SNC2D Light and Geometric Optics Teacher Demo/Student Activity: Measuring the Speed of Light Topics the speed of light Timing 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
Transcript
SNC2D Light and Geometric OpticsTeacher Demo/Student Activity: Measuring the Speed of Light
Topicsthe speed of light
Timingpreparation: 20 mindemonstration/activity: 10 min
Specific Expectations
SNC2DA1.5 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 preciselyE3.8 describe properties of light, and use them to explain naturally occurring optical phenomena
(e.g., apparent depth, shimmering, a mirage, a rainbow)
IntroductionThis 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.
Materialssafety goggles lab coat or apronheat-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.
ProcedureEnsure that all participants are wearing appropriate PPE: safety goggles, lab coat or apron, heat-proof 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 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).
5. 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.
6. 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).
DisposalDispose 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/Hints1. 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.
2. 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.
3. It is possible to substitute other food products for chocolate, such as buttered toast or marshmallows.
4. Standing waves can be demonstrated to students by using springs or beaded pull chains.
Next StepsNone
Additional Resources1. A more detailed explanation of the demonstration and related concepts -
