There are different opinions: some people say that it is nothing bad to turn-on the empty device, the others do not think so. Here we will give you a small example on should you run an empty microwave.
Imagine the following scene. In a dark room the cat is sitting (not necessarily black). You need to see it. You turn on the light and find out the cat. What happens from the perspective of physics? Light bulb (or bulbs in the chandelier) emitted photons. Some of the photons got on the cat, there was interaction (part is absorbed, the cat feels a bit warmer), part is reflected into your eyes. You see the cat. Most of the photons just got on the walls, ceiling and floor, partially absorb, partially affected, but somehow in the end they all (except those that reflected into your eyes) absorbed, ie, in terms of solving the problem (to see the cat) were lost useless.
Now imagine that you have turned on the light in the room without cat. What has changed? By and large – nothing. Bulbs also emitted the photons, they have also come to the walls and ceiling, and (see description above) were lost useless. The same situation is with the microwave with something inside or empty. Those microwaves do not differ from the light photons (both electromagnetic waves) other than the fact that the microwave wave length is much larger than the light wave in the visible range. Just those waves, which rejected an empty microwave, go to waste, as the light in the empty room (no cat). So microwave did nothing. The only loss – wasted electricity.
But there is also a different point of view on this fact. Manufactures and specialists are not recommended to run empty microwave. So let’s have a look what specialists saying about it.
The first and most important thing – you should never turn on an empty microwave. Microwave with high level of equipment has a special schemes which blocks the empty microwave. But unfortunately, sometimes these accurate and reliable circuit can not work.
What happens when you turn on an empty oven? Flow of microwaves reflects from the inner surface of the heating chamber through a waveguide and returns to the magnetron. And the oven fails very fast. There is no sense to change the magnetron, as it is a major part of the cost of the furnace, it is easier to buy a new microwave (and no longer include it in vain).
Opinions are differ, but to be in safe side we would recommend you follow guidelines from manufactures and specialists to avoid fails of technique.