Lesson Explainer: The Electromagnetic Spectrum Physics

In this explainer, we will learn how to analyze the electromagnetic spectrum by identifying and describing types of electromagnetic radiation and their sources.

When we talk about electromagnetic radiation, this is just a technical way of saying “light.”

Let’s talk about where light comes from. One source of light we encounter in our daily lives is the Sun. We also use artificial light from light bulbs and use computer screens that generate light. All of these sources have the same underlying physical mechanism producing the light: the acceleration of electrons.

Light is really the fluctuations of two fields, electric and magnetic, which is why it is also known as electromagnetic radiation.

Like other waves, electromagnetic waves carry energy. One way in which the amount of energy transported by a wave can vary is due to the wave’s amplitude.

As shown in the diagram above, the amplitude, 𝐴, of a wave is measured from zero to the peak of the wave. The larger the value of 𝐴, the more energy is being transported by the wave.

In the diagram above, a constant wave is shown by the red dashed line, and the solid blue line shows the same wave but with steadily increasing amplitude. At the left of the diagram, the red dashed wave carries more energy than the solid blue one. By the time we get to the right-hand side of the diagram, the wave represented by the solid blue line has more energy than the dashed red one.

In terms of what this means physically, the amplitude is the brightness of the light. If we are measuring the light from a lamp with a dimmer switch, the effect of the increasing brightness of the light is caused by the increase in the amplitude of the electromagnetic wave produced by the lamp. This does not change anything about the nature of the light; it just increases its intensity.

Another property that electromagnetic waves have is their wavelength. The wavelength is the distance between two peaks of a wave, and it is usually represented by the symbol 𝜆, which is the Greek letter lambda. As it is a distance, it has the same units as any other distance (e.g., metres).

Definition: Wavelength

The wavelength of a wave is the distance between two peaks (or, equivalently, two troughs) of the wave.

A wavelength has units of distance (e.g., metres) and is usually represented by the symbol 𝜆.

In the diagram above, we would measure the wavelength by measuring the distance from any peak or trough to the next one. The wavelength, 𝜆, is the same whichever peak we choose to measure from.

Waves with shorter wavelengths have more energy, and waves with longer wavelengths have less energy.

Electromagnetic waves can have a wide range of wavelengths. Changing the wavelength of the wave fundamentally changes its properties in ways that changing the amplitude does not. We therefore divide electromagnetic radiation into categories according to its wavelength. The full range of possible wavelengths of light together are known as the electromagnetic spectrum.

To help us talk about different parts of the electromagnetic spectrum, we usually divide it into seven sections. This is useful to help us understand the origin and properties of the electromagnetic spectrum, but it is important to note that there is no physical boundary between these categories.

The most common form of light we encounter is visible light. This covers the range of wavelengths that our eyes can see: approximately 4×10 m to 7×10 m. Different wavelengths within that range are what we see as different colors. The shortest wavelength of light in the visible range is 𝜆∼4×10m, which we see as violet. At the other extreme, we have the longest wavelengths in the visible range of the spectrum, 𝜆∼7×10m, which we see as red.

The full range of visible light and the corresponding wavelengths are shown in the table below.

Most visible light is generated by stars like the Sun.

The highest-energy electromagnetic radiation in the visible range is violet. If the radiation has slightly more energy, or a slightly shorter wavelength, then it falls into the ultraviolet (literally “beyond violet”) range of the spectrum. Ultraviolet, or UV, radiation has a wavelength of 𝜆∼10m. Like visible light, UV radiation in generated in the Sun and is what causes sunburn.

Radiation with even higher energy, 𝜆∼10m, is called X-ray radiation. X-rays can pass through soft matter like flesh, which is why they are used to make images of bones. They are generated when electrons decelerate.

At higher energy still, with wavelengths 𝜆<10m, are gamma rays. Gamma rays are produced when the nuclei of atoms decay.

Returning to visible light, we can also have electromagnetic radiation with lower energies, or longer wavelengths. The longest wavelength of visible light we had was red, so if we have radiation with wavelengths slightly longer than red light, or 𝜆∼10m, we call this infrared (literally “below red”).

Infrared radiation primarily comes from the thermal motion of atoms and molecules, so it is generated by anything warm. Some infrared light is produced by the Sun, but it is also emitted by planets, people, and animals. This is why infrared cameras can be used to locate people and animals in the dark.

At longer wavelengths still, we have microwave radiation with 𝜆∼10m. This is the type of electromagnetic wave produced in microwave ovens. It might seem counterintuitive that microwaves are comparatively low energy, but this is just the right wavelength to excite water molecules, which causes them to heat up. The most common source of microwave radiation is alternating electric currents (AC).

Finally, at the longest wavelengths and lowest energies, we have radio waves. Radio waves have 𝜆>1m. They can also be produced by alternating electric currents (AC) that have lower frequencies than those that produce microwaves. Radio waves are also used in communications to transmit radio and television signals. Because of this, there is a common misconception that they are sound waves. They are in fact electromagnetic waves just like visible light.

The full range of the types of electromagnetic radiation make up the electromagnetic spectrum. The details above are summarized in the table below.

The Electromagnetic Spectrum
Shortest wavelength ⟶ longest wavelength
Highest energy ⟶ lowest energy
TypeGammaX-rayUltravioletVisibleInfraredMicrowaveRadio
Wavelength,
𝜆()m
<101010101010>1
SourceNuclear decayDecelerated electronsThe SunThe SunThermal motion of atoms and moleculesAlternating currentAlternating current

Let’s get some practice thinking about the electromagnetic spectrum by going through some examples.

Example 1: Comparing Wavelengths of Types of Electromagnetic Waves

What is the name of the type of electromagnetic radiation that has the longest wavelengths?

Answer

This question is about the wavelength types of electromagnetic radiation. To answer this, we need to be able to recall the seven types of electromagnetic radiation in order of increasing wavelength. These are gamma rays, X-rays, ultraviolet waves, visible light, infrared waves, microwaves, and radio waves.

The type of electromagnetic radiation with the longest wavelength is therefore radio waves.

Example 2: Comparing the Wavelengths of Types of Electromagnetic Waves

The table shows a set of types of electromagnetic waves and the orders of magnitude of their wavelengths.

RadioMicrowaveInfraredVisibleUltravioletX-rayGamma Ray
>1m10 m10 m10 m10 m10 m<10m

  1. Using the values from the table, how many wavelengths of ultraviolet radiation would have the same total length as one wavelength of infrared radiation? Answer in standard form.
  2. Using the values from the table, how many wavelengths of the longest wavelength gamma radiation would have the same total length as one wavelength of X-ray radiation? Answer in standard form.

Answer

In this example, we are given the wavelengths of each of the different types of electromagnetic radiation and we need to compare the wavelengths of different types.

Part 1

The first part of the question asks how many wavelengths of ultraviolet radiation would fit into one wavelength of infrared radiation. First, we need to look up the relevant wavelengths: ultraviolet radiation has a wavelength of 10 m, and infrared radiation has a wavelength of 10 m.

We need to find how many times 10 m goes into 10 m, which is the same as asking for 10 m to be divided by 10 m.

A useful trick to remember when doing calculations like this one involving orders of magnitude is that 1010=10.

In this case, 5−(−8)=3, so we have 1010=10.mm

Note that the units of metres appear on the top and bottom of the fraction, so they cancel out to leave a dimensionless number.

Part 2

The second part of the question asks how many wavelengths of the longest wavelength gamma radiation would have the same total length as one wavelength of X-ray radiation?

If we look up the wavelength of gamma radiation, we can see that it is <10m, so the longest wavelength gamma radiation would be 10 m. X-ray radiation has a wavelength of 10 m.

Using the same method as before, we can write 1010=10.m

So the answer is that 10 wavelengths of the longest wavelength gamma radiation would have the same total length as one wavelength of X-ray radiation.

Example 3: Understanding the Sources of Electromagnetic Radiation

Which of the following could be a source of infrared radiation?

  1. Alternating electric currents
  2. Direct electric currents
  3. Decaying atomic nuclei
  4. Thermal motion of atoms and molecules
  5. None of the answers is correct.

Answer

Here, we are given a list of potential sources of radiation, and we need to decide which could be a source of infrared radiation.

Option A, alternating electric currents, is a source of electromagnetic radiation, but the wavelength is longer than infrared, typically in the microwave range.

Direct electric currents, option B, also produce electromagnetic radiation when they are turned on and off, but these are even longer wavelengths, generally in the radio range.

Option C is decaying atomic nuclei, which is a process known as fission, where the nucleus of an atom breaks apart. This generates enormous amounts of energy in the form of electromagnetic radiation, but the energy is too high for the radiation to be in the infrared range of the spectrum. These are gamma rays.

Option D is the thermal motion of atoms and molecules. This means the energy that atoms and molecules have due to their temperature, which causes certain types of radiation to be emitted. This radiation can be in the infrared range of the spectrum. This is why the human body glows brighter than a table when seen in an infrared camera: we emit more infrared radiation, because we are warmer than room temperature.

Therefore, option E, that none of the answers is correct, must be wrong, and so the answer is option D, the thermal motion of atoms and molecules can be a source of infrared radiation.

Key Points

  • Electromagnetic radiation is the name given to the radiation emitted when electrons accelerate. It is also known as light.
  • The wavelength of a wave is the distance between two peaks of the wave. It is usually represented by the symbol 𝜆.
  • Waves with shorter wavelengths have more energy than waves with longer wavelengths.
  • The types of electromagnetic wave, typical wavelengths, and common sources are summarized in the table below.
    The Electromagnetic Spectrum
    Shortest wavelength ⟶ longest wavelength
    Highest energy ⟶ lowest energy
    TypeGammaX-rayUltravioletVisibleInfraredMicrowaveRadio
    Wavelength,
    𝜆()m
    <101010101010>1
    SourceNuclear decayDecelerated electronsThe SunThe SunThermal motion of atoms and moleculesAlternating currentAlternating current

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