The percentage of power inside a microwave oven that is due to reflections versus direct radiation from the magnetron (the microwave source) entering through the waveguide depends on the design and efficiency of the oven's components.
In a well-designed microwave oven, the majority of the power comes directly from the magnetron into the waveguide and is then distributed into the cooking chamber. However, reflections do occur in the waveguide and cooking chamber due to the interaction of microwaves with the surfaces and the food being cooked. These reflections bounce the microwaves around, helping to ensure more even cooking.
Modern microwave ovens are engineered to minimize reflections by using various design techniques, such as smooth interior walls and proper waveguide dimensions. By reducing reflections, more of the microwave power is efficiently directed into the cooking chamber, resulting in more consistent and effective cooking.
It's challenging to provide an exact percentage breakdown because it can vary significantly based on the oven's design, construction quality, and the materials used. Manufacturers aim to maximize direct radiation and minimize reflections to improve the oven's performance and cooking efficiency.