Photographers are very concerned with light: its colour tone, its angle of incidence, if there is enough of it, if there is too much. Why? Because while our photographic focus is on the subjects we wish to memorialise when we turn our lenses on them, we do not capture our subjects directly. What we are photographing is the light that reflects from them, light that can and does very much assert its own characteristics, thereby aiding or interfering with our photographic intents. Understanding light is as important to a photographer as understanding water is to a scuba diver, or understanding air is to a pilot.
Beginning at the beginning
So where on Earth does all this light come from? Leaving aside man-made light for the moment, the answer is of course that it doesn’t come from Earth at all. Our light streams continuously at us from out of the nearby 1.4 million kilometer wide nuclear fusion reactor we know as the Sun.
Our daily starlight (better known to us as daylight) begins when energy is released deep within the Sun as hydrogen atoms are fused together to create helium atoms. For the next 100,000 years, that energy slowly migrates to the surface of the sun, where it flashes off into space at a speed so fast (almost 300 million meters per second) that it can traverse the 150 million kilometers between the Sun and the Earth in 8 minutes. 
A tiny fraction of all of this light heads in our direction, where it penetrates the sun-ward facing side of Earth’s atmosphere and encounters surfaces: clouds, ice, the wings of a bird, atmospheric gases, the leaves of trees, the ocean, a flower, buildings, a person’s face, our camera lens. When this happens, some light may be absorbed or passed through that surface, and the rest bounces off in a different direction at a speed of 3.3 nanoseconds (billionths of a second) per meter as indirect light.
We do not see anything until these reflected photons of light and the visual information they carry reach our eyes (or our camera lens). And though the time lag is vanishingly small, it exists. If you are looking at someone from 5 meters away, you are in fact seeing this person as they appeared 17 billionths of a second ago.
We are so accustomed to this vast abundance of speeding light photons that we mentally perceive them as a whole (daylight), with a beginning (dawn) and an end (sunset).
But the nature of this light stream is not homogenous and static. It is volatile, filled with eddies and swirls, angles and transformations. It can be direct (should we be unwary enough to look directly at the sun) or (more usually) indirect. Like a river of water running down a wash, each succeeding instant of light refreshes our view of the world around us in its own likeness.
Light in a bottle
Of course we cannot see light as it travels because it goes too fast, right? Actually, that is no longer true. Andreas Velten and Ramesh Raskar at Massachusetts Institute of Technology developed an experimental camera in 2011 that records images at a rate of a trillion images per second and with this camera, light in motion can be filmed. In practice, so far they have only managed something elementary, but still rather magical: a video of a pulse of light travelling through a plastic drink bottle.
You can see the video at http://newsoffice.mit.edu/2011/trillion-fps-camera-1213 but above are two screen captures from it to give you an idea. (The video is worth watching, it is less than 3 minutes and the pertinent bit is only 15 seconds from timestamp 1:47). The first image captures a pulse of light moving through darkness, and the second demonstrates how light scatters from and is shaped by a reflective surface (the inside of the bottle neck) upon contact.
There are implications inherent in understanding our photographs as being constructed from light.
Firstly, it implies that our images are a fusion of the nature of the light falling on our subjects and the light qualities of the subjects themselves. This means that we should be aware of and understand the relationship between light and subject. What proportion of our photograph is about the light representing the subject in comparison to the light representing itself?
In the images from the MIT video above the object was to capture an image of light itself. Another example of photographing light for its own sake that may be more familiar to us might be a photograph of a rainbow, where the goal is to capture the prismatic colours of the sun’s light reflected by moisture filled air.
Then there are hybrid photographs in which the quality of the light is foregrounded almost as much as the subject. For example, the ‘golden hours’ in the early morning and late afternoon are so called because the sun’s light enters our atmosphere from an angle roughly parallel to the Earth’s surface, meaning that it has to travel through much more atmosphere than when the sun is overhead. This reduces direct light and increases indirect light, creating softer shadows and gentler highlights. As a result, subjects photographed during these golden hours often seem to glow. In these moments, the quality of the light is a very important component of the photograph, merging with the representation of the scene.
But for the most part, we use light as a sort of intermediary, a medium that carries detailed visual information about objects in the landscape to our eyes and to our camera lens. The medium, light, takes a back seat to the information it contains, much like the silicone of a mask is not as important as the face it molded.
This brings us to the second implication, and that is that our original photographs are the closest representations of reality that we can achieve with a camera.
Each pixel of our photograph is an expression of the light that fell on an element of the camera sensor in that moment, and provide the highest degree of faithfulness to the existing light at the instant of the photograph. The photograph is an imprint of light, mediated by our camera sensor and settings we choose.
Any subsequent ‘tinkering’ with the photograph detracts from the essential ‘reality’ of the photograph. That is not to say that it is always inappropriate to manipulate photographs, merely that we need to understand that each operation we perform on our photographs takes them a step away from being a photograph and a step closer to being photographic art.
Lastly, on an existential note, the fact that we are unable to see something or someone until the light that first reached them reflects from them and reaches us suggests that it is not possible for us to see or photograph anything in the absolute present. Put another way, everything and everybody we see and photograph is just ever so slightly from the past.
So it behooves us to remember when we photograph our surrounds that we are at a visual remove from the objects in the landscape, linked only by the light rays that flooded the landscape and reflected from its elements.
Perhaps we should give more consideration to how the specific quality of the ambient light illuminates the scene and the role it plays in helping us or hindering us in pursuing our photographic goals. And perhaps we should think twice before manipulating our photographs, each of which is a unique record of the light that existed at a moment in time and space, to which we alone were privy.
We should treat the light that makes our photographs possible with great respect. After all, it took over 100,000 years to get to us.
Rainbow over Khancoban, NSW, 2004; Instant of lion in light and shadow, 2007 and Silver Gull in ‘golden hour’ light, 2007 by Sabrina Caldwell; other than resizing for web use, no alterations have been done to the photographs.
 Karl Kruszelnicki, ABC Science. (2012) Sun makes slow light. http://www.abc.net.au/science/articles/2012/04/24/3483573.htm Accessed June 2014
 Andreas Velten and Ramesh Raskar, MIT, Visualizing video at the speed of light – one trillion frames per second. Video by Melanie Gonick http://newsoffice.mit.edu/2011/trillion-fps-camera-1213 Accessed July 2014
 Skye Wild, Making a silicone mask with Skye Wild. https://www.youtube.com/watch?v=WfeXY4dXQm4 Accessed July 2014