The most useful type of electromagnetic wave to investigate for a device that warms up leftovers instead of a microwave would be infrared waves.
Infrared waves are effective for heating because they can be absorbed by food, causing the molecules to vibrate and generate heat, which warms the food evenly. Unlike microwaves, which excite water molecules specifically, infrared waves can warm a broader range of materials, making them suitable for heating leftovers effectively.
Gamma rays and X-rays are primarily used for imaging and sterilization purposes and are not suitable for heating food due to their high energy levels, which can be harmful. Radio waves are typically used for communication and are not effective for heating food.