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There are two cases:
1) light that is incident onto the subject, from the light source, illuminating the subject. And there is
2) light reflected from the subject, entering the camera lens (the subject itself is not a light source).
Light reflected from the subject is a different situation than light incident on the subject.
A single ray of light originates from its source with a given direction from the source, continuing on that path until interacting with another object.
The amount and color of the light reflected from the object is the result of the amount and color of light striking the object, minus the amount and color of the light being absorbed by the object. (Conservation of Energy Law)
The light striking that object is then reflected iaw AI=AR (based on the angular surface structure at the point which the object is struck by the light for an opaque object, refraction & reflection combined for a translucent object, refraction for a transparent object)
The reflected light that is captured by the camera is done so relative to angles of inclusion involved, iaw AI=AR (see Light: Science & Magic)
This is how light travels from its originating source to an object and then to either our eyes or our camera. Of course, the process involves a multitude of rays and angles that work in concert with each other.
Consider this ... most of us have used a magnifying glass to "burn leaves". When playing with a magnifying glass, the angle that we hold the light determines whether the light (energy) is being concentrated or diffused as we watch that "ring of light" change size and shape. The light is not traveling iaw with ISL after it is being refracted by the glass, it is traveling iaw AI=AR.
Whether light is reflected or refracted it continues its path iaw with AI=AR (reflection or refraction) based upon the surface's properties (opaque, translucent, transparent, refractive index, etc.)
Much of what people are trying to associate to ISL regarding reflected light is the lack of accounting for the multitude of originating rays that combine to yield an array of angles, then striking an array of surface angles. The fewer variations in the angular structure of a given surface, the more directly the light appears to travel iaw with AI=AR. The more variation in the angular structure of a given surface the more it appears to deviate from AI=AR. This seems to be when people errantly start trying to insert ISL to explain its reflected distribution based on their observance of what appears to them to be the rationale for how light travels ... rather than apply the tenets of AI=AR to the multitude of angles involved (that they cannot individually see). However, in all cases light travels iaw AI=AR.
ISL is the explanation of initial spherical distribution of a multitude of photons from PLS. Light energy is a vector quantity, thus the angles involved are paramount to understanding light as it continues to be reflected & refracted. Besides, if reflected light truly traveled iaw ISL rather than AI=AR, then parabolic reflectors would not have any different effect on light than flat reflectors, nor would changing the angle of a reflector allow you to "feather" your light any differently.
Edited on Dec 05, 2012 at 06:36 PM · View previous versions