What is the colour of radio waves?

“Well, we can’t see them, so it means they have no colour.”
We can’t see them, I agree, but does that mean they have no colour? To answer this question let’s begin with the definition of a colour.

Definition from the Encyclopaedia Britannica [1]: Colour, also spelled color, the aspect of any object that may be described in terms of hue, lightness, and saturation. In physics, colour is associated specifically with electromagnetic radiation of a certain range of wavelengths visible to the human eye. Radiation of such wavelengths constitutes that portion of the electromagnetic spectrum known as the visible spectrum—i.e., light.

Here are a few interesting keywords. Electromagnetic radiation. Visible spectrum. Wavelengths. Human eye. Let’s look into that.

Electromagnetic spectrum

Electromagnetic radiations are the waves of electromagnetic field. They are composed of photons of a specific wavelength, frequency and energy, all three characteristics linked to one another. We mostly refer to them with their wavelength and it is what I will do in this post too.

We can distinguish seven kind of electromagnetic radiation depending on their wavelength [2]:

• Gamma rays: used in PET [3]
• X rays: used in CT scans
• UV: emitted by the Sun
• Visible light: the one we can see
• Infrared: emitted by the Sun too
• Microwave: the same ones from your microwave oven
• Radio waves: generated artificially then transmitted to antennas

Human eye

The human eye treat visible light thanks to the retina that transforms it into neural signals for the brain. To do so, two different photoreceptor cells come to help: rod cells and cone cells.

Rod cells function with low-intensity light only and do not deal with colours. That is why all cats are gray in the night. They are concentrated at the outer edge of the retina. It means they mostly help with peripheral vision. This explain that we see more accurately what is on our side during the night.

Cone cells function best with high-intensity light and deal with colours. We have three different kind of cone cells designated as long, medium and short more focused on wavelengths corresponding to respectively a dominance of blue, green and red.

The cone cells receive light then transmit this information to the brain. The brain will process it in a specific way. It will translate the three colour channels into three scales: luminance (intensity of light from black to white), green chrominance (from green to magenta) and blue chrominance (from blue to yellow) [4].

Vision in animal kingdom

Eyes function in a very similar way through all animal kingdom. We find the rod and the cone cells. But animals do not have necessarily the same number of cones than we do. Marine mammals only have one kind of cone cell, allowing them to distinguish about 100 different colours. Most terrestrial mammals, like dogs, have two different cone cells making them see about 10,000 different colours. Primates, like us, marsupials and some insects have three different cones which represents 10 million colours. Most reptiles, birds, amphibians and insects have four cones. Some birds and insects have five cones and achieve to an impressive number of 10 billion colours. Besides all that, there is one animal which is the king of colour vision. Behold the mantis shrimp!

The mantis shrimp have 12 different cone cells. This crustacean can (like some other animals) see in the UV and infrared spectrum [5]. But even there, no radio waves sight.

See more than visible spectrum

To see more than visible is impossible to us but what would it be like? You already know. You already have seen it. We use of infrared camera to detect body heat or night vision goggles for instance.

A science that uses the diversity of information you can have by looking into different electromagnetic radiations is Astronomy. Indeed, stars emit the whole spectrum and a large range of wavelengths is observable and examinable. They can give us various informations impossible to know with visible spectrum only. We don’t need to observe further than the Sun to understand the benefits of such observations [6]. We distinguish the photosphere (Sun’s surface), the chromosphere (fin layer of gaz upper the photosphere), the corona and solar flares. These benefits go for any object in space we want to observe and study [7].

We are still not able to see radio waves, or any other kind of electromagnetic waves, with our own eyes for now. But we can build devices that can do it and translate it for us. In a way, we can already see the unseeable.

The question remains: what is the colour of radio waves?

Does a person able to see the whole electromagnetic spectrum see the same colours we do but expanded to the whole range? Does it make him see more shades than us? Or does he see colours we can’t even imagine?

This waves have a colour we never perceived. Colour is a way of the brain to interpret what the eye sees. Imagining a new colour is an impossible task. It is easier to know and understand what a colourblind person is able to see. Therefore, these questions might never be answered properly as long as no-one has experienced it.

References

[1] Color | Definition, Perception, Types, & Facts | Britannica.com, https://www.britannica.com/science/color

[2] Maxwell, J. Clerk (1 January 1865). “A Dynamical Theory of the Electromagnetic Field”. Philosophical Transactions of the Royal Society of London. 155: 459–512. doi:10.1098/rstl.1865.0008.

[3] All You Need Is Science, PET imaging: radioactivity to see inside the body

[4] Ali, M. A. (Mohamed Ather) & Klyne, M. A (1985). Vision in vertebrates. Plenum Press, New York, ISBN: 0306420651

[5] Thorn, Hanne H.; How, Martin J.; Chiou, Tsyr-Huei; Marshall, Nicholas Justin (January 24, 2014). “A Different Form of Color Vision in Mantis Shrimp”. Science. 334 (6169): 411–413. Bibcode:2014Sci…343..411T. doi:10.1126/science.1245824.

[6] Why NASA Scientists Observed the Sun in Different Wavelengths | NASA, https://www.nasa.gov/mission_pages/sunearth/news/light-wavelengths.html

[7] Florence Porcel, [CQFD #3] “Voir” en astrophysique – (Niveau : 2), https://youtu.be/skiShRYlAig