How come we can see a source of light extremely far away when the source only illuminates the area much closer to it?

 

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?

SHAREitHoly moly friends, thanks for the awards anAppvnd stuff. I didn't think this question would spark so much interest, lol. I am thoroughly grateful for all your replies.

Comments (5)

 
Oct 10, 2020 - 4:32pm

Cool question. I'm just going to simplify a lot of stuff here. Let's say the light is a single 3-dimensional point that radiates light waves. This means there are a large number of "light waves" spread out in the atmosphere because of this single light source. 

Why doesn't it illuminate your porch: the further you are from this point, the fewer number waves actually reach yo. This should intuitively make sense but might be clearer if you draw it on a piece of paper. For something to illuminate, you naturally need a large area to be engulfed in light waves. Hence, why we don't use a laser beam to illuminate the streets for example. 

Why do you see it: only one of these waves actually needs to reach your eye. 

 
  • Principal in HF - Event
Oct 10, 2020 - 8:34pm

Marlborough

Cool question. I'm just going to simplify a lot of stuff here. Let's say the light is a single 3-dimensional point that radiates light waves. This means there are a large number of "light waves" spread out in the atmosphere because of this single light source. 

Why doesn't it illuminate your porch: the further you are from this point, the fewer number waves actually reach yo. This should intuitively make sense but might be clearer if you draw it on a piece of paper. For something to illuminate, you naturally need a large area to be engulfed in light waves. Hence, why we don't use a laser beam to illuminate the streets for example. 

Why do you see it: only one of these waves actually needs to reach your eye. 

This is a garbage explanation. Actual explanation:

The wall takes light that was coming in straight lines from the source and scatters them in all directions (assuming the wall is not made of mirrors). So if you stand 1 foot from the wall and look at any point on the wall then you're only getting a small fraction of the light bouncing off that point of the wall. Assuming equal scattering you're getting the area of your pupil (like 1/5th of a square inch) divided by the area of a half sphere with radius 1 foot (like 1000 square inches). So each point on the wall is giving off like 1 / 5000th of the light you'd see if you were looking directly at the source.

Overall the whole wall you're looking at would be sending off about as much light into your eyes as if you were looking directly at the light source, but it's spread out over a much bigger observed area, so it's more diffuse.

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