How do streetlights make it harder for us to see the stars in the night sky?
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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.
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