This page accompanies the Lightspeed activity guide.

For help with working out averages (or *means*), take a look at either Skillswise or Bitesize.

Once you have an average for the distance between the centres of your adjacent melt patches, this page offers you three ways to work out the speed of light: Automatic calculation, Downloadable spreadsheet, Calculation step by step.

Note: your web browser must allow Javascript to run this routine.

If you have Microsoft Excel – or any other software that can open .xls files – then you may find the Lightspeed Calculator spreadsheet useful. The spreadsheet lets you examine the formulae more closely if you want to.

Open it and enter the numbers where you're asked to. You need exactly the same information as for the automatic calculation above.

The speed of a wave is found by multiplying the wavelength by the number of waves that happen every second, ie the wave's frequency.

Light waves and microwaves are energy waves that travel at the same speed but have different wavelengths and frequencies.

The distance between margarine melt patches is related to the wavelength of the microwaves (the distance between peaks in the cycle of the energy wave).

- To work out the wavelength, take your average distance between melt dot centres (in centimetres) and
**multiply it by 2**. - To convert the wavelength to metres,
**divide it by 100**.

If your oven has a label showing the frequency, you need to convert the figure from either MHz (megahertz) or GHz (gigahertz) to cycles per second, measured in Hz (hertz).

There are 1 million hertz in 1MHz and 1 billion hertz in 1GHz.

- So
**multiply**your label figure**by either 1,000,000**(if from MHz)**or 1,000,000,000**(if from GHz) to get the number in cycles per second.

No label? Many microwave ovens run at 2.45GHz, which is 2,450MHz or 2,450,000,000Hz.

- Finally,
**multiply together**your wavelength figure (in metres) and the frequency (cycles per second) figure to get a number that is the speed of your microwaves in metres per second (written m/s).

You're aiming for a number around 300,000,000m/s. Unfortunately that's such a big number that some calculators can't display it.

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