If you want to know what your microwave oven does to your food, you first need to know how it works.
Microwave radiations, both for radars and ovens, are produced by a magnetron. In the electromagnetic spectrum, the microwaves lie just above the radio waves. A conventional microwave works at a frequency of 2.45 GHz, quite close to that of WiFi. This similarity should make it easier to imagine the effect of microwaves in comparison to that of radio or wi-fi waves.
The mic works by vibrating food molecules at its frequency, causing the particles to spin. The molecular vibration results in heat due to collision with other particles. It works best with dipoles, and the best dipole in this regard is the water molecule. It works pretty well with ions too, making the heating process even faster. Items such as onions and garlic have high ion content causing them to arc. Arc is the jump of energy between particles which can be dangerous in a microwave oven.
In between the microwave absorption and molecular vibration, how does the microwave become harmful for your food? It doesn’t! All an oven does is to transfer energy to your food in the same way convection, conduction, sound or light waves would. The heating or cooking in the microwave affects the nutrients no more than the conventional way of cooking would do.
The only way you can destroy the nutrients in your food is by methods like overcooking. According to a study focused on investigating effects of cooking methods on broccoli; the ‘steaming’ method is found to retain the most nutrients. All other methods including stir-frying, boiling and microwaving caused losses of chlorophyll, vitamin C and a decrease in total soluble proteins and sugars.
In conclusion, using a microwave can cause a loss of nutrients in food, but this loss is just as much as any other cooking method. So, stop worrying and make your daily life more comfortable with this technology.
If you want to know more, here is a video that explains the working of a microwave oven.