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Calculate the speed of light

Experiment with melting chocolate to learn about the speed of light.

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You’ll need

  • Microwave
  • Pens or pencils
  • A4 paper
  • Rulers
  • Calculator or phone
  • Microwavable bowl
  • Microwavable plate
  • Heatproof worktop or surface protector
  • Oven gloves
  • 110g or 120g bar of chocolate, enough for one per group

Before you begin

  • Use the safety checklist to help you plan and risk assess your activity. There's also more guidance to help you carry out your risk assessment, including examples.
  • Don’t forget to make sure all young people and adults involved in the activity know how to take part safely.
  • Make sure you’ll have enough adult helpers. You may need some parents and carers to help if you’re short on helpers.
  • Remember to check for allergies, eating problems or dietary requirements and adjust the recipe as needed. Make sure you've checked everyone's dietary requirements and allergies then adapted the recipe as appropriate. This may include ensuring no cross-contamination during food preparation, too.
  • Check if there’re any items of food (or packaging) that people can’t touch or be near to or if there’re items that people might not be comfortable using in the activity.
  • Some people may not like certain food textures or tastes and that’s OK. Try to find an alternative for them. No-one has to use all the ingredients or be made to try foods if they’re not happy, comfortable or don’t want to.
  • Remember to have a hand washing station and take extra hygiene precautions when handling food. 
  • Read our guidance on food preparation.
  • Make sure you have all the ingredients ready. 
  • You may want to run a kitchen safety talk and show people how to use the equipment safely, such as for cooking or chopping ingredients. You could run our kitchen safety and hygiene activities, Kitchen risk bingo and Home kitchen hygiene prior to running this session.
  • Depending on your meeting place, you may wish for groups to prepare the chocolate in a wider, more spacious area, then invite each group into the kitchen to use the microwave one at a time. Remember the groups not using the kitchen will still need to be supervised, always following the Yellow Card.

 

Planning and setting up the activity

  • Remember to give a safety briefing for the cooking equipment and methods you’re using. You may wish to demonstrate the methods or activity before you all start cooking.
  • Be aware of anyone with Bluetooth medical devices, as being near to turned-on microwaves may cause interference. 
  • Remove the turnstile plate and wheels from the microwave and place them somewhere safe.
  • Place a bowl upside down in the microwave to rest the plate on.
  • Put the pens, paper, ruler, calculator, chocolate bar and plate on a table, enough for one set per group.
  • Make sure to have a heatproof surface to place the hot plate on, such as a worktop protector or trivet. 

The speed of light’s a universal physical constant that is exactly equal to 299,792,458 metres per second. This is approximately 186,000 miles per second. All forms of electromagnetic radiation, including visible light, travel at the speed of light and nothing goes faster than it.

When you look at the stars in the night sky, you’re seeing light that has travelled millions of miles through space to get to you, including our closest star, the Sun. The distance that sunlight needs to travel to get to Earth is huge. The distance is so large that it takes approximately 8 minutes and 20 seconds to reach Earth. If you were to travel the 93 million miles to the Sun, it’d take you around 176 years to get there without stopping.

When looking at scientific calculations and equations, ‘c’ is the universal constant for light speed. 

Albert Einstein’s Theory of relativity unified energy, matter and the speed of light in the famous equation: E=mc².

For this experiment, you’ll be looking at calculating the speed of light using the following equation: c=^f. 

  • C = the speed of light 
  • ^ = the wavelength of light 
  • f = the frequency at which the light wave moves (hertz)

Most microwaves have a frequency of 2.45 gigahertz, which means that the microwaves inside the microwave move up and down 2.45 billion times per second.

For the purposes of this equation, 2.45 is represented by 2,450,000,000.

 

Running the activity

  1. Gather everyone together and ask if anyone knows what the speed of light is. Make sure to explain what it is, how it can be measured and how it can be tested. We’ve included some information on this page that you can use.
  2. Explain that you’ll be learning about the speed of light by melting chocolate. In groups, you’ll be using the microwave to slightly melt their chocolate. Each group will then measure the spaces between the melted spots on the chocolate bar and complete the calculation to work out the speed of light.
  3. Everyone should get into groups, with each group at a different table. 
  4. Ask each group to unwrap and place their chocolate bar on their plate. Make sure there’s no metal or foil wrappers and no metal on the plate, as this metal can’t go in the microwave.
  5. The group’s who are waiting could be doing another supervised science activity.
  6. The first group should place their chocolate into the microwave. Ask them to place the plate on top of the up-turned bowl and run the microwave for 20 seconds. 
  7. When the microwave stops, use the oven gloves to take the plate out of the microwave and place it on a heatproof surface. 
  8. Ask everyone to look for melted spots on the chocolate bar. 
  9. If there are melted spots, they can take their chocolate bar back to their table. 
  10. If there are no melted spots, put it back in for another 20 seconds and repeat until there are melted spots, making sure not to burn the chocolate.
  11. After the group get back to their table, they should measure the distance between two of the melted spots. If there are more than two melted spots, take several measurements and find the average between the pairs. 
  12. Explain that the distance between melted spots is part of a microwave wave – it's half a wavelength. Scientists use this to figure out how fast light travels.
  13. Multiply your measurement by two to get the full wavelength and note this down.
  14. Now, use the formula: Speed of Light = Wavelength × Frequency
  15. Convert your wavelength from centimetres to metres (so divide by 100), then multiply by 2,450,000,000. The frequency of microwaves in most ovens is 2,450,000,000 Hz (or 2.45 GHz).
  16. The actual speed of light is 299,792,458 metres per second (m/s). How close was your result?
  17. After they’ve finished their experiment, you could melt the chocolate and make hot chocolate spoons. 

Reflection

In this activity, you had the opportunity to learn about the speed of light using chocolate. Can you remember anything about the speed of light, such as what it is? How did you measure it? 

You also tried to experiment with the speed of light. Microwaves are a type of electromagnetic wave, just like light, but with a longer wavelength. Was your experiment successful? The speed of light is 299,792,458 m/s. How close did you get to the speed of light number? Why do you think it might be slightly different? You might have had measurement errors or chocolate melting inconsistencies. Is there anything you would do differently next time?  

Safety

All activities must be safely managed. You must complete a thorough risk assessment and take appropriate steps to reduce risk. Use the safety checklist to help you plan and risk assess your activity. Always get approval for the activity, and have suitable supervision and an InTouch process.

You must run your activities in line with the Safeguarding Code of Conduct for Adults (Yellow Card) and report any concerns to the UK HQ Safeguarding Team.

Electrical equipment

Inspect cables for any damage before each use. A responsible adult should supervise people using equipment, and people should follow instructions on how to use them correctly and safely. They should be properly maintained and stored. Be extra cautious of trailing cables and water when using electric equipment.

Science

Supervise young people, and only do science activities that are advised and age appropriate for your section. Test activities first, to make sure you’re confident you can lead them safely. Use protective clothing where necessary.

Cooking

Teach young people how to use cooking equipment safely. Supervise them appropriately throughout. Make sure it’s safe to use and follow manufacturers’ guidelines for use.

Food

Remember to check for allergies, eating problems, fasting or dietary requirements and adjust the recipe as needed. Make sure you’ve suitable areas for storing and preparing food and avoid cross contamination of different foods. Take a look at our guidance on food safety and hygiene.

To make this activity easier, you could try the experiment using one chocolate bar and work out the equation as a whole group. 

Make it accessible

All Scout activities should be inclusive and accessible.

Scouts Space Activity Badge

Try this experiment using different types of chocolate or cheese. Do different materials affect the accuracy of your results?