Microwave ovens are a common sight in the kitchen.
An ordinary oven produces thermal or infrared radiation. The radiation heats the food in the oven directly, but also heats everything else, so a lot of the energy is wasted heating things that do not need to be heated.
Microwave ovens work by producing radiation at a wavelength that can be absorbed by water molecules. When the radiation is absorbed by the molecules, they vibrate more quickly. Energy of vibration is heat energy, so the food is heated. Less energy is wasted by a microwave oven because the radiation does not heat the walls of the oven, since these contain no water. Instead, radiation not absorbed immediately by the food bounces around inside the over, until eventually it is absorbed by the food and becomes heat energy. When you look inside the oven, you can see the food turning, but the door of the oven is threaded with a perforated metal screen which allows visible light to pass through, but no microwaves.
