How Does a Microwave Oven Heat Food?

A microwave oven works by generating electromagnetic waves at a frequency of 2.45 GHz (gigahertz) using a vacuum tube called a magnetron. These waves are channelled into the cooking chamber, where they penetrate food and cause polar molecules — primarily water, but also fats and sugars — to rapidly rotate back and forth, trying to align with the oscillating electric field 2.45 billion times per second. This molecular agitation generates heat through dielectric heating (molecular friction). Microwaves penetrate food to a depth of about 1-3 cm; deeper layers are heated by thermal conduction from the outer layer. This is why thick foods can be hot on the outside but cool in the centre. The microwave oven was invented accidentally in 1945 by Percy Spencer, a Raytheon engineer, who noticed a chocolate bar in his pocket melting while he worked near an active radar magnetron. The first commercial microwave (the Radarange, 1947) was 1.8 metres tall, weighed 340 kg, and cost $5,000.