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.

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