To introduce students to the electromagnetic spectrum, focusing on visible light. Students will be introduced to the idea that all light travels as waves, and that wavelength defines the various regions of the electromagnetic spectrum.
Many middle-school students do not think of light as something that travels from one place to another, and most tend to identify light with its source (e.g., light is in the bulb) or its effects (e.g., a patch of light). As a result, students have difficulty explaining the direction and formation of shadows, as well as reflection of light by objects.
Middle-school students usually understand that mirrors reflect light, but have a hard time believing that of objects which do not reflect their image. Many students do not believe that their eyes actually receive light when looking at an object. Students' ideas of vision vary from the notion that light fills space and the eye sees without anything linking it to the object, to the idea that light illuminates surfaces that we can see by the action of our eyes upon them. The notion that the eye can see without a link to the object is a notion that can even persist after basic instruction in optics. (Benchmarks for Science Literacy, pp. 338-339.)
There are three Science NetLinks lessons in this series on light:
Light 1: This lesson is an introduction to light, preparing students to address issues like those discussed above. This lesson introduces students to the electromagnetic spectrum (focusing on visible light) and the wave nature of light.
Light 2: This lesson focuses on the idea that we can see objects because they either emit or reflect light. It discusses the way light is reflected, absorbed, and scattered to allow certain wavelengths to reach the eye, leading to a perception of different colors.
Light 3: This lesson includes an Internet exploration that focuses on the roles of the eye and brain in the perception of color. It includes an introduction to the anatomy of the eye, including the functions of rods and cones.
Hand each student or group of students a prism, and ask them if they know what it is. Give them time to use the prisms to break up the white light of the room, a flashlight, or sunlight coming into the room. If you have an overhead projector, set a prism on it to project the visible light spectrum onto the projection screen.
Ask students the following questions:
- What do you see when you play with the prism?
- What are the colors of the "rainbow"? (Red, orange, yellow, green, blue, indigo and violet. Students may be familiar with the phrase "Roy G. Biv"; if not, you could introduce them to it.)
- Are these colors distinct, or do they seem to blend together? (They blend together. If students didn't notice this, allow more time for exploration with the prisms.)
Review with students that visible light is light they can see, and tell them that now they will learn about light they cannot see, as well as about how light travels.
Have students access the Making Light of Science student esheet. Although they will use this sheet online, they may want to follow along with a print copy.
As outlined on the esheet, students will navigate through three websites for the remainder of the lesson. As students move through the sites, you will lead class discussions to help them make sense of the information, as well as assess their understanding. Discussion questions are included below.
After students complete Step 1 on the esheet, help them relate what they have read back to the prism activity. That is, remind them that the prism was a demonstration of the visible light spectrum, within the electromagnetic spectrum about which they just read.
Then ask students these questions:
- What is the electromagnetic spectrum? (It is the entire range of all kinds of light, including light the human eye cannot see.)
- What is the light that humans can see called? (Visible light.)
- How does light travel? (It travels as waves.)
- What are the high and low points of a wave called? (They are called the peak and trough.)
- What is wavelength? (It is the distance between two nearest peaks or troughs.)
- How is wavelength important in defining the regions of the electromagnetic spectrum? (Different parts of the electromagnetic spectrum are made up of light of different wavelengths.)
- What is a common measurement of wavelength? (Angstroms.)
Next, have students move to Step 2 on the esheet. Allow students to move through the slides at their own pace, following the links embedded in the text.
Review the concepts listed on the final page, including asking students these questions:
- How are light and ripples in a pond similar? (They travel as waves.)
- How does wavelength change as you move from left to right along the electromagnetic spectrum? (Wavelength gets shorter.)
- What region of the electromagnetic spectrum consists of light humans can see? (Visible light.)
- What are the other parts of the electromagnetic spectrum? (They are radio, microwaves, infrared, ultraviolet, X rays, and gamma rays.)
- How do our eyes perceive color? (Each color of visible light that we see has a different wavelength. Our eyes respond differently to each wavelength.)
- What type of light does the ozone layer protect us from? (Ultraviolet.)
- Which type of light has the shortest wavelength and the most energy? (Gamma rays.)
- How are energy and wavelength related? (Shorter wavelengths carry more energy than longer wavelengths.)
Now do the classic Slinky demonstration to further explore the nature of waves. Stretch the Slinky across the room so everyone can see it; you should hold one end and have a student volunteer hold the other end. Slowly move your end of the Slinky to form waves. Practice with the Slinky until you are able to generate standing waves, or waves that don't appear to move.
Generate a standing wave and ask these questions:
- Where is the peak of the wave? (Have a student point to the peak.)
- Where is the trough of the wave? (Again, have a student point it out.)
- If we were going to measure the wavelength of this wave, where would we measure? (From one peak or trough to the next.)
- Have students use a ruler to actually measure the wavelength.
- According to what we learned on the websites, do long or short waves have more energy? (Short.)
Now, move the Slinky slowly to generate long wavelengths and tell students to observe closely, then make waves of much shorter wavelength by moving the Slinky very quickly. (Be sure to move the Slinky approximately the same distance each time so that the waves all have about the same height.) If done just right, this may demonstrate that the short waves have more energy. If so, ask students if they can draw that conclusion from this demonstration.
This demonstration touches on the concept of frequency, which can be explained further if appropriate for your students. Other concepts related to waves could also be demonstrated and discussed, such as amplitude, echoes, and waves adding and subtracting. In addition, students could practice measuring wavelength and frequency at Making Waves, on the Star Light, Star Bright site.
Have students complete Step 3 on the student esheet. The information at this website reviews and builds on what they have learned in this lesson.
Then give students the prisms, allowing them to once again explore and see the various colors of the rainbow. At this time, students should better understand and be able to explain in scientific terms how the prism works.
Ask students these questions:
- In what century was the visible light spectrum first observed? By whom? (It was first observed in the 17th century by Isaac Newton.)
- Explain how the prism works (i.e., how it breaks light into a rainbow). Explain this using words and an illustration. (As stated on the "Light Facts" page, students should understand that in sunlight, all the wavelengths [colors] are mixed together, and they travel at the same speed. When light passes through the prism, it changes its speed just a little. But each wavelength, which makes up the light, changes its speed by a different amount. The differences in speed cause each wavelength to bend a little differently when the light comes out the other side of the prism, resulting in the appearance of different colors.)
Complete the second and third Science NetLinks lessons in this series:
At Optics For Kids, students can explore things such as simple lenses, prisms, and mirrors.
At CD Spectroscope, on the Little Shop of Physics site, students use old CDs and card stock to make a simple spectroscope. The spectroscope can then be used to separate light from various light sources.
The Light Stuff, at the NOVA Online site, is an activity where students discover how light can be slowed down when it passes through certain objects or substances.
The Exploratorium includes a large variety of light-related activities and labs.