How do streetlights make it harder for us to see the stars in the night sky?
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Artificial lights like street lights do two things:
1. Artificial lights illuminate particles of dust, water vapor, etcetera in the atmosphere, causing the atmosphere to glow hazily. This hazy glow washes out the light of the stars coming from beyond the atmosphere. It’s like looking through a veil of cotton gauze.
2. Artificial lights impede our eyes from acclimating to the darkness. In darkness, our vision slowly adapts to the dark, but any bright lights reverse this process. You may have experienced temporary blindness before when car headlights or flash lights shined in your eyes, but it actually take several hours for the eyes to adapt to full darkness sensitivity.
Because the light reflects on everything (the ground, the walls of the houses, the cars, etc…), and that light is reflected by the air and the dust/water particles that fly around. Even when you’re miles away from the closest lamppost, you can tell a city is behind a hill because air above the horizon will have that white/yellow halo, even though you don’t see the lights or any building in that city. You only see air.
Air isn’t completely transparent, sometimes light will bounce off air and back at us. There are also tiny particles suspended in the air. This means that if you have enough air in an area it will look like a sort of haze when illuminated by light.
When there is a lot of light being blasted up into the night sky it effectively adds “noise” to the incoming light from space. If you imagine your eyes like a bucket and the light falling into it like rain, some of that rain of photons is going to come from the stars and some will come from light bouncing off air/particulate and back into your eyes.
It isn’t always easy to tell which photons are which. If 90% of the photons that hit your eye are from reflected street lights and only 10% come from the stars you will have a really hard time seeing stars. The result is that as ambient light (such as from streetlights) increases the stars seem to fade away.
This phenomenon is also the reason “one way mirrors” work. Such a mirror is actually just a partial mirror, allowing some light through while reflecting the bulk of it. If you are in a bright room and 90% of that light is reflected while 10% passes through from a dark room beyond, you may end up seeing 99% of the light being reflection and only 1% being the other room. You won’t see the other room at all! But in the dark room even the 10% that comes through from the bright room is way more than the 90% reflected from the dark room, so the mirror seems like a slightly dimmer window into the bright room.
The atmosphere isn’t perfectly transparent, especially around cities where the air pollution makes it even less so. What happens in “light polluted” areas is the light from the ground is reflecting off the air above you and back at your eyes. Since most starlight is quite dim, that reflected light overpowers it so you can only see the brightest objects. There’s also your night vision. If you are surrounded by artifical lighting your eyes will never get a chance to adjust to the dark (takes about 30 minutes), so any faint starlight making it through the light pollution will be invisible to you anyways.
If you look at an eye when strong light suddenly goes into it, will you see the hole in the middle get smaller, this does it to reduce the amount of light that goes into it. Now if it is dark does the hole get as big it can to gather every bit of light.
Now imagine it’s dark, and you light a candle, it will light up the surroundings and appears bright, if you now turn on a flashlight directly toward your face, will you now only be able to see the flashlight and the candle will be nearly totally dark, this is because you hole in the eye has gotten small again to keep out the excessive amount of light.
The result is that the candle’s light is nowhere near enough to be seen by you. The same is it with the stars, the tiny point of light, compared to the bright light from a light pole, makes your eyes get small holes and keep out too much light.
Light is a stream of weird things called photons that we can just imagine as tiny balls for this answer. Your eye has a bunch of “buckets” inside of it, and the eye sort of works by counting how many balls hit each “bucket” in an instant.
A dim light doesn’t shoot very many balls at your eye. A bright light shoots a lot more. If you think this through, you can see how bright lights hide dim lights.
So when you’re looking at a star, your eye is noticing that some small cluster of “buckets” is getting hit by a few photons and you see a dim small light corresponding to that. A streetlamp blasts millions of photons in every direction. Now your eye sees that all of your “buckets” have a small amount of balls in them. The parts where the star should be have a few more balls inside, but your eye isn’t very good at telling the difference between 3,000 photons and 3,010 photons, so you can’t really see the star anymore.
The stars you can still see under streetlamps are the ones bright enough to emit so many photons your eye can still tell that part of the sky is brighter than others.
Think it’s called light pollution. The same reason it’s hard for you to see things when someone flashes a light into your face. Other lights get in the way.
Most of the issue is with dark adaptation in your eyes. Your pupils can dilate (grow large to let in more light) in just a few seconds but your eyes also have a chemical adaptation process that can take over 20 minutes to complete. Then they’re much more sensitive and you can see more things in the dark, including dimmer stars.
Just a quick glance at a bright object will ruin this “night vision” meaning you have to wait the 20 minutes all over again. Amateur astronomers use dim red torches (flashlights) to do things like read charts and adjust telescopes because this can avoid losing night vision. Street lights near an observing site are a disaster unless you’re viewing bright objects like the moon and planets.
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