Physics
Together
The physics of sunlight: The Electromagnetic Cocktail
The sun is secretly green. That is not a metaphor. If you measured the peak energy output of the sun, the needle would land in the green-blue part of the colour range. Yet the sun looks white, or yellow — never green. Why? Because what your eyes see is only part of what the sun sends. The light hitting your eyes is a thin slice of everything the sun produces. The rest is invisible. Some of it arrives as warmth. Some of it tightens your skin. Some of it is so powerful that the atmosphere has to stop it before it reaches you — or life on Earth would not survive. This is the electromagnetic cocktail. The full mix of energy that leaves the sun and arrives at Earth.
The cosmic ruler
To understand the cocktail, you need one simple tool: the wavelength.
Light does not travel in a straight stream. It moves in waves — like ripples on water. The distance from the top of one wave to the top of the next is called the wavelength. Short wavelengths carry more energy. Long wavelengths carry less.
We measure wavelengths in nanometres (nm). One nanometre is one-billionth of a metre. A single human hair is about 80,000 nanometres wide. Light waves are far smaller than that.
The entire electromagnetic spectrum is just wavelengths arranged from shortest to longest. Your eyes can only detect a narrow band in the middle. The rest — most of it — is invisible.
Infrared: the invisible blanket
More than half of the energy the sun sends to Earth is infrared — and none of it is visible.
Infrared has long wavelengths, from about 700 nanometres up to 1 millimetre. It carries gentle energy that you feel as warmth. When you stand in sunlight and feel heat on your skin, that is infrared doing its work.
It is not dangerous. It is not dramatic. It is simply the sun warming the world — quietly, invisibly, and in enormous quantities.
Most of a sunny day's warmth is energy your eyes cannot detect.
Visible light: the green illusion
The part of the spectrum your eyes can detect runs from about 380 to 700 nanometres. That is the visible range — violet, blue, green, yellow, orange, red, all in sequence.
Here is the part most people do not know. The sun's peak energy output lands in the green-blue part of that range. The sun produces more green light than any other colour.
So why does it not look green?
Because the sun floods Earth with every colour at the same time. Your eyes blend all those colours together into white. The yellow and orange tints you see near the horizon come from the atmosphere scattering blue light away. Remove that scattering and the sun would look almost pure white.
The sun is a green star that we have never seen as green.
Ultraviolet: the lethal light shield
Below 400 nanometres, wavelengths get shorter and the energy gets higher. This is the ultraviolet range — UV.
UV comes in three types. UV-A (315–400 nm) penetrates deep into skin. UV-B (280–315 nm) is more powerful — it is the wavelength that triggers vitamin D production, but in high doses it damages DNA. UV-C (100–280 nm) is the most dangerous of all.
UV-C would sterilise the surface of this planet. On Earth, we use it to disinfect hospital equipment. The sun fires it at us constantly — and it never arrives.
Earth's ozone layer absorbs 100% of UV-C before it reaches the ground. It also absorbs most UV-B. Without that layer, life on land would not exist.
The atmospheric window
The sun also sends X-rays and radio waves. Neither reaches you — not because your body ignores them, but because the atmosphere blocks them first.
The atmosphere acts like a very precise filter. Scientists call the gaps in that filter the atmospheric window. Only certain wavelengths pass through: most visible light, some UV-A and UV-B, and a broad range of infrared.
Everything else stops above you. X-rays, UV-C, most radio waves — all blocked. This filtering is one of the reasons life thrives on Earth and not on Mars, where the atmosphere is too thin to do the same job.
You live under a shield. You just cannot see it.
1,361 watts on every square metre
Here is the scale of what actually arrives.
At the top of Earth's atmosphere, every square metre receives 1,361 watts of solar energy. Physicists call this the solar constant. That is enough to run a high-end gaming PC — on every square metre of the planet, simultaneously.
By the time that energy passes through the atmosphere and reaches the ground, some is absorbed and some is reflected. On a clear day, around 1,000 watts per square metre still arrives at the surface.
The sun is not a gentle light source. It is a star 150 million kilometres away, and you can still feel it through a window.
What this means
Most of what the sun sends is invisible. Over half arrives as heat you feel but never see. A narrow slice in the middle is the light you use to read these words. And at the edges, the energy becomes powerful enough to reshape your biology — or destroy it, if the atmosphere did not intervene.
This precise mix is what the body was made for. It expects the full cocktail. When it does not get it, something is missing.
That is what makes sunlight different from any lamp ever made.
Go Deeper
If you want to explore this topic further:
- Electromagnetic spectrum — Wikipedia: Electromagnetic spectrum
- Solar irradiance — Wikipedia: Solar irradiance
- Ultraviolet — Wikipedia: Ultraviolet
- Ozone layer — Wikipedia: Ozone layer
- Solar constant — NASA: Solar Radiation and Climate Experiment (SORCE)
Some of the best conversations happen shoulder to shoulder, in the last good light of the day.
On the after-dinner lap
Make it the same time, the same loop, and it stops being a decision and starts being a ceremony. That is when it sticks.
Heidi Fink
Content and project lead, Sollees
Heidi is on a mission to share honest, well-grounded information about sunlight and health. She is especially drawn to the biology of it all — how our bodies are wired to respond to light, and what that means for the way we live.
