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.
Think about a slice of toast which you want to spread cream cheese on. The toast represents an object at a distance (d) from the light. This toast is special in that as you move it away from the light it gets larger. You move it twice the distance (2d) it’s sides grow twice as large.
The light is like the cream cheese we want to spread on the toast, but the light is a fixed amount, it doesn’t grow like the toast.
The surface area (A) of the side of toast facing the light is calculated by multiplying the width of the toast (w) by the height of the toast (h). Then (A=w*h). At distance (d) there is (A) amount of cream cheese spread over it, but remember the amount of cream cheese is fixed.
Now at twice the distance (2d) the toast is twice as wide (2w) and also twice as tall (2h). This means that the area of this slice of toast at (2d) (let’s call it B to not confuse it with the size of A) is (B=2w*2h), or in other words (4wh) which is 4 times the original slice (recall originally A=wh), thus we can say (B=4A).
The amount of cream cheese is fixed at A, but now we have 4 times the toast to spread it across! So we see that the further we get the thinner we must spread the cream cheese to cover the ever growing surface.
Now, our eyes are a fixed size, much like our mouths. Imagine taking a bite out of that slice of toast. The closer you are to the source the more cream cheese would be in this fixed size bite, and similarly the further away you are the less cream cheese there will be.
In the same way our eyes will be hit by less light the further we are from the object.
The name of this rule is called the inverse square law. If you were 3 times the distance at (C), then we get (C=3w*3h=9A).
There is an inverse square Law describing the drop off in light intensity. Basically each time the distance doubles the light intensify is just a quarter.
Our eyes see by detecting photons and they are pretty darn good at it. Does not take a ton of photons to see ( detect) light.
Car lights. When you are looking directly at them, your getting ALL the photons hitting your eyes. blinding up close, less far away but at a long distance your sill getting some
If your sitting in the car. The photons are not going straight to your eye. The photons are going out and bouncing back to you. Many are absorbed by material before coming back to your eyes. ( like the light is white, BUT you only see the colours of the stuff you can see. ). This means there are less photons hitting your eyes, Vis a Vis less light.
For starters, if I shine a light at you and you are 100 meters away, that’s only 100 meters it has to travel for you to see it.
If it then bounces back from you, so that I can see it (and therefore see you), thats another 100m. That’s 200m now.
The light weakened on its way to you, by bouncing into particles and whatnot, and then kept weakening on its way back to me.
On top of that, not all the light is going to be bouncing back in the first place, unless you happen to be a perfect mirror.
Simply put; it doesn’t take much light to be seen, but it takes a lot of light to project and reflect off of other things.
Let me see if I understand what you’re asking, because the question is awkwardly phrased:
Are you asking why we can see the light shining from far away (like seeing car lights from far away), but can’t see things illuminated by the same light just as well (like when driving)?
The lights it projects does go so far – that’s how you see the light in the first place. But for something to be illuminated, the light has to make it to the object, reflect, and make it all the way back to your eye. That’s a much more taxing journey on the intensity of the light than simply shining at you.
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