Evan of Tauranga asks :-

Is radiation still present in food for several minutes after the food has been cooked in a microwave oven?

Martin Gledhill, a physicist at the Ministry of Health's national Radiation laboratory in Christchurch, responded.

No. Microwaves in a microwave oven are absorbed by polar molecules (that is, molecules which have an uneven distribution of electric charge) which makes them oscillate. One of the most important polar molecules is water, which is present in almost all foods. Some biological materials, such as amino acids which make up proteins, are also polar.

The increased oscillation of these polar molecules is what is more familiarly called heat. Hence these interactions convert the microwave energy into heat energy, which cooks the food.

When some recipes call for the food to be left standing for a few minutes after you have finished cooking, this is merely to allow the heat to dissipate evenly through the food. When the microwave oven is turned off, there are no longer microwaves present in the oven, as absorption of microwaves is almost instantaneous.

This is similar to what happens when the light is turned off in a windowless room. Any photons produced while the light was on are immediately absorbed by the walls and objects in the room. No more light energy is produced by the light bulb, so the room becomes dark immediately.

More information on matters pertaining to radiation can be found on our web site www.nrl.moh.govt.nz