in astronomy, why does flux decrease with distance but not intensity?
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Imagine you’re spraying glitter from a bottle onto your bedroom floor. If you spray close to the floor, the glitter is all bunched up in one spot. But if you stand on your bed and spray, the glitter spreads out more. That’s like flux; the farther away, the more spread out and less glitter in one spot. But each piece of glitter is still as shiny no matter where it lands. That’s like intensity; each little light bit is just as bright no matter how far away it is.
The intensity of light remains constant because it is a measure of the power of the light source, while the flux decreases with distance because it spreads out over a larger area.
It depends on what you mean by flux and intensity; different fields and subfields annoyingly use the same words to describe different things.
In the way you probably mean it, ‘flux’ means how tightly packed the light is. As you get farther away, the light has more room to spread out.
In the way that you probably mean it, ‘intensity’ refers to how tightly packed the light in one particular direction is. Since the light is all going in one direction, it does not spread out as you get farther away.
Intensity of light has two different definitions depending on which branch of physics you are in. It is a bit messy when there are photometry, radiometry, optics, astrometry and thermodynamics that all handle same things but may not use same names for them.
Definition 1: Intensity is power transferred per unit area. Also called flux. This gets smaller as you get further away from the light source as the same power is spread out over larger surface area.
Definition 2: Intensity is radiant power coming out per unit solid angle (solid angle is like normal angle but in 3d instead of 2d). This is property of the light source and so does not change with distance.
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