Metals placed into a microwave oven have undesired effects, can you explain please.

Microwaves are designed to cook food by the absorption of microwave energy by water molecules in food.

Metal (in general) is reflective to microwaves.

Firstly this may damage the magnetron tube that produces the microwaves, due to feed back.

Also it increases the amount of microwave energy in spots in the microwave. The energy may become enough to ionize air particles (knock off an electron). This makes the air particles conductive and so you can see sparking or arcing within the microwave in order to neutralize the charge. This arcing can heat the air and microwave cavity further and damage the oven.

When tin foil is placed in the microwave around food (for example as a lid on a container), it normally is not much of a problem, as the foil is thin enough to not be totally reflective, and also as it is in contact with the food and water, this will enable stray microwaves to be absorbed.

If the food is completely surrounded by tin foil, it will hardly cook, and you will get some sparking, as the stray microwaves cannot be absorbed.

I hope this answers some of your questions, feel free to enquire further :)