The health and safety message on the wall above our office microwave includes the advice that we should not look closely into the microwave while it is heating food. What would happen if we ignored this advice? And how close is it safe for me to get?
• We can see food cooking inside a microwave oven through the metal mesh incorporated into the glass of the door. This is because visible light has a wavelength of around 500 nanometres – about 5000 times smaller than the holes in the mesh – and so passes through unhindered.
But as far as microwaves are concerned, the mesh might as well be an impenetrable sheet of metal. This is because the microwave radiation used in ovens has a wavelength of around 12 centimetres, about 60 times the size of the holes in the mesh. It is perhaps easier to visualise the situation if we replace the waves with their particle equivalents, photons. Photons of light are skinny enough to squeeze through the holes in the mesh whereas fat microwave photons cannot.
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However, when any electromagnetic radiation is reflected in this way, an “” extends slightly beyond the reflector. The intensity of this radiation falls off exponentially and is virtually zero by the time it reaches the outer surface of the glass.
If you hold your hand against the window, you will not expose your skin to significant microwave radiation. But if a bigger hole were torn in the metal mesh, the evanescent wave would extend further out and you would risk burning your hand. A bigger risk comes from microwaves leaking through any gaps around an ill-fitting door.
Unless there were an obvious manufacturing fault or subsequent damage to the oven, it would probably be safe enough for you to stick your nose up against the glass. But why take an unnecessary risk? I suspect the message is a potential defence against a lawsuit in what is becoming an increasingly litigious society.
In defence of your employer, it might be unreasonable for them to keep a constant vigil on the microwave oven to ensure it is fault-free, but it might be helpful if the message urged users to report any damage, particularly to the metal mesh or the door seal.
Mike Follows, Willenhall, West Midlands, UK