OpenStax-CNX module: m38778
1
Electromagnetic Radiation:
∗
Electromagnetic spectrum
Free High School Science Texts Project
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 3.0†
1 Electromagnetic spectrum
Figure 1: The electromagnetic spectrum as a function of frequency. The dierent types according to
wavelength are shown as well as everyday comparisons.
Electromagnetic radiation allows us to observe the world around us. Some materials and objects emit
electromagnetic radiation and some reect the electromagnetic radiation emitted by other objects (such as
the Sun, a light bulb or a re). When electromagnetic radiation comes from an object (whether the radiation
is emitted or reected by the object) and enters the eye, we see that object. Everything you see around you
either emits or reects electromagnetic radiation or both.
Electromagnetic radiation comes in a wide range of frequencies (or wavelengths) and the frequencies of
radiation the human eye is sensitive to is only a very small part of it. The collection of all possible frequencies
of electromagnetic radiation is called the electromagnetic spectrum, which (for convenience) is divided into
sections (such as radio, microwave, infrared, visible, ultraviolet, X-rays and gamma-rays).
The electromagnetic spectrum is continuous (has no gaps) and innite. In practice, we can only use
electromagnetic radiation with wavelengths between (very roughly) 10−14 m (very high energy gamma rays)
and 1015 m (very long wavelength radio waves) due to technological limitations in the detectors used to
receive electromagnetic radiation and in the devices used to produce or emit electromagnetic radiation.
The various frequencies (or wavelengths) of electromagnetic radiation coming from a particular object or
material depends on how the object or material reects and/or emits electromagnetic radiation.
1.1 Wave Nature of EM Radiation
1. List one source of electromagnetic waves. Hint: consider the spectrum diagram and look at the names
we give to dierent wavelengths.
∗ Version
1.1: Jul 25, 2011 3:39 am -0500
† http://creativecommons.org/licenses/by/3.0/
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OpenStax-CNX module: m38778
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Click here for the solution1
2. Explain how an EM wave propagates, with the aid of a diagram.
Click here for the solution2
3. What is the speed of light? What symbol is used to refer to the speed of light? Does the speed of light
change?
Click here for the solution3
4. Do EM waves need a medium to travel through?
Click here for the solution4
Table 1 lists the wavelength- and frequency ranges of the divisions of the electromagnetic spectrum.
Category
Range of Wavelengths (nm) Range of Frequencies (Hz)
gamma rays
X-rays
ultraviolet light
visible light
infrared
microwave
radio waves
<1
> 3 × 1019
1-10
10-400
400-700
700-105
3 × 1017 -3 × 1019
105 − 108
3 × 109 -3 × 1012
> 108
< 3 × 109
7, 5 × 1014 -3 × 1017
4, 3 × 1014 -7, 5 × 1014
3 × 1012 -4, 3 × 1019
Table 1: Electromagnetic spectrum
Examples of some uses of electromagnetic waves are shown in Table 2.
Category
Uses
gamma rays
used to kill the bacteria in marshmallows and to
sterilise medical equipment
used to image bone structures
bees can see into the ultraviolet because owers
stand out more clearly at this frequency
used by humans to observe the world
night vision, heat sensors, laser metal cutting
microwave ovens, radar
radio, television broadcasts
X-rays
ultraviolet light
visible light
infrared
microwave
radio waves
Table 2: Uses of EM waves
1 http://www.fhsst.org/l2c
2 http://www.fhsst.org/l2x
3 http://www.fhsst.org/l2a
4 http://www.fhsst.org/l2C
http://cnx.org/content/m38778/1.1/
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1.2 EM Radiation
1. Arrange the following types of EM radiation in order of increasing frequency: infrared, X-rays, ultraviolet, visible, gamma.
Click here for the solution5
2. Calculate the frequency of an EM wave with a wavelength of 400 nm.
Click here for the solution6
3. Give an example of the use of each type of EM radiation, i.e. gamma rays, X-rays, ultraviolet light,
visible light, infrared, microwave and radio and TV waves.
Click here for the solution7
2 The particle nature of electromagnetic radiation
When we talk of electromagnetic radiation as a particle, we refer to photons, which are packets of energy.
The energy of the photon is related to the wavelength of electromagnetic radiation according to:
Denition 1: Planck's constant
Planck's constant is a physical constant named after Max Planck.
h = 6, 626 × 10−34 J · s
The energy of a photon can be calculated using the formula: E = hf or E = h λc . Where E is the energy
of the photon in joules (J), h is planck's constant, c is the speed of light, f is the frequency in hertz (Hz) and
λ is the wavelength in metres (m).
Exercise 1: Calculating the energy of a photon I
(Solution on p. 4.)
Exercise 2: Calculating the energy of a photon II
(Solution on p. 4.)
Calculate the energy of a photon with a frequency of 3 × 1018 Hz
What is the energy of an ultraviolet photon with a wavelength of 200 nm?
2.1 Exercise - particle nature of EM waves
1. How is the energy of a photon related to its frequency and wavelength?
Click here for the solution8
2. Calculate the energy of a photon of EM radiation with a frequency of 1012 Hz.
Click here for the solution9
3. Determine the energy of a photon of EM radiation with a wavelength of 600 nm.
Click here for the solution10
5 http://www.fhsst.org/l23
6 http://www.fhsst.org/l2O
7 http://www.fhsst.org/l2i
8 http://www.fhsst.org/l2H
9 http://www.fhsst.org/l26
10 http://www.fhsst.org/l2F
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OpenStax-CNX module: m38778
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Solutions to Exercises in this Module
Solution to Exercise (p. 3)
Step 1.
E
=
=
=
hf
6, 6 × 10
−34
× 3 × 1018
(1)
2 × 10−15 J
Solution to Exercise (p. 3)
Step 1. We are required to calculate the energy associated with a photon of ultraviolet light with a wavelength
of 200 nm.
We can use:
E=h
c
λ
(2)
Step 2.
E
http://cnx.org/content/m38778/1.1/
=
h λc
=
6, 626 × 10−34
=
9, 939 × 10
3×108
200×10−9
−10
J
(3)