Eli5 why are you able to see lights far away but the light it projects doesn’t go nearly as far? (Car lights, street lights, etc)
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Oh! That’s a fun one. Let’s restate it more clearly.
It’s nighttime at your beach house, where you have a big floodlight that lights up the beach. Your friend, in a boat a half-mile away, can see your floodlight clearly and use it to navigate back to you, but you can’t see the boat at all.
We know that light travels in generally straight lines. And human eyes (especially when “used to” the dark) are very sensitive. Your friend in the boat sees the light clearly – a straight line from your floodlight to her retinas.
The entire boat, however, is receiving a very small fraction of the light emitted by your floodlight. Most of it hits the beach: some hits the water, some goes into the sky and is visible from, say, airplanes.
But, of the tiny bit of your light that does make it to the boat, an _even smaller_ fraction makes it back to your eyes. The boat absorbs some light – more if it’s painted a dark color – but even if it’s painted white, it scatters what’s left in all directions. The light that makes it back to you is well below your retinal sensitivity and the boat just blends in with the dark background.
A reflector (like a mirror) trades apparent brightness for viewing angle. Your friend could use a big mirror to reflect your floodlight back to you, but it only works if precisely aimed.
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