The Solar Electromagnetic Spectrum
The Sun radiates energy across a continuous spectrum of wavelengths, from low-frequency radio waves at one end to high-frequency gamma radiation at the other. Biological significance varies with position on this spectrum: what matters is not just that radiation arrives, but how much energy it carries.
Energy that travels as oscillating waves through space, spanning radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays.
Three portions of the spectrum are especially relevant to life on Earth:
- Infrared radiation (~700 nm to 1 mm) warms the surface and lower atmosphere, maintaining temperatures that allow liquid water and metabolic activity.
- Visible light (400–700 nm) drives photosynthesis and therefore underpins almost all food webs, both terrestrial and aquatic.
- Ultraviolet (UV) radiation (100–400 nm) carries enough energy to damage biological molecules, making it a significant hazard to living organisms.
The relationship between wavelength and energy is direct: shorter wavelength means higher frequency and higher energy per photon. UV radiation sits at shorter wavelengths than visible light, which is precisely why it is biologically dangerous in a way that infrared and visible light are not.
UV Radiation Categories and Biological Risk
UV radiation is not a single type: it spans a range of wavelengths that the ozone layer filters very differently, producing three biologically distinct categories.
| Type | Wavelength | Ozone filtration | Key biological effects |
|---|---|---|---|
| UVA | 315–400 nm | Mostly passes through | Skin ageing, contributes to cancer |
| UVB | 280–315 nm | Mostly absorbed; some reaches surface | DNA mutation, skin cancer, sunburn |
| UVC | 100–280 nm | Completely absorbed by ozone | Would be lethal at surface |
The pattern is clear: shorter wavelength, higher energy, greater danger, and greater absorption by stratospheric ozone. UVC carries the most energy and would be acutely lethal to most life if it reached the surface. The ozone layer absorbs all incident UVC and most UVB, so what reaches the biosphere is predominantly UVA with a fraction of UVB.
UVA is often assumed harmless because it causes a tan rather than immediate burning. In fact, UVA penetrates deeper into skin, contributes to cancer and causes immune suppression and premature ageing. Lower energy does not mean safe.
The mechanism of damage is the same across UV types: UV photons carry enough energy to break chemical bonds in biological molecules. DNA absorbs UV strongly, and absorbed energy can fuse adjacent bases, producing mutations. Proteins and lipids in cell membranes are also vulnerable. The higher the frequency, the more efficient this molecular disruption becomes, which is why UVC is so much more destructive than UVA per photon.