The question of what happens to small creatures in microwave ovens is a hardy perennial. No end of explanations will surface in the answers and discussions. Some are based on nonsense, some on half sense, and many on perfectly good facts in incorrect contexts and interpretations.

The operative facts are fairly simple. The absorption of, or, for that matter, interaction with, energy from photons of any wavelength, depends critically on the size of the object relative to that wavelength. There are other factors as well, of course, such as the conductivity of the object, but for our purposes we may ignore them. We regard them as constant in this context. For instance, if the subject happens to be a juvenile German cockroach a few millimetres long, in contrast to say, a mature Madagascan cockroach several centimetres long, then we regard them as structures of roughly the same material, only on different scales.

Now, a typical specification for a domestic microwave oven is a frequency of 2.45 GHz, which implies a wavelength of roughly 12 cm. As a rule of thumb, anything with an effective length of much less than a quarter wave, 3 cm, will couple poorly with the energy. Essentially a conductor with a suitable length may act as an effective aerial; at other lengths, especially very small lengths, aerials are ineffective and cannot absorb much energy from the photons.

A 1-cm cockroach might well be practically unaffected by the radiation, while a 3 or 6 cm animal might go pop. The question often is asked why ants can go crawling casually around inside a microwave oven while food is cooking there.  Actually they only can do so as long as they are not either in a place where something else is getting very hot, or where they touch some other conducting material and thereby become part of an effective aerial. If you could train about a dozen three-millimetre ants to stand in a row in a working microwave oven without touching each other, and then at a signal, each to touch the next one in line, then they all would go pop.

For a less heartless, but far easier experiment, take a couple of dozen currants and soak them in water overnight. Dry them all gently. Put several in a row in a microwave oven, not touching each other. A few centimetres away put a few rows in which all the currants of touching their neighbours. Try a rows of 3 cm, 6 cm, 9 cm, 12 and 24 cm. In fact, you can have a lot of fun with different configurations, such as curved, or kinked lines or circles.  See how much the effects differ between items in different parts of the oven.

NB: To avoid ruining your oven, whenever you perform this kind of experiment, put a glass of cold water into the oven as well. That will absorb dangerous excesses of energy.

As controls, you can use various other food items, such as dried fruit of various sizes, both dry and soaked, or slices of carrot or celery. A few such experiments might give you some good insights into the behaviour of microwaves, and for that matter of radio waves and light in general.

How much it would take to kill a cockroach?  Enough to heat it up, if properly coupled to that cockroach as an aerial, or if the cockroach were inside a food item that was heating, of course!

Why not let us know how you get on?

Cheers,

Jon