You’ll probably hear from someone the that light falls off as the inverse square of distance. This suggests that doubling the distance from light source to plant leaves will yield only 1/4 of the light intensity (1/(2^2) = 1/4). Tripling the distance would then yield only 1/9 of the light intensity (1/(3^2) = 1/9).
This is true but only for a “point” source. If you are using a single metal halide or high pressure sodium bulb this law does apply since the bulb looks like a point source for the typical four to six foot distance from bulb to leaves. Thus the light on the lower leaves will be less than the top leaves but not by much because the light is already at least four feet away to start with. Assuming a one foot plant height and another four feet to the bulb the light intensity on the bottom leaves compares to the top is:
1/((5 ft / 4 ft)^2)
which comes out to approximately 65% of the light which gets to the top leaves.
This law is often incorrectly applied to fluorescent light fixtures. The common lore claims that you must keep the plants less than two inches from the bulb or they won’t get enough light and grow spindly. There is something to be said about keeping the growing tips in as much light as possible, but the light falling on the lower leaves is not near as dim as the inverse square law would predict.
Fluorescent tubes more closely resemble a “line” source which falls off linearly with distance (not distance squared). This is because the light arriving at any point is the sum total of light coming from many points along the tube. If however the light is measured as a function of distance beyond the ends of the tube, the light intensity quickly reverts back to the inverse square relationship. This is why many sources say to keep plants away from the ends of the tubes.
Actually though, fluorescent lights are even more complex than that. The 1/(distance) relationship above ignores the presence of the reflector above almost every fixture I’ve seen. With good focusing and reflection coefficients you could have a fixture whose intensity falls off as 1/D for some distance then leveling off to a constant value as long as the plant is in the focused beam. [To illustrate this effect try shining a flashlight on an object a dozen or so feet away in a darkened room. Now try it again after removing (or masking off) the reflector behind the flashlight bulb. You will notice a huge difference.] In practice the fluorescent fixture reflectors are not as focused as a flashlight but it still is a significant contributor to overall light.
But it is even more complex. Most fluorescent fixtures have multiple bulbs, and most plant setups have multiple fixtures. Now we have a light source that is approaching a “plane” source, which does not fall off with distance at all. This plane source model would only apply when the distance to the plant leaves is much greater than the distance between tubes (and not at the edge of the tube array). The light would still be greater at distances closer to a tube (compared to inter-tube distances). At greater distances the “plane” tube array shines much more light on the lower leaves (which obviously can’t be kept close to a tube).
Keep the lights as close as possible to the plants (a few inches). This gives the maximum possible amount of light to the growing plant tips. And, if you are using multiple fixtures try to arrange them such that the light pattern is a rectangular block rather than a long linear row of fixtures. This second suggestion will yield the maximum light intensity for the lower leaves