Lesson: Visible Light and the Electromagnetic Spectrum

Summary

During this lesson, the electromagnetic spectrum is explained and students learn that visible light makes up only a portion of this wide spectrum. Students also learn that engineers use electromagnetic waves for many different applications.

Engineering Connection

Engineers use many types of electromagnetic waves. Gamma radiation is emitted by fuel rods in nuclear power plants, x-rays are used to see inside our bodies, ultraviolet light can be used to sanitize things, microwaves are used to cook, and radio waves allow us to communicate over large distances.

Pre-Req Knowledge

This lesson follows Lessons 1-6 of the Sound and Light Unit.

Learning Objectives

After this lesson, students should be able to:
• Explain that light can be considered an electromagnetic wave.
• Give two examples of how engineers use electromagnetic waves.
• Explain that we can only see a small portion of all electromagnetic waves.

Introduction/Motivation

*Note: These concepts might be very difficult to visualize; if possible, create transparencies of the lesson figures for students to look at during class discussions.
Let's think about our brave adventurers for a minute – who remembers what Angie and Harmon need to do next to find the treasure? That's right – they want to use lights underwater to find the sunken treasure ship. But first, they need to learn something about light waves so that they can choose the right kind of lights to use in their exploration.
When we learned about sound, we discovered that sound travels in waves. Light can also be thought of as a wave – light is a form of an electromagnetic wave. That's a big word, so let's write it on the board.
An electromagnetic wave is a type of wave that can travel through empty space... Yes, you heard correctly, empty space. Unlike sound waves, which need "something" to travel through (for example, water or air), electromagnetic waves are able to travel through "emptiness" or a vacuum.
This picture (show Electromagnetic Transparency #1– in the Attachments section) shows different kinds of electromagnetic waves. Engineers use electromagnetic waves for many different purposes. Gamma rays (nuclear power plant radiation), x-rays, light, microwaves, and radio waves (including cell phone waves) are all electromagnetic waves. What makes all these waves different from each other are their wavelengths and frequencies.
Who would like to remind the class what frequency is? That's right! The frequency of a wave is the number of times a crest occurs each second. Some waves have really big — or even really small — frequencies. If a wave has a higher frequency (many waves in a certain amount of time), it has more energy. And, if a wave has a smaller frequency (fewer waves in a certain amount of time), it has less energy.
Let's look at another picture (show Electromagnetic Transparency #2) to see if we can figure out which waves have the most energy. Which waves do you think are the most powerful? That's right! gamma waves have very high frequencies and, consequently, have a lot of energy. This extreme amount of energy is one reason why gamma waves are very dangerous if improperly used.
Have any of you ever had an x-ray? X-rays are not as strong as gamma rays, but they are still very powerful. A sunburn? Have any of you ever burned your skin when out in the hot sun (or overcast as well) too long? Sunburns come from ultraviolet light, which we cannot see, but can still burn our skin. Radio waves and microwaves have a smaller frequency, so they are much less powerful than x-rays or ultraviolet light. Waves are fascinating, that's for sure!
We know that waves with high frequencies have a lot of energy. And, the waves that have smaller frequencies have less energy — think of these wave types as energetic waves that move very fast and lazy waves that move slow. Did you know that we cannot see most electromagnetic waves? The small section of the spectrum with the waves that we can see is called the visible spectrum, and the wavelengths that we can see allow us to see the colors of red, orange, yellow, green, blue, indigo, violet — the colors of the rainbow! We do not usually think of visible light (the visible spectrum) as being an electromagnetic wave, but it is. Figure 2 shows where visible light falls on the electromagnetic spectrum.
Engineers use electromagnetic waves in all sorts of different inventions. We cannot see most electromagnetic waves, but you do observe these waves every day when you see visible light. Today we are going to build a fascinating invention so that we can see the different colors that are a part of visible light. Let's get started!

Lesson Background and Concepts for Teachers

A wave is a change (a variation) that travels through a substance (or medium). You can often see the change, such as the increased height of a traveling ocean, but what is important to understand is that the medium itself does not travel with the wave.
Ripples in a pond are good examples of waves. If no wind, a pond is smooth until a rock is thrown in and disturbs the water. Then ripples, "disturbances" in the pond, travel to the edges. The medium in this case is the water, through which the ripples travel. The water is not actually moving, but the waves (ripples) are.
Waves move in two ways: longitudinally and transversely. Transverse waves oscillate (move back and forth) in a direction perpendicular to their motion. Our pond ripples, for instance, oscillate up and down but move horizontally towards the edge of the pond. Because the ripples oscillate perpendicular to their horizontal motion towards the edge, they can be classified as transverse waves.
An electromagnetic wave is a transverse wave that can travel through empty space or a vacuum. Literally, electromagnetic waves are able to travel through "emptiness," unlike sound waves, which need "something" to travel through (for example, water or air). Electromagnetic waves have two parts to them: electric and magnetic. Both of these parts are considered transverse waves.
Waves with high frequencies have a lot of energy, and, waves with smaller frequencies have less energy. Most electromagnetic waves are not visible. However, a small section of the spectrum includes waves that we can see — it is called the visible spectrum (see Figure 3). These visible wavelengths allow us to see the colors of the rainbow: red, orange, yellow, green, blue, indigo and violet. We do not usually think of visible light (the visible spectrum) as being an electromagnetic wave, but it is.
The electromagnetic spectrum can be considered from another perspective — in nanometers — specifically from 400 nm to 700 nm. Each color in this visual range has a different wavelength. Red has the longest wavelength (700 nm) and violet has the shortest wavelength (400 nm). Green occurs near the middle at about 550 nm. A prism divides light into the wavelengths that make it up. Seen together, color waves make white light. White light is especially dramatic because many different colors of the visual spectrum can combine to make white light. Two "white" light sources can have very different spectral compositions. When white light shines on a prism, the colors in white light separate from each other because they refract at different angles depending on their wavelength (see Figure 4). Water droplets in the air refract sunlight to create rainbows.
Light can be absorbed, reflected (or diffused) and refracted. Some materials can affect how light bends in more than one way, refracting and reflecting at the same time. Objects made of more than one substance usually have different reflective, refractive and absorptive properties.
Light reflects at a predictable angle: the angle of the light that strikes a surface equals the angle of the light that bounces off the surface. Rough surfaces scatter — or diffuse — light, which can cause glare, blur an image or prevent us from seeing an image.
Light changes speed and direction — refracts — when it moves from one transparent medium to another. The refractive property of transparent materials can be used to make lenses that focus light (for example, cameras, eyeglasses, telescopes).
Since visible light is the part of the electromagnetic spectrum that our eyes can see (remember, we cannot see most electromagnetic waves), our whole world is oriented around it and the colors that are produced through this visible spectrum. Understanding these visible electromagnetic waves has enabled engineers to develop many instruments that can see farther and more clearly than our eyes. That is why we use satellites to look at the Earth, and telescopes to look at the Sky!

Vocabulary/Definitions

 A transverse wave with electric and magnetic parts that travels at 300,000 km/sec. Examples are gamma waves, x-rays, light waves, microwaves, and radio waves. The range of the electromagnetic spectrum — between 400 nm and 700 nm — that our eyes can see.

Associated Activities

• Building a Fancy Spectrograph - Students create and decorate their own spectrographs using simple materials and holographic diffraction gratings. A holographic diffraction grating acts like a prism, showing the visual components of light. After building the spectrographs, students observe the spectra of different light sources as homework.
• The Visual Spectrum - Students make simple spectroscopes (prisms) to look at different light sources. Using the spectroscopes, students see differing spectral distributions of different light sources

Lesson Closure

You all did a great job today of being good listeners and thinking hard about light! We learned that light can be thought of as a wave, just as sound is a wave. We also learned that our eyes can only see some wavelengths of electromagnetic radiation. What do we call the set of wavelengths that our eyes can see ? That's right---the visible spectrum. We also discovered that engineers use many different electromagnetic waves for many different applications. In our next lesson, we are going to learn about rainbows, and continue to follow the adventures of Angie and Harmon. What do you think will happen to them next?

Assessment

Pre-Lesson Assessment

Discussion Question: Turn on a radio, and tune it to a station. Next turn on a lamp. Ask the students what the two devices (radio and lamp) have in common. (Answer: They both use electromagnetic waves.) Tell students that today we are going to learn about a few different types of electromagnetic waves.

Post-Introduction Assessment

Fill in the Table: On the left side of the classroom board, list the types of electromagnetic waves. Then work with the students to fill in the right side with ways that engineers use each type of wave. Examples of waves and their uses are:
• Gamma waves – emitted by nuclear power plants
• X-rays – used for healthcare
• Light waves – for seeing visible light
• Microwaves – for cooking
• Radio waves – for communication

Lesson Summary Assessment

What I Learned Today: At lesson end, give students time to think about what they learned. Invite a few students to volunteer something new that they learned through the lesson. (If students do not mention it, remind them that they learned that visible light is an electromagnetic wave and the only electromagnetic wave we can see; also it is only a small portion of the overall spectrum of electromagnetic waves.)

Lesson Extension Activities

Bring in small prisms for students to experiment with and explore how they break light into different wavelengths/colors.
Have students create an electromagnetic wave journal of different electromagnetic waves they experience throughout the week (for example, microwaving food, seeing a rainbow, listening to the radio, etc.).
Invite an engineer to visit the class and talk about how s/he uses electromagnetic waves in his/her research or work.

References

Dunbar, Brian. National Aeronautics and Space Administration, Langley Research Center, Multimedia, April 13, 2007. Accessed June 1, 2007. http://www.nasa.gov/centers/langley/images/content/114284main_EM_Spectrum500.jpg

Ferebee, Michelle T. National Aeronautics and Space Administration, Atmospheric Sciences Data Center, April 10, 2006. Accessed April 17, 2012. http://science-edu.larc.nasa.gov/EDDOCS/images/Erb/wavelength_figure.jpg

Sample, Sharon. National Aeronautics and Space Administration, Science@NASA, Science Fun, November 22, 2004. Accessed June 7, 2007. http://science.hq.nasa.gov/kids/imagers/ems/waves3.html

Contributors

Luke Simmons, Frank Burkholder, Abigail Watrous, Janet Yowell