Electromagnetic waves (2/4)
Let us investigate further the properties of plane electromagnetic waves. We found on
page 1 of this chapter that monochromatic
waves are characterised in space by the wavelength λ,
and in time by the period T or frequency f with f=1/T.
These features can be described with a sinusoidal function for the electric field E, oscillating around E=0
and taking on a maximum value which is called the amplitude Eo of the wave.
The spatial periodicity of E(x) along the x-axis can thus be written as
...and the temporal periodicity along the t-axis by
Combining the spatial and temporal features into one equation yields finally the electric field which is a function of two variables, x and t:
Polarised light ↓ ↑
Electromagnetic waves are often polarised. Read in Supplement 3.1 in more detail about the different types of polarisation.
Equations ↓ ↑
Mathematical equations are shown using the
Mathematical
Markup Language (MathML), which is supported by Mozilla Firefox
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The sine function is periodic in multiples of 2π of its argument. Hence, with
or
or
where n is an integer number, the sine functions in the left column take on the same value
.
More specifically, the displacement E(x,t) of the electric field always takes on the same value
(not necessarily zero) when the wave has propagated along the x-axis by multiples of λ and along the
t-axis by multiples of T. The condition for a constant value of the displacement is a constant value of the argument
(or: the phase) of the sine function:
We calculate the velocity of this constant displacement by differentiating x with
respect to t:
Derivatives
↓
↑
In mathematics, you are familiar with functions f of a variable x, i.e., f(x).
Differentiating f with respect to x is then written as the derivative f’.
In physics there are many different functions of interest, e.g. the position, the velocity, the electric and the magnetic field and so on,
which you might differentiate with respect to time t, position x and other variables. Then the
“ ’ ” as a symbol to denote the differentiation is no more
useful since it is unclear as to which variable you want to differentiate. This ambiguity can be easily solved when writing the derivative as a quotient of two differentials
which include explicitely the function and the variable. In the case discussed here the differentials are dx and dt,
and the derivative of x with respect to t is then:
The symbol “d” preceeding the quantity which follows denotes an infinitely small difference
of that quantity, i.e., a differential.
Read more about functions of several variables and their derivatives in Supplement 2.6.
The term dx/dt is the so-called phase velocity c of the wave, and it follows:
This result has been obtained from purely geometrical considerations. It therefore holds with all kinds of wave phenomena, i.e.
waves on water, sound waves,... With electromagnetic waves this is the speed of light, and its value in vacuum is
c=2.998·108 m/s
or 300 000 km/s, approximately. The speed of light in air is slightly below the vacuum value,
in water it is around 225 000 km/s, and glass it is around 200 000 km/s. Read more about the speed of light in
supplement 1.3.
Question: Time lapse of sunlight and moonlight reaching the earth ↓ ↑
a) The distance between the sun and the earth is approx. 150·106 km. How much time does it take
(in seconds and in minutes) for the light emitted by the sun to reach the earth?
b) The distance between the moon and the earth is 384 000 km, hence much shorter. How much time does it take
for the sunlight reflected by the moon surface to reach the earth?
Check your results
on this page!
Question: The light year and the distance to Alpha Centauri ↓ ↑
The closest star to the sun is called Alpha Centauri. Its distance is given as 4.247 light years. A light year is the distance
travelled by light in one year, a unit of length often used in astronomy. How many kilometres is a light year? How far away in
kilometres is Alpha Centauri? You will realise that light years are a very practical unit of length to avoid making distances
to distant celestial bodies too unwieldy!
Check your results
on this page!
The magnetic field B of the electromagnetic wave is given by the same type of equation as
the electric field:
Both E and B oscillate perpendicularly to each other, as shown in the graph on the previous page. Moreover,
both oscillate perpendicularly to the direction of propagation, here: the x-axis.