Video: Electromagnetic Wave Applications

Which of the following statements correctly explains why microwave ovens strongly heat food inside them but do not strongly heat the glass plates that the food rests on? [A] Glass reflects microwaves better than food does. [B] Microwaves are more strongly absorbed by water molecules than by molecules in glass. [C] Food has higher density than glass so it absorbs more microwave energy. [D] Glass emits microwaves more easily than it absorbs them.

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Video Transcript

Which of the following statements correctly explains why microwave ovens strongly heat food inside them, but do not strongly heat the glass plates that the food rests on? A) Glass reflects microwaves better than food does. B) Microwaves are more strongly absorbed by water molecules than by molecules in glass. C) Food has higher density than glass so it absorbs more microwave energy. D) Glass emits microwaves more easily than it absorbs them.

All right, we want to pick which of these four options best explains a phenomenon that many of us have probably seen before. When food is put inside a microwave, the food heats up much more rapidly than the glass plates that the food rests on. So, if this is our glass plate and this is some food sitting on the plate, then when we turn the microwave oven on, microwaves land on the food as well as the plate. But despite the fact that both the food and the plate receive these rays, it’s the food that strongly heats up and the plate that does not. Let’s go through our answer options one by one and see which one correctly explains this.

Option A says that glass reflects microwaves better than food does. In fact, neither glass nor food is a very good reflector of microwave radiation. Pure glass would simply let this radiation pass right through. It would transmit it. And it’s only the impurities in glass that would have a tendency to absorb these rays. Reflection of microwaves is not something that glass tends to do. We’ll cross option A off our list.

Option B says that microwaves are more strongly absorbed by water molecules than by molecules in glass. The idea here is that in virtually any food we might eat, there are water molecules. And this option is saying that these water molecules are better at absorbing microwave radiation than glass molecules. Now, it makes sense that if we have a water molecule that absorbs a microwave, the energy carried by the microwave will be given to that water molecule. The water molecule will be excited. This will lead to increased motion and heat. And in fact, this is the mechanism by which food is heated up in a microwave.

It’s not that all of the food absorbs these microwaves very well, but the water in it that does. And indeed, these water molecules do absorb microwaves more strongly than the molecules in a glass plate that the food is on. This is why the water molecules and therefore the food heats up while the plate doesn’t very much. Option B looks like our correct answer. Let’s consider options C and D just to make sure.

Option C says that food has higher density than glass, so it absorbs more microwave energy. We can tell right away that this answer option can’t be correct because food in general doesn’t have a higher density than glass. For example, imagine putting a bowl of soup or a cup of tea in the microwave to heat it up. Those items don’t have a higher density than glass. If we put a bit of glass in the soup or in the tea, it would sink. And yet, they do heat up more strongly than the glass. Option C won’t be our choice.

Option D says that glass emits microwaves more easily than it absorbs them. In theory, if a glass did absorb enough microwaves, eventually it probably would emit some. It would give some off. But as we’ve seen, pure glass transmits microwaves and it’s only the impurities in the glass that tend to absorb them. And more than that, glass or any object needs to be energized by absorption before it can possibly emit or give off radiation. This option speaks of an energy imbalance where more is being emitted or given off than absorbed or taken in. From an energy-conservation perspective, this doesn’t make much sense, so we’ll cross off option D.

So then, to explain why food in microwaves is more strongly heated than the glass that the food rests on, we choose option B. Which says that microwaves are more strongly absorbed by water molecules, which are in food, than by molecules in glass.

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