Microwaves: How do they Heat Food without Heating the Container?

Why does an empty plastic or glass dish remain cool to the touch when removed from a microwave oven?

Assume that your electric bill has been paid.

Answer:

An empty plastic or glass dish being removed from a microwave oven remains cool to the touch because it does not absorb microwaves and is not heated directly. The food inside the dish heats up and then transfers some of that heat to the dish by conduction.

When food is heated in a microwave, the microwaves excite the water molecules present in the food, causing them to vibrate and generate heat, which in turn cooks the food. However, most plastic and glass dishes are transparent to microwaves, which means that they do not absorb microwaves and do not get hot. This happens because the microwaves have a much larger wavelength than the size of the molecules in the plastic or glass, so they pass through the container without heating it.

The reason why the plastic or glass container remains cool to the touch is that the microwaves do not interact with the container material in the same way they interact with the water molecules in food. As a result, the container does not absorb the microwaves and does not get hot, allowing it to be safely handled immediately after being removed from the microwave oven.