No, it is not true that microwaves are more efficient when they have less food inside them. Microwaves work by emitting electromagnetic waves that excite water molecules and other polar molecules in food, causing them to vibrate and generate heat. The heat produced then cooks the food.
Microwaves are designed to distribute the electromagnetic waves evenly throughout the cooking chamber. When there is less food inside the microwave, the waves can bounce around and potentially cause uneven heating. This can result in hot spots and cold spots within the food, leading to inconsistent cooking.
In fact, microwaves are generally more efficient when there is a larger quantity of food inside. The reason is that more food absorbs the microwave energy, converting it into heat. With less food, the microwave may have to operate for a shorter duration, which can result in less even cooking and potentially waste energy.
To achieve optimal cooking results and energy efficiency, it is recommended to follow the manufacturer's instructions for the appropriate cooking times and power levels based on the quantity and type of food being prepared.