by Philip Lepoutre
Have you ever seen tribes in the desert wearing black and thick clothes? If so, have you wondered why? Why would they wear black, a colour that is known for its high heat absorption properties? In this article, we will find out why, the scientific way. For this short research, I teamed up with a Romanian physicist, Mircea Hincu. He has worked in almost anything you can think of, from micro to macro physics, and has been involved in projects about nuclear and solar energy. He blew my mind when we had our discussion about this topic and will hopefully blow yours once you read this.
To answer the initial question, we must understand why each colour has a specific heat absorption property. But, why is a colour, a colour? For instance, why is red, red?
• Why is a colour, a colour?
What we think of as “light” are waves or packets of energy, part of the electromagnetic spectrum, with specific wavelengths that the human eye can pick up on. The human eye can perceive electromagnetic waves that are located between the 380 and 740 nanometres. Depending on the wavelength, our eyes will recognise a certain colour. For instance, what we think of as purple is actually a wavelength of around 400 nanometres, red has a wavelength of around 720 nanometres and so on. Wavelengths longer than that of red are called, you guessed it, “infrared” and the ones shorter than that of violet are called “ultraviolet”.
Credits: Munsell Color- Illustration of the specific energy level and wavelength of each electromagnetic wave in the electromagnetic spectrum.
When light hits a red object, all the wavelengths of the light will be absorbed by the object, except for the red wavelength, which is directed onto our retina. For this same reason a white shirt is white, because it reflects all the wavelengths that hits it and a black shirt is black because it absorbs all the wavelengths it receives.
Credits: Mohit Verna- Picture illustrating how our eyes perceive colour
• Heat Absorption Property of Colours
One of the properties that surfaces have is that they all generate heat, based on its colour. This was discovered by William Herschel (1738-1822), one of the greatest astronomers that ever lived. He performed a famous experiment where he tried to measure the temperature of different colours in the spectrum by taking a prism, breaking down white light into all the colours of the spectrum and putting a thermometer next to each of the colours he could see. To his amazement, he found that the hottest temperatures were reached in a region beyond the red colour, where there was no colour at all. He reasoned that there must be a colour beyond/below red that we just cannot see with our eyes, and so he was right, this invisible colour became known as “Infrared light” (from the Latin prefix infra, meaning below).
Credits: Khan Academy- Illustration of William Hershel’s experiment
The heat absorption properties of a surface vary on the quantity of wavelengths that it absorbs. The more it absorbs, the hotter it will get. For instance, this is why white is used as the colour of choice for the fuselage of planes, because it generates less heat.
Despite all this, why do tribes in the Sahara Desert like the Bedouins and Tuaregs choose to wear black clothes, that generate the highest amount of heat?
The desert is a very arid place, with temperatures reaching 58°C. As it is so hot and dry, you can imagine that any sort of humidity evaporates in an instant. This humidity can come from plants or your skin, in the form of sweat.
Credits- Heat News Field
Sweat is a way for your body to regulate its internal temperature. To say this in other words, it is a way for your body to dispose of its surplus heat. It releases water molecules which are packed with energy, and their evaporation would cause them to leave your skin. The more you sweat, the more energy you will dispose of. As a result, sweating followed by instantaneous evaporation of these water molecules will result in feeling cooler. This is precisely why we can observe tribes in deserts from all over the world wearing black clothes, because they release energy (heat) that gets absorbed very quickly by the environment. It may have seemed counter intuitive at first, but all this comes down to is simple thermodynamics.
This is why physics is amazing, it gives us the power to explain everything around us, from how birds fly to how your laptop works and even to the so said “strange behaviours” of populations around the world. Scientific literacy really is something special and should be fundamental to a society. Let’s all encourage investing in the frontier of science, however remote it may seem in its relevance to what you’re doing today, since it is a way of planting the corn seeds of future harvests of our nations.
“The good thing about science is that it’s true whether or not you believe in it.”
Credits- Kwekudee, “Trip Down Memory Lane”