For example, I’m sitting on my front porch which overlooks the town. Miles away I can see streetlights, signs, etc. How does the source project light to my location, yet doesn’t illuminate my location?
Holy moly friends, thanks for the awards and stuff. I didn’t think this question would spark so much interest, lol. I am thoroughly grateful for all your replies.
In: Physics
Well if you see them they are illuminating your location.
But if far away the light is so little it gets lost in the lighting reflected from closer or brighter objects, nearby lights, the moon, etc.
In fact look at a cloudy sky over a city at night, you can see the clouds quite well due to city lights.
Missing from this explanation so far is also a mention of just how amazing the eye is. Your retina cells can detect single individual photons. So even at great distances we can see lights shining, even if only a few of those photons make it to our eye. However there are not enough photons making it to the area to illuminate, which requires them bouncing off the surroundings, and not being absorbed.
I think the answer is how light and other signals use the inverse square law. the light intensity. Meaning the light density is spread out over an area that is increasing in proportion to the square of the distance from the source. Hence, the intensity of radiation passing through any unit area (directly facing the point source) is inversely proportional to the square of the distance from the point source
Absorption and scattering.
Unless your location is a perfect white, some of the light hitting it will be absorbed, reducing its intensity.
Then, unless your location is a perfect mirror perfectly angled to redirect the light to your eye, the light is scattered in every direction, further reducing its intensity.
The light *is* still illuminating your location, and if you could somehow turn off all the other lights illuminating your location, turn around to look away from the last distant street light, wait a while for your eyes to adjust, and then start flipping that streetlight on and off you should be able to see a difference (depending on just how far away the streetlight is).
But in the real world, turning off all other sources of light isn’t really possible, most of the time.
That light is illuminating an area far away from the source – specifically the rods and cones inside your eye.
Pretty crazy when you think of a photon emitted a long (thousands of years) ago by star made its way through the universe and bam hits the rod in your eye so you can see the star.
Because the amount of diffuse light required to illuminate large surfaces through scattering is disproportionately higher than the amount of direct light needed for our perception of light.
I just want to add this: in a night with no moon and no artificial light sources, the light of the star Sirius can project a faint shadow on the ground. Sirius is 8,6 lightyears distant.
There are a lot of things going on, but here a few:
1) our perception of light is non-linear,
Meaning that something twice as bright (twice the light photons) appears just a “shade” lighter.
But it also means that something super dark just needs a little bit of light to visually become much brighter.
2) the sun is _really_ bright – just remember that street lights during the day are hardly noticeable when on, but at night they can be seen from miles away.
3) bouncing light (reflecting against walls etc) is actually also pretty bright, we just don’t think about it. Just remember your entire house is lit by sunlight bouncing around through a few windows during the day. You know those weird umbrellas at photo shoots? Those are only there to bounce light around that’s already there. Lots of outside shoots use bog white reflectors to just bounce sunlight around – don’t even need lamps!
Also remember seeing streetlights on during the day; they look super dim – almost everything around you is as bright as those lamps just from the sun reflecting on random surfaces.. You’re effectively as bright as a lamp yourself!
4) another perceptual thing: humans are really good at zooming in on details in the distance. making them look bigger in our head. just look up a model scale of earth and the moon. The moon is way smaller and way further away than you would think, it looks bigger in the sky.
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This all combined has the effect of making small lights in a relatively dark area to be bigger and light up stronger than you would think. Stuff like leds on an old stereo are actually pretty strong because they need to be visible in the daylight. In the dark those things can light up a room. You can see lights from a distance because they stand out and our eyes are really good of noticing small bright things. But also, you’d be surprised how much a small light actually does light up surroundings, during the day it is completely washed out by the sun, and we also just never notice it.
Tl;dr non linear perception, lights are pretty bright but the sun blows everything away during the day – and actually small lights are lighting up stuff more than you’d think
It’s a long rant but i think it’s a super interesting, phenomenon – I’ve had a decade of light rendering experience and one thing I’ve learned is that what we see, what we think about and what we notice are completely different things.
Well, not denying what others have said, but i have a slightly different take on this problem.
Light aka photon, likes to move in a straight line unless other forces or medium changes occur. Now take an average street lamp, when its turned on, gazillions of photon is rushing out on a straight line at every possible angle as long as it can go unobstructed. So from your perspective, say a kilometer away, some photons are directly reaching you ( on a straight line ) from the lamp, with nothing in between to stop them and some are reflecting towards you also on a straight line, somewhat brightly enough to see, basically the most illuminated area just below the light.
Lets make it more simple, Im pretty sure you have played with lasers once in a while, and they can reach extreme distances because all of there photons are running in parallel, with the least possible scattering, so it illuminates very low amount of area, but can withstand more absorption ( air particles ) to give a visible dot miles away, fun fact, the red dot of the laser you see is basically another extension of the laser that is reflected directly to your eyes, but the light loses most of its energy in the way and does no harm to your eye.
To add, some photons are possibly reaching your surroundings, but have no energy left to be reflected back to your eyes and make an impression of the object, only those that are directly reaching your eye is still powerful enough to make sense in the brain
The light goes away from its source in all directions. North, South, East, West, Up, Down, and every angle in between.
Think about all the light that leaves the source at the same time in all directions as a sphere of light. As the light travels further away from the center that sphere gets bigger. It’s kind of like blowing up a balloon; as the balloon gets bigger and bigger the rubber of the balloon’s surface gets thinner and thinner because the same amount of rubber is surrounding a bigger volume.
Now pretend that the balloon is a special kind of balloon that doesn’t pop so you can keep blowing it up forever. You will still be able to feel the balloon because you can touch its surface, but eventually the rubber will get so thin that it will be hard to see.
It’s like that with light, as the “balloon” of light gets further from the center, there will still be some light that has travelled in a straight line from the source to your eye. Our eyes are very sensitive to light hitting it directly, so the distance the light can be perceived is much much further than the distance that the density of light will be good at illuminating your surroundings.
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