EMR and Matter Interactions
Physical Science, Physics Recommended for grades 9–12.
Teacher Prep: 30 min
Student Activity: 5 min
Student Analysis: 30 minutes
Safety Goggles Required
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Phenomenon
What interactions between microwaves and matter can you identify in the photograph on the left? What interactions between X-rays and human tissue can you identify in the photograph on the right?
As a reminder, wave properties include:
The bands on the electromagnetic spectrum include:
Essential Question
What are the effects of interactions between matter and different frequencies of radiation?
Activity Objectives
- Identify the wavelength of a microwave using changes in candies produced by interactions between the microwave radiation and candy.
- Use the wave equation to calculate the speed of the microwave.
- Use the activity data, observations, and proportional reasoning to make an argument regarding different frequencies of EMR, interactions with matter, and possible human health concerns.
Next Generation Science Standards* (NGSS)
PE HS-PS4-4. Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter.
Science and Engineering Practices
Obtaining, Evaluating, and Communicating Information
- Evaluate the validity and reliability of multiple claims that appear in scientific and technical texts or media reports, verifying the data when possible.
Disciplinary Core Ideas
PS4.C: Electromagnetic Radiation
- When light or longer wavelength electromagnetic radiation is absorbed in matter, it is generally converted into thermal energy (heat). Shorter wavelength electromagnetic radiation (ultraviolet, x-rays, gamma) can ionize atoms and cause damage to living cells.
Crosscutting Concepts
Cause and Effect
- Cause and effect relationships can be suggested and predicted for complex natural and humandesigned systems by examining what is known about smaller scale mechanisms within the system.
Safety Procedures and Precautions
Candy will be hot and sticky out of the microwave and could cause burns. Handle carefully. If completing this activity at home, parent supervision is necessary.
Teacher Preparation and Disposal
Dispose of candy and the microwave mounting tray in the trash.
Procedure
- If the microwave has a carousel (turntable), remove it.
- Can be a teacher demonstration or student activity.
- Construct a carboard mounting tray for the candy by measuring the dimensions of the floor of the microwave.
- Add 5 inches to the width measurement. Draw a line on each side of the cardboard, 2.5 inches from the edge (see image below).
- Score lines on the edges of the cardboard, but do not cut the sections off.
- Fold the end sections down to make a stand for the cardboard tray.
- Draw a straight line across the middle of the cardboard, from left to right. This line will be used to position the candy.
- Make sure the tray fits easily in the microwave.
- Remove the tray from the oven and make any necessary changes.
- Place the candy along the positioning line, making sure that the candy pieces touch and cover the full width of the tray.
- Place the tray in the microwave and microwave on full power for 5 to 7 seconds.
- Check the softness of each candy piece with the craft stick. If the candy has softened, place a mark on the mounting tray where the candy has softened.
- If there is no indication of softening, repeat the procedure until it is evident that some pieces of candy have become soft and are beginning to melt.
Data & Observations
For groups of students or a class.
1. Beginning on the left edge, measure the distance from the edge to each mark, in centimeters. Use the following image as a reference.
Analysis & Discussion
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Average the distances between marks. Record the distance in centimeters.
6.0 cm
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Use the average to determine the wavelength of the microwave.
6.0 cm × 2 = 12.0 cm
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Convert the microwave wavelength into meters.
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Locate the frequency of the oven’s microwave. This is found on a sticker or plate inside the oven or on the oven door.
2.5 GHz
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Convert gigahertz (GHz) to hertz (Hz).
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Calculate the speed of the microwave using the wave equation, velocity equals wavelength times frequency. v =λ x f
v = 0.12 m × 2.5 x 109 Hz = 3 × 108 m/s
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All electromagnetic waves travel at the speed of light, rounded to 3 x 108 m/s. Calculate and evaluate the percent error for the value calculated above.
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You have established the speed the electromagnetic radiation above, which applies to all types of EMR. Use the candy as a rough model for living tissue, since it is made from sugars, proteins, and fats, and make a claim relating possible tissue damage to wave frequency and wavelength. Begin with a mathematical statement of proportionality.
The wave equation is v = λf. If wavelength increases, then frequency decreases. Conversely, if wavelength decreases, then frequency increases. Planck’s formula, E = hf, establishes the relationship between wave energy and wave frequency; as frequency increases, the energy of the wave increases.
So, the smaller the wavelength, the higher the frequency, and the greater the wave energy. Additionally, as wavelength decreases to the nanometer and smaller scales, the wavelength becomes small enough to interact with molecules and even electrons. These small wavelength-high frequency waves are referred to as ionizing radiation because they can remove electrons from outer electron levels, resulting in ions or free radicals, which can be very damaging to tissue.
*Next Generation Science Standards® is a registered trademark of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of, and do not endorse, these products.
