You
get light from a candle, from the sun etc. How? Why? Until 1860, we had
no idea. It is amazing that refrigerator magnets, static cling, or electric
generators have anything to do with light.
Electromagnetic radiation is a transverse wave, composed
of two parts, an electric and a magnetic component.
Both are oriented 90° from each other. Each wave might have its polarization
in a different direction.
Both parts keep the wave going. The origin of the wave is the acceleration
of an electric charge. We know that this in turn induces a magnetic field.
If the acceleration of the charge is, say up and down, then the electric
field and the magnetic field will oscillate.
There is a slight delay between the two. Say the electric goes first, then
the magnetic follows closely behind. The magnetic creates an electric and
so on, as a continuous cycle. The slight delay keeps it going.
The velocity of all electromagnetic waves is the same, 300,000
km/sec or 186,000 mi./sec in a vacuum. This is a result of the oscillating
nature of the electromagnetic waves. One pushes the other.
In this way the wave does not need a medium, but moves by its own internal
When Maxwell proposed the idea of the waveform of light, he was not able
to conceive of the fact that it could travel through a vacuum. This is
an example of how pre-conceived notions are sometimes difficult to overcome.
He assumed that the whole universe was filled with a substance called ether.
It is a sort of tenuous, Jell-O-like material. An accelerating electric
charge would shake the ether and the wave would rush off into space.
The concept was first invented by the Greeks, and accepted by everyone,
Newton, etc. until 1887.
In 1887, Michelson and Morley designed an experiment to look for ether.
They assumed that if the universe was filled with ether then the travelling
earth should leave a wake behind it, like the disturbance to the air when
a ball travels through air. They looked for disturbances in light interference.
They were unable to find this wake, and so concluded that there is
Michelson won the first Nobel prize by an American in 1907.
Ethernet, named after the concept of the ether, is the medium in which e-mail
The speed of light
is a constant. We hear this all the time. Well it is
really not true. The speed of light is a constant in a vacuum. Its speed
is different in other media. It can go slower in crystalline material.
In fact the slower it goes the greater the brilliance of a cut gemstone. Diamond
slows the speed of light by 2 ½ times, from 186,000 mi/sec down
to 75,000 mi/sec. Some lead glasses slow it down even more. This is just
like sound, going different speeds through different media.
slowed the speed of light down to 38 mi/hr by
passing it through a new form of matter called
Bose-Einstein condensates. (picture)
Two years ago, scientists slowed the speed of light down to 0 mi/hr,
stopping it dead in its tracks.
Imagine that you have a charged comb and you wave it up and down. Then
you are creating electromagnetic radiation. If you move it up and down
once per sec then you are creating a wave with wavelength equal to 300,000
km. This is a very low energy wave.
If you wave it up and down 300,000 times per sec then you have an electromagnetic wave with a 1 km wavelength.
It took more energy to wave it up and down 300,000 times per sec.
(We moved the mass of a comb up and down 300,000 times in a second, compared to just once in a second). So the
lesson here is that the more energy in the wave, the shorter the wavelength.
As another example, think about visible light. The red glow of cooling embers
in a campfire has less energy then the blue white flame of a blowtorch.
(The blowtorch is hotter). So the red wavelength is longer than the blue.
The red is 700 nm, blue is 400 nm.) nm=10-9 m. Red is the longest
wavelength that our eyes can see, blue is the shortest.
To get a red light by waving the comb around, we would have to wave it
up and down 400 trillion = 400,000,000,000,000 times per sec. (4x1014
Question: What do we have to do in order to make a real bright red light, (versus dim)?
Once a wave has been produced then its motion is independent of the source.
In other words, all electromagnetic radiation behaves in the same way,
regardless of how it was produced.
This is the same for sound too.
Suppose that the sound emitter was moving towards you. Then the wave would
travel towards you, and the wavelength would appear to be shorter than
if the emitter was standing still.
Called the Doppler effect. Definition of the Doppler effect:
If wave source is moving towards you then the wavelength gets shorter,
if it is moving away from you then the wavelength gets longer.
occur when an object is travelling at the speed of
sound. The object is producing waves faster than they can travel away and
so the crests all overlap and the interference makes a large amplitude
wave, i.e. louder. Click here
to see computer based demonstration, at the new site click on "Doppler".
Light does the same thing. If the source of light is moving towards you
then the light will be shifted in frequency towards the shorter blue wavelengths. If it is
moving away from you then the light will be shifted to the red. Such an
effect is found when looking at the wavelengths of light emitted from distant
galaxies. For instance, helium produces certain wavelengths of light. It
is observed that these wavelengths are shifted towards the red for objects
that are far from us. This means that they are receeding from us at high
speeds. This is the experimental evidence for the expansion of the universe.
Police radar also works off the Doppler effect. The unit in the police car sends out a signal of a certain frequency.
This signal is absorbed by the metal parts of your car, and retransmitted back at the same frequency as recieved.
The police unit then compares the frequency that it emitted with the frequency that it recieves.
If the recieved frequency is greater than the one that they emitted, then that means the recieved wavelength
must be shorter, and the car is approaching the police unit. The greater the change, then the greater the speed
of your car.
Transmission, absorption, and scattering
We are made of matter, therefore the only way that we can detect electromagnetic
radiation is by observing its interaction with matter. There are 3 ways
in which this can occur.
Transmission: this occurs when the light waves passes through transparent
materials. The material does nothing to the light except to slow it down
(e.g. diamond) or to change its direction (e.g. eye-glasses).
Absorption: The light interacts with the atoms in the object and instead
of passing through, it gets converted to heat, and the energy stops being
transmitted. Dark objects absorb visible light. E.g. black pavement
Scattering: the light is absorbed by the electrons in the object, making
them go to higher energy states. The electrons then pop back down in energy
and re-emit the light. A mirror re-emits all the light. If the color gets
changed then it is because some light is absorbed or transmitted and not re-emitted.