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20 DESIGN007 MAGAZINE I AUGUST 2020 The speed of light is the one universal physi- cal constant that we are yet to break. It is the limit of the velocity at which conventional matter and information can attain in our uni- verse—without warping space-time, of course. At a lightning 299,792,458 m/s, it is the max- imum speed at which massless particles (or waves) of light, electromagnetic energy, and gravitational waves travel in a vacuum. In this month's column, I will look at how to simply measure the speed of light and how the wave- length of electromagnetic energy relates to the multilayer PCB. One morning recently, whilst eating my vegemite toast (as Australians do), I was read- ing my weekly New Scientist Magazine and came across an interesting article on how to measure the speed of light using a chocolate bar and a microwave oven. Here's how it works. A microwave oven's magnetron (RF trans- mitter) oscillates at 2.45 GHz. Electromagnetic energy in this frequency range has an inter- esting property: It is absorbed by water, fats, and sugars. The microwaves, in the turned cavity, penetrate the food and excite the mol- ecules heating the food throughout—provided the turntable is rotating. But for this exercise, the chocolate bar needs to be stationary, so remove the turntable. Since the chocolate bar is not rotating, the microwaves are not evenly distributed throughout the bar, and regions of chocolate will begin to melt in the high-intensity areas. Chilling the bar first makes the molten areas more distinct. This will take approximately 50 seconds on high power. Take care not to exceed 60 seconds, or you may have a mess to clean (lick) up. E l e c t r o m a g n e t i c energy travels in a wave through a vac- uum or air at the speed of light. The distance between the peaks of the wave is the wavelength of the energy. As the wave travels, the peaks and troughs heat the choc- olate. By measuring the distance between these hot spots, one can determine the half wavelength of the energy (Figure 1). You The Wavelength of Electromagnetic Energy Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Half wavelength between hot spots. (Source: NASA Goddard Space Flight Center)

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