When I put a cold plate of cold soup in the microwave, I take out a cold plate of hot soup in a minute. How does the microwave know what to heat?
The microwave oven is an example of an invention that was created not on the basis of a theoretical design, but happened by chance. While working on military radar components, Percy Spencer discovered that a chocolate bar in his pocket had melted. He researched further and the result was a microwave. Warm bodies differ from cold ones in that the molecules move faster in them. The faster the molecules move, the higher the temperature of the body. We can move molecules in different ways (e.g. collisions of surrounding molecules — so-called conduction) but we always need some form of energy for that. When a molecule receives energy, it starts moving. Microwave radiation has energy just like any other radiation. So why don't we shine ultraviolet light on soup? Because water absorbs a particular frequency significantly better than others. The 2.45 GHz frequency is an example which belongs to microwave radiation. If we put such waves into water, the water molecules oscillate with the same frequency (resonance occurs). The vibrating molecule will strike neighbouring molecules and move them, which we observe as a rise in temperature. Each molecule is composed differently and therefore oscillates at a different frequencies. So the radiation that is generated in a microwave will be absorbed only by the water molecules (and increase their temperature), while it will either pass through or bounce off other materials. Similar to light passing through transparent glass, the light does not heat the glass but only passes through it.
By finding out the frequency that water absorbs, it was possible to heat only the water and not other materials.
And because the chocolate bar contains water, it spreads. On the other hand, the fired ceramic plate does not contain water and therefore does not heat up… at least not by microwaves. The fact that the plate is warm after being removed from the microwave is caused by the fact that water molecules hit the plate molecules and thus speed up their movement which we again perceive as an increase in temperature. In this case, it is heat conduction.
Finally, we are composed of water and therefore it is not safe to be exposed to such microwave radiation. It would increase our temperature — specifically all parts of the body that contain water. This is used by so-called non-lethal weapons — microwave radiation can be used to disperse a crowd.
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