Download Zealey Phys-in-Cont

E X T E N S I O N
3
Chapter 2 The World Communicates: Waves, Energy and
Information Transfer
Unit 2.3 X-rays, Light, and Radio Waves are all
Electromagnetic Waves
A Modern View of Electromagnetic Radiation
The wave description of light is most useful in describing the way in which electromagnetic
disturbances propagate and interact. It can explain the phenomena of reflection, refraction,
diffraction and interference.
Electromagnetic waves are not usually infinite in length. They have a beginning and end. In
some emission processes the accelerating charge may produce electromagnetic waves for a very
long time; in others the process may last less than 10–8s.
The physical length of a wave train produced is equal to the speed of light multiplied by the
emission time. For emission times of 10–8s these lengths are about 3 m. Each wave train of visible
light (λ = 500 nm) contains 6 × 106 cycles.
We can therefore also think of light as a stream of wave packets. Travelling at the speed of light
these wave packets will take about 10–8 s to pass the observer. Each wave train carries an energy,
∆E, which is related to its frequency by ∆E = hf = hc/λ. These wave trains can be likened in their
behaviour to particles and are called photons.
Figure 1
Light photons are
produced by
emission from
billions of excited
atoms. Our eyes
average these
short wave trains
so that we
observe
continuous
emission.
Light from a source such as a light globe is composed of emissions from billions of atoms.
Each atom emits a short wave train carrying a small amount or quantum of energy. The emission
from individual atoms occurs at random. This means that the wave packets are not in step when
they arrive at the observer. The eye cannot distinguish individual wave packets or photons.
Instead we average the individual wave packets and interpret them as a continuous wave.
The penetrating power of electromagnetic radiation
We are used to visible light being unable to penetrate solids and metals. However, these materials
are transparent to other forms of electromagnetic radiation (for example X-rays).
Imagine trying to knock down a brick wall by throwing peas at it. Even if you threw 1000 peas
an hour it would be an impossible task. To have any effect on the wall you would need to use
bowling balls or objects capable of carrying more energy. The same applies to electromagnetic
radiation. It is not the total energy that matters, it is the energy in each wave packet.
Electromagnetic waves interact with materials in several different ways depending on the
energy of each wave packet or photon. The energy of each wave packet is given by
λE = hf = hc/λ
where h is Plank’s constant and is 6.63 × 10–34 Js.
Energy is usually measured in a unit called the joule. However this is rather a large unit when
dealing with atomic physics and it is sometimes easier to talk in terms of energy units of electron
volts (eV) rather than joules.
1 electron volt = 1.6 × 10–19 joules
Although the energy associated with a wave packet appears small, visible light can kick the
electrons in atoms into higher energies. Ultraviolet light with wavelengths of 90 nm has sufficient
energy to knock the electrons out of the atom altogether (ionise).
Whenever the electromagnetic wave has sufficient energy to either remove electrons or push
electrons up into higher energies in atoms in its path, or cause molecules to take up energy
through vibration or rotation, then that wave has a high chance of being absorbed by the
material.
The energy that has been absorbed may reappear at a later stage as light or heat.
Table 1
Examples of the photon energy needed for some atomic and subatomic interactions
Absorption event
Energy (eV)
Radiation
Knock an electron from the innermost shell of a tungsten atom
Kick an electron from the helium atom
Kick an electron from the hydrogen atom
Excite an electron in a hydrogen atom
Be absorbed by silicon
Increase the energy of rotation in a diatomic hydrogen molecule
Cause vibration in a CO molecule
Cause the proton in hydrogen to flip
72k
25
13.6
1.9
1
1
0.001
0.000006
X-rays
extreme ultraviolet
extreme ultraviolet
visible
infrared; 2 microns
infrared; 2 microns
millimetre; mm
radio; 21 cm
In general it can be said that the penetrating power of high energy electromagnetic radiation
such as ultraviolet, X-ray and gamma rays increases with frequency. Gamma rays will penetrate
a thin sheet of lead, and X-rays a thin sheet of aluminium, whereas ultraviolet waves will not
penetrate a sheet of paper. However, at longer wavelengths we observe more complex effects
which are responsible for absorption of radiation.
Scattering of light and infrared waves by small grains of dust actually decreases as wavelength
increases. Blue light is more scattered than red light. The molecules and dust in the sky scatter
the blue light more than the red light of sunlight as it passes through the atmosphere. When we
look away from the Sun this scattered light makes the sky its familiar blue. When looking towards
the setting or rising sun, the loss of the blue wavelengths makes the Sun appear red.
Electromagnetic wave characteristics
Light has some properties that are wave-like. Energy is transferred from the source of the light to
the observer as electromagnetic waves. These waves move out from the source in all directions
as a changing electric field (accompanied by a changing magnetic field). Electromagnetic radiation from a point source can be thought of as series of spherical wave crests, each expanding at
the velocity of light, 3 × 108 m s–1.
As these wave crests move past charged particles, the particles experience a variable force and
begin to oscillate as they take energy from the electromagnetic wave. At any given time we can
join up all of the wave crests, which started from the source at the same time, as a wavefront. At
any instance the wavefront represents a narrow region in which all of the particles are vibrating
in phase.
We can represent the direction in which the wavefront is moving by arrows or rays, drawn at
right angles to the wavefront. If we follow one set of arrows from the source outward we may join
them to represent a ray.
Wavelength and frequency
Electromagnetic waves, like all waves, can be described by their wavelength, period and
frequency.
• The wavelength (symbol λ measured in metres) is the distance between adjacent
wavefronts. If we take a photograph of charged particles in the path of the wave, the
wavelength is the distance between charges that are oscillating in step (e.g. charges at the
extreme of their vibrational motion).
• The period (symbol T, measured in seconds) is the time that elapses between successive
crests of the wave passing an observer. If you imagine you are a small charged particle in
the path of an electromagnetic wave this is the time it takes to move through one complete
cycle.
• The frequency (symbol f, measured in cycles per second or Hz) is the number of complete
wavelengths passing an observer in one second.
Frequency and period are different ways of looking at the same thing. They are related by:
frequency =
f=
1
period
1
T