What materials reflect microwaves?

Metals are materials that reflect microwaves due to their electrical conductivity and free electrons. When microwaves, which are a form of electromagnetic radiation, encounter a metal surface, the electrons in the metal can move freely, allowing the waves to bounce off. This reflection is critical in microwave ovens, where metal components, such as the walls and the turntable, help to keep the microwaves contained within the cooking chamber and direct them towards the food being heated.

In contrast, plastics and glass do not possess the same properties that allow them to reflect microwaves effectively. Plastics can absorb microwaves and may, in some instances, heat up during the cooking process. Glass typically allows microwaves to pass through without significant reflection or absorption, making it a common choice for microwave-safe containers. Human tissues can absorb microwaves as well due to their high water content, which can lead to heating, rather than reflection.

Thus, metals stand out as the correct answer because of their ability to effectively reflect microwaves, making them essential in the design and functionality of microwave appliances.