Summary

In this lesson students use spectroscopes to analyze the colors of sunlight reflected off a white surface, and determine various colors of light emitted by a multicolor LED bulb at different color settings. They predict what color an object will appear when observed under a certain color of light, and test their predictions with the LED bulb. Students explore how a color wheel can provide a useful model for determining the colors of light an object may absorb or reflect. Finally, students will investigate the color yellow. Many objects that appear yellow are not reflecting yellow light, but reflecting green and red. Students use their spectroscopes to search for objects that truly reflect yellow light.

Grade Level

Middle School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• MS-PS4-2. Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Developing and Using Models
  • Engaging in Argument from Evidence
  • Obtaining, Evaluating, and Communicating Information

Objectives

By the end of this lesson, students should be able to

• Explain that the color of an object depends on the colors of light that the object is exposed to, which wavelengths are absorbed by the atoms/molecules in that object, and which wavelengths are reflected.
• Predict the color of an object under different colors of light.

Chemistry Topics

This lesson supports students’ understanding of

• Energy
• Electromagnetic Spectrum
• Emission Spectrum
• Wavelength
• Absorption
• Reflection
• Light
• Color

Time

Teacher Preparation: 1 hour (initial gathering/purchasing of materials), 10 minutes to set-up

Lesson:

• Engage: 10 minutes
• Explore: Part 1 - 45 minutes, Part 2 - 45 minutes
• Explain: Part 1 - 15 minutes, Part 2 - 15 minutes
• Elaborate: Part 3 - 60 minutes
• Evaluate: 10 minutes

Materials

• Spectroscopes: 1 for every 2 students
  Options:
  • Hand-held, diffraction grating spectroscopes are available for purchase from Flinn Scientific (Item #: AP8696)
  • For purchase, build-your-own spectroscope kits are available from Stanford Solar Center and Astronomers Without Borders
• Color Pencils, with color label: 1 pack for every 2 students
• Multicolor LED light sources: 1 light source for every 8 students (options below)
  • The Crayola Multi Color LED bulb with remote control is available from Toys ‘R Us.
  • Other remote controlled multicolor LED bulbs are available from Home Depot, Amazon, and Walmart. Choose a bulb that at a minimum allows for white, red, orange, yellow, green, blue, and violet light.
  • LED strip lights and remote control with options for white light, primary colors, and secondary colors.

• Different colored objects – 1 set of 8 colors per group (white, black, primary colors, and secondary colors) for each group of 4 students. Options:
  • Paint color sample cards from hardware store
  • Legos
  • Primary and secondary color candies

Safety

• Never look directly at the sun, even with a spectroscope. To observe the colors in sunlight, point the spectroscope at a piece of white paper reflecting the sunlight, or at a white cloud in the sky.
• Use only ground-fault circuit interrupt (GFCI) protected electrical receptacles for power.
• Inspect electric cords for damage before plugging in.

Teacher Notes

• Engage: Before class on the first day of this lesson, set up your multicolor LED light sources, and turn them all to green. Minimize all other sources of light by pulling down window shades. Open the door and let the students into the “green room”. Once the students are seated, hold up three pieces of colored paper – red, blue, and green. The red and blue paper will appear black/dark under the green LED light. The green paper will appear green. Ask the students what color paper you have. Write their responses on the back of the papers. Do not tell them whether they are correct or not, but let them know that you will return to this challenge in the conclusion section of Part 2 of the lab. Put the papers in a folder, out of sight of the students.
• Explore: Students will use spectroscopes to observe the emission spectra of different light sources (Part 1), and observe the colors reflected by different objects (Part 2). They will complete drawings of the spectra, search for patterns, and make predictions. Please note that Part 1 conclusion question #3 asks students to compare how their classroom lights and LED lights produce light. If your classroom light source is also an LED, you can modify this question. Bring in a compact fluorescent (CFL) or incandescent bulb and ask students to compare the LED to one of these sources instead of the classroom lights.
• Explain: Students will draw and label emission spectra. They will develop a model to explain the relationship between the colors of light reflected and those observed by the human eye. They will determine which colors of light are being absorbed. The color wheel will be introduced to help clarify patterns. The three pieces of color paper from the “Engage” portion will be revisited, and in Part 2 Conclusion question #3, students will be allowed to change their color predictions. Once students submit their Part 2 conclusions, the color papers will be viewed with white light and the true colors revealed to the class.
• Elaborate: Part 3 of this activity, “The Search for True Yellow” asks students to find objects that reflect a significant amount of yellow light. Many objects appear yellow because they reflect large amounts of red and green light, which additively make yellow. It is difficult to find objects reflecting a large fraction of the incoming yellow wavelengths. Students will observe various yellow objects with their spectroscopes and record the spectra. They will decide if these are reflectance or emission spectra, and if the color is coming from yellow wavelengths or a combination of red and green.
• Evaluate: This is structured so that students will produce a written report of their findings, including data tables and answers to analysis questions. A short summative assessment quiz is also included.
• Acknowledgements: Part 3 is an extension of the “Search for True Yellow” activity originally developed by Mark Schlawin at Princeton Charter School, Princeton, NJ.
• Answer Keys for Student Part 1, 2 and 3 as well as the Summative Assessment/Quiz are available for download as a separate document.
• Images used in the student documents are all available on Wikimedia Commons, and are hyperlinked to their source.