[ATJ]

Light is made up of photons. Each photon has a single wavelength. The wavelengths of photons in visible light range from 400 to 700 nm. A photon of one wavelength is identical to every other photon of the same wavelength regardless of how the photons were produced whether it be from a star (including our Sun), an artificial light source or bioluminescence. They have the same energy level, which is dependent on the wavelength. They travel at the same speed, which is also dependent on the wavelength but also on the optical density of the medium through which they pass.

The intensity of light is defined as the amount of light striking a certain area over time. The only two factors that affect intensity are the number of photons striking the specific area over time and the energy level of those photons. When using quantum units, such as Einsteins, only the number of photons is relevant. When energy units are used, such as watts or joules, the number of photons is the dominant factor with wavelength having a much smaller influence.

The number of photons reaching a surface will be influenced by the amount of light (total number of photons) emitted from a light source, the area over which those photons are spread and any attenuation by the medium through which the light passes. Many of the stars we can see from Earth are much larger than our Sun and put out, but our Sun is much closer and so proportionally more of our Sun's photons reach the Earth than those from other stars. Even with our Sun, only a small proportion of the photons reach the Earth with the others heading off in other directions. And, yes, it is to do with the spreading out of the light, but it still affects the intensity. The Earth is closest to the Sun every January and more of the Sun's photon's reach the Earth.

Attenuation of light occurs with the Earth's atmosphere as photons are absorbed, reflected or refracted by the air and particles in the air. The shorter the distance of atmosphere the light passes through, few photons that are lost. This is why the intensity is greatest when the Sun appears directly above the Earth and it decreases when the Sun appears just above the horizon.

Attenuation of light also occurs with water and at a much greater rate than in air. The light intensity can be reduced to 50% in around 5 m of clear water.

The inverse square rule "decrease is inversely proportional to the square of the distance" only applies to point light sources. The light intensity DOES decrease as there are fewer photons reaching the area and it IS because the light is spread out over a greater area. When you double the distance, the light is spread over four times the area and so the intensity is reduced to one quarter. Triple the distance, the area of spread is 9 times and the intensity is one ninth. Note that the Sun and the stars are all point light sources.

A fluorescent tube is not a point light source, at least not at the distances we use them over. While light intensity does decrease with distance, it does not decrease as quickly as with a point light source. This is because the tube is effectively a row of point light sources. If you take each point individually, you will get the inverse square rule but the light from each point overlaps with the next point and so the "spread" includes the other light. The further you go from the tube, the more light you pick up from other points along the tube.