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Question:

The condition under which a microwave oven heats up a food item containing water molecules most efficiently is:

Infra-red waves produce heating in a microwave oven

Microwaves are heat waves, so always produce heating

The frequency of the microwaves must match the resonant frequency of the water molecules

The frequency of the microwaves has no relation with natural frequency of water molecules

Solution:

Microwave ovens operate at a frequency of 2.45 GHz and this is NOT the resonant frequency of a water molecule. This frequency is much lower than the diatomic molecule resonant frequencies mentioned earlier. If 2.45 GHz were the resonant frequency of water molecules the microwaves would all be absorbed in the surface layer of a substance (liquid water or food) and so the interior of the food would not get cooked at all.