On your wavelength

Visible spectrum: On our wavelength

Post by Mark Daly

The Earth’s Sun emits a tremendous amount of electromagnetic radiation in the Earth’s direction. Even though the entire spectrum of light is incident on this planet, why is it that we humans only see in a tiny band that we have — rather appropriately — named the visible spectrum?


At first, we could simply dismiss the problem as being purely evolutionary, but there must have been a driving factor — some evolutionary pressure — that led us down this path. With a little thought and some ‘light’ physics, we can use the process of elimination to lead us to a logical solution.

Let’s begin with the source of all our light, the aforementioned Sun. The Sun can be described as a ‘black body’. Without getting too bogged down in the somewhat confusing name for such a bright object, let’s just say it means that the Sun is capable of emitting light continuously over the entire frequency spectrum. (More interested readers can take a look at the Wikipedia entry for more information).  Luckily, our good friend Max Planck worked out the distribution of light across the different frequencies emitted by such a black body back in the 1900s. Using Planck’s law, we can input our Sun’s temperature and discover that the peak of its emission just so happens to be in the visible spectrum. You might be forgiven for thinking ‘Ah-ha! This is why we see in this range’, and you would be partially correct. Yet we can still dig a little deeper.


Atmospheric absorption of light from the sun. Image credit: NASA, SVG by Mysid

Not all of the light that is incident on Earth makes it down to the ground in any meaningful abundance. In fact, there are huge absorption bands in our atmosphere. The only bands of light that could be candidates for vision are ultraviolet, visible, near-infrared, and longer radio wavelengths.

Technically speaking, all of these types of light would be viable candidates for providing some information about the world around us. However, the scale of the fine details on Earth limits it further. Radio wavelengths are very long, ranging from a few millimetres to kilometres! Because of their long wavelengths they turn out to be very useful, because they aren’t absorbed much by thin obstructions. However, because of their large dimensions, if we relied on radio wavelengths to see we wouldn’t be able to make out any fine details in our surroundings, so we can rule them out.

What about the infrared spectrum? Some animals do see in the infrared, but these animals are typically cold-blooded. Why? Well, hot objects emit quite a bit of infrared radiation, so we would be all but blinded by the heat from our own bodies if we could see in this regime. Ultraviolet light contains a lot of energy – the shorter the wavelength, the higher the energy of a single photon. Although some animals do see in the UV, we humans cannot. We know that even on a seemingly cloudy day, UV radiation can damage our skin, so it’s best not to focus this powerful light onto our delicate retinas.

Ultimately, after exhausting all other options, we are left with one contender: the good old reliable visible spectrum.


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