Maybe you’ve read about specific wavelengths of ultraviolet (UV) light being used to sanitize objects and inactivate the SARS-CoV-2 virus. Or perhaps you’ve seen something about how certain wavelengths of light are especially bad for your skin. Or maybe you were just wondering why light has a wavelength at all. In any case, by starting from the basics and glossing over some of the more difficult-to-understand physics, allow me to explain what a wavelength of light is, and how we measure it.
Visible light is a form of radiation that falls within a really specific portion of the electromagnetic spectrum. Electromagnetic radiation is just energy, and it travels in a wave.
These waves are characterized by two different parameters. Their wavelength: the distance from one peak to the next, and their frequency: how many wavelengths pass a given point per second.
Wavelength is a distance, so it can be measured in metres, centimetres, or if you’re American, in inches. However, electromagnetic radiation wavelengths are usually so small that we measure them in nanometres, denoted nm. One nanometre is equal to 0.00000010 centimetres. Frequency is measured in hertz, denoted Hz. One hertz is equal to one wavelength passing per second, also termed one cycle per second. That means that signal for CBC Radio One Montreal, which broadcasts at 88.5 MHz (megahertz, or one million hertz), is cycling 88.5 million times per second!
The higher the frequency of a wave of light, the higher it’s energy, since the waves must travel faster and faster as the number of cycles per second increases. You’ve probably already noticed that frequency and wavelength are inversely related. The higher the frequency (and therefore the energy) the shorter the wavelength, and vice-versa. This is a simple point, but one that leads to a lot of confusion, because radiation with a long wavelength has low energy, and radiation with a short wavelength has high energy.
Short wavelength = high frequency = high energy
Long wavelength = low frequency = low energy
If we plot all the known possible wavelengths of electromagnetic radiation, we get the electromagnetic radiation spectrum:
Visible light is only a very small portion of the spectrum, from roughly 400 nm to 750 nm. The shortest wavelengths (and therefore those with highest energies) are violet and blue, the longest are red. So, I can know that my purple laser pointer is giving off light of approximately 400 nm. But, getting to the point of this article, how do we measure that?
With a spectrometer. Specifically, an optical spectrometer, since the wavelengths of light we’re interested in fall within the visible range. The basic way that a spectrometer works can be seen in the diagram below.
Essentially a light source is focused on a mirror that reflects the light onto a grating that separates it by wavelength. The light then hits another mirror that focuses it onto some kind of detector. The detector can identify the wavelengths of the light based on some physical properties of the system (like the spacing of the grating, the focal point of the mirrors and their angles) and some complicated equations that I won’t go into .
The grating works just like a prism, which Pink Floyd fans will be familiar with, except that the light bounces off it instead of travelling through it. It’s necessary because most sources of light, like the sun, a light bulb, or a flame, are actually made up of many different wavelengths of light. There are some sources of light that emit just one wavelength, like lasers, but they tend to be expensive.
The wavelengths of light that are effective at inactivating SARS-CoV-2 particles range from approximately 260-265 nm. That’s because wavelengths this small are able to disrupt the viruses’ nucleic acids.
Radiation of that wavelength is emitted by the sun, which could be problematic for us if we weren’t lucky enough to have the ozone layer to filter it out for us! This is, however, why we can’t count on sunny summers to help combat COVID-19, our world just won’t get enough of the right kind of light.