After a late night of studying leaves you with a hole in your stomach, you decide to head to the kitchen and warm up a slice of left-over pizza in the microwave. You see it spinning and wonder how this little box can cook your food, well the answer is in the name itself- microwaves! Microwaves are electromagnetic waves that fall within a particular wavelength range on the electromagnetic spectrum (figure 1) that have oscillating electric and magnetic fields (Sun, Wang and Yue, 2016). This oscillating wave is responsible for the cooking of the food.
Figure 1. The electromagnetic spectrum that ranges from gamma rays (0.1 to 0.000001 nanometers) to radio waves (10mm- 10,000+ km). As seen in the figure radio waves have longer wavelengths than the visible spectrum and are typically 1mm to 300mm in wavelength (Citeli and Citeli, 2020).
So how do these waves cook the food? The key- water! Most food has water inside of it, and as water is a polar molecule, it has two hydrogens at a 104.5° angle from each other that are connected to an oxygen molecule (Hu et al., 2021). This causes the water to behave in a dipole manner. When a uniform electric field is induced from the oscillating wave, the water molecule that is within the space of the electric field wants to align the positive end of the dipole with the direction of the induced field. As previously mentioned, the electromagnetic field is oscillating. This causes a “flipping” of directions of sorts of the electric field. When the field is flipped, the dipole matches this motion to align its positive end with the direction of the field. This turning motion generates torque (Dudley, Richert and Stiegman, 2015). As this torque force generates rotational motion of the dipoles, the molecules of water rub against each other causing the kinetic energy to be transformed into friction, and thus thermal energy.
In order to sustain a constant source of kinetic energy to fuel the thermal energy generated it is essential to use the electromagnetic waves in the most efficient way possible. This is done via a process known as resonance cavity. Resonance cavity works from the utilization of the structure of a microwave, when an incident wave is introduced from the oscillating electric field it propagates towards the opposite side of the inner microwave (Mehdizadeh, 2024). Once it reaches the opposite side it is reflected off the metallic surface back towards the incident wave. This wave is known as the reflected wave. In order to utilize the maximum power of these waves it is essential to use the reflective wave to induce constructive interference. Constructive interference occurs when the incidence wave and the reflective wave overlap (figure 2).
Figure 2. Two waves are interfering at points where their maximum amplitude aligns, this results in a greater overall amplitude in the resultant wave, thus forming constructive interference (phys.uconn, 2024).
To formulate this constructive interference a metal plate is placed directly at the location that is a multiple of equation 1
Equation 1. Denotes how to find constructive interference for a particular resultant wave. Where N is the number of nodes, and l is the wavelength. This value depends on the width of the microwave in order to facilitate constructive interference The value must be a whole value i.e., equal to one, two, or three etc… to ensure a nodal point is reached when the wave is reflected (Mehdizadeh, 2024).
The issue with the process of constructive interference waves is the unevenness of the cook. Where the constructive interference occurs, hot spots, which are areas where food is cooked more, will appear. This occurs due to the locations in the cavity that have areas of higher amplitude subsequently having higher amount of energy being induced into the food, and thus generates more torque, and higher amounts of friction in those areas (Thostenson and Chou, 1999). To combat this, a spinning plate is placed inside the microwave to evenly distribute the constructive interference that is occurring throughout the food.
In essence, the physics behind microwaves not only illuminates the extraordinary efficiency of these kitchen staples but also underscores scientific principles that provide to our everyday convenience, forever changing the way we approach meal preparation.
References
Citeli, M. and Citeli, N., 2020. Electromagnetic Spectrum. In: J. Vonk and T. Shackelford, eds. Encyclopedia of Animal Cognition and Behavior. [online] Cham: Springer International Publishing. pp.1–4. https://doi.org/10.1007/978-3-319-47829-6_1794-1.
Dudley, G.B., Richert, R. and Stiegman, A.E., 2015. On the existence of and mechanism for microwave-specific reaction rate enhancement Electronic supplementary information (ESI). Chemical Science, 6(4), pp.2144–2152. https://doi.org/10.1039/c4sc03372h.
Hu, Q., He, Y., Wang, F., Wu, J., Ci, Z., Chen, L., Xu, R., Yang, M., Lin, J., Han, L. and Zhang, D., 2021. Microwave technology: a novel approach to the transformation of natural metabolites. Chinese Medicine, 16, p.87. https://doi.org/10.1186/s13020-021-00500-8.
Mehdizadeh, M. 2024. Applicators and Probes based on the Open End of Microwave Transmission Lines. Applied Sciences Publishers. https://doi.org/10.1016/B978-0-8155-1592-0.00006-5.
Mumtaz, S., Rana, J.N., Choi, E.H. and Han, I., 2022. Microwave Radiation and the Brain: Mechanisms, Current Status, and Future Prospects. International Journal of Molecular Sciences, 23(16), p.9288. https://doi.org/10.3390/ijms23169288.
phys.uconn,. Constructive and Destructive Interference. [online] Available at: <https://www.phys.uconn.edu/~gibson/Notes/Section5_2/Sec5_2.htm> [Accessed 15 March 2024].
Sun, J., Wang, W. and Yue, Q., 2016. Review on Microwave-Matter Interaction Fundamentals and Efficient Microwave-Associated Heating Strategies. Materials, 9(4), p.231. https://doi.org/10.3390/ma9040231.
Thostenson, E.T. and Chou, T.-W., 1999. Microwave processing: fundamentals and applications. Composites Part A: Applied Science and Manufacturing, 30(9), pp.1055–1071. https://doi.org/10.1016/S1359-835X(99)00020-2.