Two regions of the EM spectrum that will be focused on in this unit are the infrared and microwave regions. A conventional oven heats food using infrared radiation while a microwave oven uses microwaves to do the same job. In order to discuss how these types of radiation heat food, the unit will first need to discuss what heat is.
Heat can be described as an increase in temperature but one can take this a step further. Increasing the temperature of a system is an increase in the average kinetic energy of a system. Kinetic energy is the energy of molecular motions. This means that to increase the temperature, molecules and atoms in the system need to move faster. There are three types of molecular motion: translational, rotational and vibrational motions.15 Microwaves are able to excite rotational motion of water while infrared radiation excites the vibrational motion. It is due to the resonance effect that this excitation is possible. The resonance effect is when the frequency of radiation matches the frequency of the molecular motions. This allows the radiation’s energy to be absorbed and transferred to the molecules, further exciting the molecular motions.
To explain the resonance effect, one can consider an analogy. When a child on a swing, the back and forth movement of the swing is at a natural frequency of the swing, determined by the length and weight of the swing. If the child wants to swing higher you need to push at the right time or frequency to make that happen. In this analogy, the swinging motion of the child already happening before you push is at the natural frequency. You pushing the child at a particular frequency making the child swing higher is the resonance effect.16 Each object has a natural frequency of motion and when a small periodic force synchronizes with this frequency the motion can be excited to a greater degree or larger amplitude.
In the microwave oven, the electric and magnetic fields of microwaves oscillate at the frequency matching the natural frequency of rotational motion of water in food. Hence, the microwave radiation can be absorbed by exciting the rotational motions and thereby heating up food. Similarly, the EM wave of infrared radiation of the conventional oven oscillates at the frequency matching the vibrational motions of molecules in food. Thus, energy of infrared radiation can be absorbed by food, leading to excitation of vibrational motions and thus increasing the temperature of foods17