- SECS, Chapter 3: Laws of Light, beginning sections through "Light is Directional"
Remember how we stated that light is directional? Light also has a "life" including the birth and death of a photon. Electromagnetic radiation is emitted from a surface and will encounter objects in its many paths. The fate of electromagnetic radiation (as a photon) depends on wavelength of the photon at hand and the physical composition of the objects along its path. Again, the photon is a packet of radiation and can interact with molecules or atoms in its path, being transmitted, absorbed, or reflected.
The action of "reflection" is more appropriately described as "back-scattering" by atmospheric physics, which is why you see that term in the figure of possible paths of light. We also have seen a shorthand diagramming technique from the textbook to describe the life events of ensembles (really big groups) of shortwave and longwave photons.
I would like to pull the concept of photon lifetimes in the following video.
Video: Diagramming Light Intro (2:03)
Each of these phenomena is happening simultaneously, and the next video opens up that discussion.
Video: Light Interactions Descriptive (4:31)
Emission ($\epsilon$), absorption ($\alpha$), reflection ($\rho$), and transmission ($\tau$) can all occur at the same time (in fact it always does). We also know that there is a relationship between light energy balances and temperature. When a receiving surface or material absorbs more energy than it emits, the internal temperature of the material will increase (effectively pumping up the system's energy density). Consider a bright summer day, where your rooftop absorbs much more solar radiation than it emits (or conducts away). The roof surface temperature will rise from just after sunrise to the late afternoon.
Video: Light Diagramming Example (5:49)
Recapping: Electromagnetic radiation (as light) is first emitted, and the emitted photons have an opportunity to be transmitted, absorbed, or reflected by an intervening surface. If that surface is a particle or an assemblage of particles, there will be a tendency for the reflection to be more accurately described as "back-scattering."
In solar energy resource assessment, clouds are the most important atmospheric factor influencing device performance, followed by aerosols (dispersed atmospheric particles).