SNC2D Light and Geometric Optics
Teacher Demo/Student Activity: Measuring the Speed of Light
Topics
Timing
the speed of light
preparation: 20 min
demonstration/activity: 10 min
Specific Expectations
SNC2D
A1.5 conduct inquiries, controlling some variables, adapting or extending procedures as
required, and using standard equipment and materials safely, accurately, and effectively, to
collect observations and data
A1.12 use appropriate numeric, symbolic, and graphic modes of representation, and appropriate
units of measurement (e.g., SI and imperial units)
A1.13 express the results of any calculations involving data accurately and precisely
E3.8 describe properties of light, and use them to explain naturally occurring optical phenomena
(e.g., apparent depth, shimmering, a mirage, a rainbow)
Introduction
This demonstration allows students to experimentally determine the speed of light by heating a
piece of chocolate in a microwave oven. This demonstration is best used as an enrichment
activity as the concepts involved go beyond the curriculum expectations. Still, the demonstration
offers a very interesting and intriguing look at electromagnetic waves.
Materials
safety goggles
lab coat or apron
heat-proof gloves or oven mitts
microwave oven with rotating platter (with
platter removed)
large chocolate bar (at least 10 cm long)
microwave-safe dish (large enough to hold
the chocolate bar)
ruler
Safety Considerations

Take proper precautions when operating the microwave oven. Never put any metal objects
or metal foil in an operating microwave oven.
Procedure
Ensure that all participants are wearing appropriate PPE: safety goggles, lab coat or apron, heatproof gloves or oven mitts.
1.
Record the frequency of the microwave oven. A typical value would be 2450 MHz.
2.
Place the unwrapped chocolate bar in the microwavable dish and place it in the microwave
oven. Be sure to remove the rotating platter first.
3.
Close the door and turn the microwave oven on at full power for about 60 s.
4.
Observe through the door until soft spots start to form on the chocolate bar. The time
required varies with the oven used but the time range should be between 15 and 45 s. After
5.
6.
at least two soft spots have formed on the chocolate bar, take the bar out and measure the
distance between the centre of the spots. You may have to poke and prod the chocolate to
find the soft spots. Convert this distance to metres (m).
The distance between the soft spots represents half of the wavelength of the microwaves
used. Multiply the distance between the spots by 2 to get the wavelength of the
microwaves used.
Multiply the wavelength of the microwaves by the frequency of the microwaves to
calculate the speed of the microwaves. Compare this answer with the accepted value of the
speed of light in air. (The speed of light in a vacuum and in air is 299 792 458 m/s).
Disposal
Dispose of the chocolate in the municipal green waste recycling bin or in the regular garbage.
Students must not be allowed to eat in the lab.
What happens?
A microwave oven heats food by passing standing microwaves through the food. Standing waves
have points known as antinodes and at these points the energy delivered is a maximum (Fig.1).
These are the points where the chocolate starts to melt first (the soft spots).
How does it work?
Microwaves, just like light waves, are electromagnetic waves within a certain frequency and
wavelength range. The wavelength of the wave is the length between successive peaks (crests) of
the wave. The symbol for wavelength is the Greek letter “lambda” (λ). The number of waves per
second is known as the frequency (f). Frequency is measured in hertz (Hz) – one hertz is one
wave or cycle per second. The speed of any wave (v), measured in metres per second, can be
determined by using the universal wave equation which says that the speed of a wave in is equal
to the wavelength in metres times the frequency in hertz or v = f λ.
In this demonstration, you can determine the frequency of the microwaves used by finding the
frequency information on the manufacturer’s label at the back of the microwave oven.
Fig.1 Diagram of microwaves in a microwave oven
The distance between successive antinodes is equal to one half the wavelength of the
microwaves. Therefore, if we measure the distance between the soft spots (each one occurring at
an antinode) this distance represents half the wavelength of the microwaves. If we multiply this
by two we get the wavelength of the microwaves. Be sure to convert the wavelength to metres.
Finally, we can determine the speed of the microwaves by multiplying the wavelength by the
frequency.
Teaching Suggestions/Hints
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This demonstration is simple and yields surprisingly good results. Be warned that some of
the concepts involved (standing waves, antinodes) are beyond the curriculum expectations.
With this in mind, use this activity as enrichment or extension.
You will need to know the frequency of the microwaves used in the oven. This information
should be on a label on the back of the oven.
It is possible to substitute other food products for chocolate, such as buttered toast or
marshmallows.
Standing waves can be demonstrated to students by using springs or beaded pull chains.
Next Steps
None
Additional Resources
1.
A more detailed explanation of the demonstration and related concepts -
2.
http://morningcoffeephysics.com/measuring-the-speed-of-light-with-chocolate-and-amicrowave-oven/
Online video showing this demonstration http://www.youtube.com/watch?v=7WXW2bBWBEg