Lesson 11: The Electromagnetic Spectrum

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Seems like quite a jump from quantifying chemical reactions to learning about the electromagnetic spectrum, but it will soon become clear that in order to understand chemical reactions, we really need to understand electrons and to understand electrons, we need to learn about the electromagnetic spectrum.

Textbook reading:  Chapter 9, Sections 9.1 – 9.4. The Electromagnetic Spectrum and the Bohr Model of the Atom, pp. 285-294.



Before we jump into the electromagnetic spectrum, let’s spend a minute learning about waves. You may have read that light and its relatives in the spectrum travel in waves. What does that mean?

Think about physical waves, like the waves in the ocean. Electromagnetic waves exhibit a similar motion. Check this interactive website from Colorado to see how you can create varying waves with different motions.

Of course, Bozeman Science also has an introduction to waves with cool animations to help you understand what you did at the interactive site.


(Illustration from NASA)

Although electromagnetic radiation forms a continuous ribbon of wavelengths, we cut the ribbon into segments and name different regions based on their size and related properties. Electromagnetic radiation has been mined over the years to produce many useful gadgets, which helps us relate to the different types.

Moving from left to right, we find that the first piece of the ribbon, radio waves, are the largest (the illustration suggests as big as buildings). Besides sending sound to radios, radio waves are are used in astronomy. For example, radar and radio waves were used to investigate the recent asteroid that passed by the Earth. Currently the largest telescope to detect radio waves from space is the Arecibo Radio Telescope in Puerto Rico.

Microwaves consist of wavelengths shorter than radio waves. They are used for all sorts of things besides cooking, including remote sensing of weather for forecasting, as well as for monitoring space. Some forms are also used for communications.

Nearer to the visible spectrum (forms of electromagnetic radiation we can see), are the infrared waves. We can’t see infrared radiation with our eyes, but we can sometimes feel it is as heat. Certain snakes, called pit vipers, have organs on their heads that can detect infrared radiation as a way to find their prey in the dark. Humans rely on equipment, such as specially designed cameras, to observe infrared radiation. Your TV remote works using infrared waves.

We are most familiar with the wavelengths in the visible spectrum. Once again, the wavelengths are continuous, not discrete regions. Sir Isaac Newton paved the way by breaking white light into its components with a prism and naming bands of wavelengths as colors. Although realistically they could be broken into any number of colors, he gave us seven: red, orange, yellow, green, blue, indigo and violet.

Ultraviolet radiation has a slightly shorter wavelength than visible light. Humans can’t see ultraviolet light, but many other animals can, including honey bees and butterflies. In fact, many flowers have spots we can not see in visible light, but that show under ultraviolet light. Scientists call these spots and patterns “nectar guides” because they are thought to attract bees and other pollinators to the flower.

X-rays are a form of electromagnetic radiation with a very short wavelength. X-rays are considered to be “ionizing radiation,” which means its energy levels are high enough that when it strikes a molecule it can remove an electron, thus forming an ion. Because of this property, exposure to X-rays should be limited.

Certain gamma rays have the highest level of energy of all the forms of electromagnetic radiation, as well as the shortest wavelength. Here on earth gamma rays are only produced by lightning, radioactive decay of certain radioactive elements, and nuclear explosions. In space they are produced by high-energy events such as supernovas. Gamma rays are mostly harmful, but they can also be helpful because they are used to treat certain cancers and to kill bacteria in food.


The various forms of electromagnetic radiation were long known to behave like waves, but also they also act like particles. Scientists call these elementary particles photons. They are odd particles because they have no mass or charge, and could be thought of as bundles of energy or “quanta.” At motion in a vacuum, photons travel at a constant rate: the speed of light c = 2.998 x 108 m/s. One way photons behave like particles is when they collide with electrons.

Bohr Model of the Atom

Where is all this going? Niels Bohr used data from light emission spectra of elements to develop his famous model of the atom.


Visible lines in the hydrogen emission spectrum (public domain image from Wikimedia)

Bohr’s observations that atoms of different elements emit light at only certain wavelengths lead him to conclude that electrons are fixed positions from the nucleus. Check out the ladder analogy in Figure 9.10 on page 293 in your text.


(Public domain image from Wikimedia)

Although Bohr’s model has been widely adopted in popular culture, it only had the power to explain the hydrogen emission spectrum, not those for elements. Obviously, something more was needed. We will learn about how the model has be modified over the years in our next lesson.

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