Chapter 2 — Exam Question 1
List the two models used to describe electromagnetic energy.
Answer
Electro magnetic (EM) energy can be modelled by waves or by photons. The
first model describes EM energy as sinus-like waves which propagate in space
at constant speed C, given by
C =
λ
·
υ
where
C =
3 · 10
8
[
m · sec
−
1
]
λ
=
wavelength [m]
υ
=
frequency [cycles · sec
−
1
]
C
is called ‘the speed of light’,
λ
is the distance between two successive crests
on the wave (measured in meters),
υ
is the number of given
λ
cycles passing a
fixed point in space per unit time (usually 1 second) and is measured in Hertz
(Hz). The relationship between
λ
and
υ
relate inversely since C is constant (the
longer the wavelength the lower the frequency).
The second model is the Quantum (or ‘photon’ or ‘particle’) model of New-
ton which describes EM energy as discrete units called “photons.” Where the
amount of energy in a photon is a function of its frequency, given by
Q = h ·
υ
where
Q =
energy of one photon [J]
h =
6.62 · 10
−
34
[
J · sec
−
1
]
υ
=
frequency[cycles · sec
−
1
]
The relationship between Q and
υ
is positive since h is constant. When com-
bined with the wave model we see that the relationship between Q and
λ
is also
inverse since (h · C) is a constant. When the models are combined
Q = h ·
υ
=
h · C/
λ
This implies that photons of low frequency (and thus long wavelength) con-
tain less energy compared to that of shorter wavelengths.