Putting a metal bowl in the microwave can be potentially dangerous and should be avoided. While nothing happened in your specific case, it's important to note that microwaving metal can lead to several hazards:
Fire hazard: Metal objects in the microwave can cause sparks and potentially start a fire. The metal reflects the microwaves, which can result in electrical arcing and heating. This can lead to melting, igniting nearby materials, or damaging the microwave itself.
Damage to the microwave: Microwaving metal can damage the interior of the microwave, including the walls, ceiling, and turntable. It can also cause damage to the magnetron, which is responsible for generating the microwaves.
Electrical hazards: Metal objects in the microwave can cause electrical arcing, leading to a short circuit or damaging the electrical components of the appliance. This can result in malfunctions or even electrical shocks.
Regarding the concern about releasing cancerous waves, microwave ovens are designed to contain and direct microwave energy within the cooking chamber. The microwaves themselves are non-ionizing radiation, which means they do not have enough energy to alter the structure of atoms or molecules in a way that can cause cancer. However, it's crucial to avoid improper use of the microwave, such as microwaving metal, as it can cause other safety hazards as mentioned above.
In your case, since you only microwaved the metal bowl for a short time and nothing apparent happened, the risk of immediate danger is low. However, it's still a good practice to carefully inspect the microwave and the metal bowl for any signs of damage. If you notice any abnormalities or experience any issues with the microwave's performance, it would be wise to consult a professional for further assessment or repair.