My students in Stage Lighting at College of Charleston are required to write short journal entries at various points during the semester in order for them to look at light in a way that they perhaps have not before. I promised my class this semester that I would participate in this activity (perhaps with the intention of getting them to actually drop by the blog once in a while). One is due tomorrow, so I best get my homework done.
This photo was taken while I was on a walk across campus the other day. It was a nice, clear, sunny day, resulting in sharp shadow-play from the lamppost and tree. This is a good example of the shadows thrown off by a single light source (in this case, the king of all single-source lights: the sun). The contrast here that I’d like to point out is between the shadows cast by the lamppost and the branches of the unseen tree. The lamppost is reproduced sharply on the pavement- the shape of the lamp is very detailed, and very clear. Here is a detail: 
However, despite the clarity that the shadow of the lamppost gives, the branches are much fuzzier. The light source is the same- so why do the shadows not share the same clarity? The difference between these items is not the light source, but the distance that the surrounding light travels between the point at which it is blocked by the opaque object and the point that it lands on the pavement next to the shadow. The branches are much farther away from the pavement than the lamppost is. What difference does this make? The answer to this conundrum lies in that which the light passes through.
Light bounces around everywhere. Anything that gets in the way of light causes it to bounce off in a different direction. That’s why objects appear brighter when a light is shined at it- the light is being reflected off of that object and towards our eye. Notice the highlights that help to shape the lamppost in our eyes:
All those bright white parts are where the most light is being reflected (bounced) directly back towards the camera’s lens. The same thing is happening more subtly in the air between the tree branches and the pavement. Light is bouncing off of particles in the air- dust, gas, all kinds of stuff- and part of where that light bounces is into where the cast shadow of the branches land on the pavement, slightly brightening up those areas of the brick. For what it’s worth, this phenomenon is happening between the lamppost and the pavement too, but because there is less distance between the lamppost and the pavement than there is between the branches and the pavement (and therefore less air), it is much less noticeable, and less of that light is reflecting back into where the cast shadow lands- leaving us a much clearer image of the outline of the lamppost than that of the tree branches.
If our tree and lamppost were in a vacuum, the shadows would be equally clear.
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