Lab

Summary

In this lab, students will see the wave nature of light and calculate the speed of light with the use of marshmallows and a microwave.

Grade Level

High school

Objectives

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

• understand the electromagnetic spectrum.
• verify the speed of light.

Chemistry Topics

This lesson supports students’ understanding of

• Electromagnetic spectrum
• Energy
• Atomic theory

Time

Teacher Preparation: 10 minutes

Lesson: 30 minutes

Materials

• Microwave
• Jumbo marshmallows
• Microwave safe dish
• Ruler

Safety

• Always wear safety goggles when working in a lab setting.
• Food in the lab should be considered a chemical, not for consumption.

Teacher Notes

• This is a simple way to show how microwaves are part of the electromagnetic spectrum.
• This could be done as a homework assignment, assuming students have access to a microwave.

Downloads

• Teacher Guide.doc
  
• Teacher Guide.pdf
• Student Activity.doc
  
• Student Activity.pdf
  

Submitted by:
Josephine Golcher
Rosary High School in Fullerton, California

For the Student

Lesson

Background

Microwave ovens cook unevenly because there is a pattern of standing waves that forms inside the oven chamber. The pattern creates an array of hotspots throughout the oven's volume. An operating frequency of around 2,000 MHz should produce a wavelength of about 10 cm, and the hotspots should be at half wave points, or every 5 cm, but in a complex 3-D pattern.

The speed of light is 3x10⁸ m/sec.

Materials

• Microwave
• Jumbo marshmallows
• Microwave safe dish
• Ruler

Procedure

1. Take the marshmallows and place them in the dish, completely covering it with a layer one marshmallow thick. Make sure that the flat side of the marshmallow is facing upwards.
2. Take the turntable and any rotating device out of the microwave.
3. Put the dish of marshmallows in the microwave and cook on low heat. The microwave will not cook evenly after the removal of the turntable and the marshmallows will begin to melt at the hottest spots in the microwave.
4. Heat the marshmallows until they begin to melt in four or five different spots.
5. Remove the dish from the microwave oven and observe the spots.
6. Use a ruler to measure the distance between the melted spots. You should find that one distance repeats, which will correspond to the wavelength of the microwave.
7. Assume the frequency of the microwave is 2,450 MHz, which is common for domestic microwave ovens.

Data and Calculations

Analysis

1. Carry out the simulation at:  http://www.colorado.edu/physics/2000/applets/oven.html
2. Did you get a similar result?
3. Sketch the pattern of the cooked and uncooked marshmallows in your dish.

This picture below  is from the site: http://www.ap.smu.ca/demos/content/modern/speed_of_light/speed_of_light.htm.  Do you agree with the measurements? Explain.

There is a website that disproves this method but is quite complicated to follow. Give two reasons based on your observations as to why this method may not be accurate.

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