Sunlight photons travel about 150 million kilometers from the sun’s surface and then meet one of several fates: they scatter off air molecules, get absorbed by atmospheric gases, bounce off Earth’s surface, power photosynthesis, warm the ground and oceans, or pass through materials without interacting at all. Roughly one-third of incoming solar radiation reflects straight back to space, half is absorbed at the surface, and the remainder is absorbed by the atmosphere itself.
Scattering in the Atmosphere
The moment sunlight enters the atmosphere, photons begin colliding with gas molecules. Nitrogen and oxygen molecules are tiny compared to visible light wavelengths, so they scatter light through a process called Rayleigh scattering. This type of scattering is far more effective at short wavelengths, which is why blue light (with its shorter waves) gets redirected in every direction across the sky while longer-wavelength red and yellow light passes through more directly. That’s the entire reason the sky looks blue.
When photons hit larger particles like water droplets in clouds, something different happens. Cloud droplets are much bigger than air molecules, so they scatter all visible wavelengths roughly equally. This is why clouds appear white or gray rather than blue. The shift from wavelength-dependent scattering (tiny molecules) to wavelength-independent scattering (large droplets) is the dividing line between a clear blue sky and an overcast white one.
Absorption by Atmospheric Gases
Not all photons make it through the atmosphere intact. Different gases selectively absorb photons at specific wavelengths, effectively destroying them and converting their energy into heat or chemical changes.
Ozone is the most important filter for high-energy photons. It absorbs nearly all ultraviolet radiation while remaining essentially transparent to visible light and most infrared. Without this ozone shield, far more DNA-damaging UV would reach the surface. Water vapor has a complex absorption pattern with numerous broad peaks in the infrared range between 0.8 and 10 microns, making it one of the most significant greenhouse gases. Carbon dioxide absorbs at narrower infrared bands, notably around 2.6 and 4 microns, and blocks nearly all outgoing infrared radiation beyond about 13 microns. Together, these gases act like selective filters: they let most visible photons through while trapping much of the infrared energy that the surface later emits.
Reflection at the Surface
Photons that survive the trip through the atmosphere still face a coin flip of sorts at the surface. Some bounce right back. The fraction reflected, called albedo, varies enormously depending on what the photons hit. Fresh snow reflects nearly all visible light, acting almost like a mirror. Deep, clean ocean water does the opposite, absorbing almost everything. Forests and asphalt are also strong absorbers, while sand, ice, and concrete reflect significantly more.
These differences matter on a global scale. When Arctic ice melts and exposes dark ocean water, for example, the surface suddenly absorbs photons it used to reflect, accelerating warming. Seasonal shifts in vegetation, dust storms over deserts, and urban development all change how much sunlight energy stays on Earth versus bouncing back to space.
Conversion to Heat
When a surface absorbs a photon, the photon ceases to exist as light. Its energy gets transferred to the atoms and molecules of the material, increasing their vibration. That vibration is what we experience as heat. A high-energy visible photon absorbed by dark soil, for instance, becomes thermal energy warming the ground.
Here’s where things get interesting. The warmed surface doesn’t stay warm forever. It re-emits energy, but at much longer wavelengths than the original sunlight. The ground radiates infrared photons, invisible to the human eye, with far less energy per photon than the visible light that was absorbed. This is the fundamental mechanism behind Earth’s energy balance: the planet receives mostly visible-wavelength photons from the sun and re-emits lower-energy infrared photons back toward space. Greenhouse gases in the atmosphere absorb many of these outgoing infrared photons and re-emit them in all directions, including back toward the surface, which is the basic physics of the greenhouse effect.
Powering Photosynthesis
Plants intercept photons for a very specific purpose: building sugar from carbon dioxide and water. Chlorophyll absorbs light between 400 and 700 nanometers, with peak absorption in the blue range (400 to 500 nm) and the red range (650 to 680 nm). Green wavelengths are mostly reflected, which is why leaves look green.
The process requires a surprising number of photons. Plants use two photosynthetic systems working in series. In each system, four photon absorptions occur for every molecule of oxygen produced and every molecule of carbon dioxide reduced. So producing one oxygen molecule and converting one carbon dioxide into sugar requires at least eight photons total. Each absorbed photon kicks an electron to a higher energy state, and this chain of excited electrons ultimately drives the chemical reactions that store solar energy in the bonds of glucose. It’s a remarkably efficient energy conversion, though plants only capture a small fraction of the total sunlight hitting their leaves.
Penetrating the Ocean
Photons that hit the ocean don’t just stop at the surface. Different wavelengths penetrate to very different depths. Red photons (600 to 700 nm) disappear within the upper few meters, absorbed quickly by water molecules. Blue and green photons travel much farther, defining the “euphotic zone” where enough light exists for photosynthesis. This zone is typically defined as the depth where light drops to 1% of its surface value.
Blue-green light actually penetrates 30 to 40% deeper than the commonly cited euphotic zone boundary. In clear, nutrient-poor open ocean waters, about 10% of surface ultraviolet-A radiation (around 360 nm) still remains at depths of 50 to 70 meters. This deep-penetrating light supports phytoplankton, the microscopic organisms responsible for roughly half of all photosynthesis on Earth. In murky coastal waters, the same wavelengths may be fully absorbed within just a few meters.
Triggering Vitamin D Production
One of the most direct effects sunlight photons have on the human body is triggering vitamin D synthesis in the skin. Ultraviolet-B photons in the narrow wavelength band of 290 to 310 nanometers are the ones that do this. When a UVB photon strikes a cholesterol-related molecule called 7-dehydrocholesterol in skin cells, it has enough energy to break open one of the molecule’s carbon rings. This creates a precursor molecule that then slowly rearranges itself into vitamin D3 through body heat alone.
The process is self-regulating. With continued UV exposure, the precursor molecule converts into inactive byproducts instead of more vitamin D3, which prevents toxic overproduction from sun exposure alone. The initial conversion is relatively rapid, reaching a maximum within hours of UV exposure. This is why brief, regular sun exposure is more effective for vitamin D production than occasional long sessions.
Generating Electricity in Solar Cells
Solar panels exploit the same basic interaction as photosynthesis: a photon transfers its energy to an electron. In a silicon solar cell, incoming photons need to carry at least enough energy to push an electron across silicon’s “band gap,” the energy threshold that separates bound electrons from free-flowing ones. Photons with energy below this threshold pass straight through the cell without generating electricity. Photons with energy above the threshold do knock electrons free, but any excess energy beyond the band gap is wasted as heat rather than converted to current.
This is the core limitation of single-material solar cells. A significant portion of the solar spectrum is either too low-energy to be useful or too high-energy to be fully captured. Silicon’s band gap is well-matched to the peak of the solar spectrum, which is why it dominates the solar industry. Multi-junction cells stack materials with different band gaps to capture a wider range of photon energies, improving overall efficiency.