Prep: 30 mins | Activity: 5 mins
In this activity, students observe and collect data on the interaction of microwave radiation and soft candy. The wave-matter interaction causes the rotational energy (kinetic energy) of water to increase, which in turn softens or even melts the candy. Patterns in candy softness or degree of melting provide the data necessary to calculate the wavelength of the microwave.
Additional calculations using the wave formula allow the speed of the wave to be determined. A discussion of electromagnetic wave frequency and matter interactions leads students to claim that higher frequency, shorter wavelength electromagnetic radiation (EMR) produces more energetic interactions with matter and more damage to living tissue.
The calculated microwave speed should be the same as the speed of light and all other EMR, 3 × 10^{8} m/s. If there is only one microwave in the classroom or lab, the teacher will perform this activity as a demonstration.
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:
What are the effects of interactions between matter and different frequencies of radiation?
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.
Obtaining, Evaluating, and Communicating Information
PS4.C: Electromagnetic Radiation
Cause and Effect
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.
Dispose of candy and the microwave mounting tray in the trash.
For groups of students or a class.
Average the distances between marks. Record the distance in centimeters.
6.0 cm
Use the average to determine the wavelength of the microwave.
6.0 cm × 2 = 12.0 cm
Convert the microwave wavelength into meters.
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
Convert gigahertz (GHz) to hertz (Hz).
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 10^{9} Hz = 3 × 10^{8} m/s
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.
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.
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