If you mean “why are the insides of microwave ovens dangerous when on”, it just comes down to the power levels.
My microwave puts (allegedly) 1000 watts of radio-frequency energy into the cooking part.
My mobile phone puts out (at maximum) 3 watts.
If I wrap my hand all the way around my phone, covering the antenna part, 3 watts is all I’m going to absorb, worst case.
If I put my hand into the microwave oven, having somehow got it to operate with the door open, I’ll be absorbing hundreds of watts.
It’s just differing power levels.
Latest Answers