Why can microwaves travel through both empty space and through air and food?

Answer 1

That shouldn't be too surprising when you recall that in addition to empty space, radio

can travel through walls, heat can travel through air, light can travel through glass and

water, and x-rays can travel through steel and bones.

All electromagnetic radiation is absorbed to some extent by passing through a material

medium. The amount of absorption depends on the material, and also on the wavelength

of the radiation. In the case of microwave radiation at 2.5 GHz, we are very happy that

some foods absorb it quite well, otherwise we couldn't cook a turkey in 3 minutes in the

microwave oven.

Microwaves travel through empty space unimpeded just as light. If truly empty space they should theoretically travel forever. Air is not truly empty and microwaves can interact with the air molecules, losing energy as it does so, but will still travel a great distance before being totally attenuated. However, material objects, such as food, have dielectric properties that affect the way microwaves interact with them and will attenuate them in shorter distances. While air has a relative dielectric constant of 1.0, water is 78 and beef 80 at room temperature. The microwave energy will be strongly attenuated by these materials and will travel a relatively short distance. In water, for example, 63% of the microwave energy is attenuated in 1.2 cm (< 1/2") and so will effectively about 95% absorbed in 6 cm. (the number is accurate because it doesn't take into account the change in the dielectric constant with temperature, but is good enough to demonstrate the effect.)