Old 08-17-2016, 09:04 PM   #1
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Default Reflection of Light Stupidity

Ok,

So I'm probably stupid and overlooking something obvious, but here we go.

Situation 1:
The sun is in front of you at whatever angle, let's say 30 degrees.
Light from the sun (pure white) hits the gray floor in front of you. It reflects gray light. That light reaches your eyes, so you see the gray floor in front of you.
So far so good.


Situation 2:
You turn around.
The sun is behind you at prementioned angle.
Light from the sun (pure white) hits the gray floor in front you. It reflects gray light. That light goes away from you, never reaching your eyes. You don't see the gray floor behind you.


We know we can see floor behind us, so there's something wrong with this oversimplified idea.

=====

Idea 1: Light reflects off of something else behind you before it reflects off the floor behind you.

Say that other object is a red wall a few hundred meters away, the reflected light would then be red light. This would make the light reflected off the gray floor a dark red colour. So you'd see a dark red floor. Which isn't true.

Idea 2: Light hits our atmosphere, so light comes from all different angles, and by the sheer amount of light coming from the sun, some of it will hit your eyes from every spot and any angle.

Could be. But this wouldn't apply on the moon. Although I have no references, I get the feeling you're able to see the floor that's away from the sun on the moon.

Idea 2.2: The moon does have an atmosphere, although a lot less dense.
Light reflecting similar to situation 1 on the moon would have a certain intensity of light. A million molecules per cubic centimetre is a lot, but would that be enough to make the light reflected from behind have the same intensity?

Idea 3: The straightforward path in situation 1 never happens. Light reflects off of the inconceivable amounts of molecules so that neither the light from the front nor the back has a considerably shorter distance or considerably more or fewer reflections.
This seemed very plausible. But (even though I haven't done the math for it) shadows seem too dark for that to be viable. Light does get blocked by matter, creating a shadow in direct line with the sun. Following the same argument, shadows would change in shape sporadically (I think? This could be false).

Idea 4: The earth isn't a perfectly smooth globe. Light can reflect off of all kinds of microscopic differences in terrain.
This rings truth for me. But I'd like to know if it would be impossible to look behind you on a perfectly smooth globe.

ON THE OTHER HAND
I am probably overlooking something stupidly obvious.
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Quote:
Originally Posted by bluguerilla
So Sexy Robotnik (SKG_Scintill) {.0001/10} [--]
___
. RHYTHMS PR LAYERING
. ZOMG I HAD TO QUIT OUT TERRIBLE
.

Last edited by SKG_Scintill; 08-17-2016 at 09:06 PM..
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Old 08-17-2016, 09:08 PM   #2
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Default Re: Reflection of Light Stupidity

Consider placing a mirror on the ground in front of you and you would see whatever is in the mirror
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Old 08-17-2016, 09:11 PM   #3
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Default Re: Reflection of Light Stupidity

I am imagining it.
Sun behind me, mirror in front of me. I would see perhaps a red wall, as in idea 1.
Then light would have come from that red wall, reflected only red light off of it, hitting the mirror (which is a gray tinted mirror in this case), reflecting red/gray light into my eyes.
So I'd see a red/gray floor.

Edit: I also see blue sky and white clouds.
Off the blue clouds the floor would be blue/gray.
Off the white clouds the intensity of the light would be dimmer than if I were to look the other way.

Edit v2: But the clouds are perfectly white, so it's fair to assume that it would reflect fairly brightly. Doesn't explain a cloudless sky though.

Edit v3: Sun behind me, mirror in front of me on the moon is weirding out my head.
Wouldn't that be the black emptiness of space? (assuming you don't see the earth)
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Quote:
Originally Posted by bluguerilla
So Sexy Robotnik (SKG_Scintill) {.0001/10} [--]
___
. RHYTHMS PR LAYERING
. ZOMG I HAD TO QUIT OUT TERRIBLE
.

Last edited by SKG_Scintill; 08-17-2016 at 09:17 PM..
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Old 08-17-2016, 09:18 PM   #4
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Default Re: Reflection of Light Stupidity

The difference in light intensity is negligible in your scenario because the source is the sun aka millions of miles away.
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Old 08-17-2016, 09:24 PM   #5
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Default Re: Reflection of Light Stupidity

Fair enough, didn't consider the relative brightness, since it's already lost like 99.9% of its brightness.
But the colour is still bothering me here. Reflecting off of either a blue sky, a red wall before reflecting off of the gray ground (or gray mirror).

Also, the moon still.
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Quote:
Originally Posted by bluguerilla
So Sexy Robotnik (SKG_Scintill) {.0001/10} [--]
___
. RHYTHMS PR LAYERING
. ZOMG I HAD TO QUIT OUT TERRIBLE
.

Last edited by SKG_Scintill; 08-17-2016 at 09:25 PM..
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Old 08-18-2016, 05:36 AM   #6
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Default Re: Reflection of Light Stupidity

Slept over it.
Light coming onto the supposed mirror didn't reflect from the blue sky, so it's still pure white.
The blue sky was its own mirror, unrelated to the light trajectory to the mirror in front of us.

Still the moon though...
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Quote:
Originally Posted by bluguerilla
So Sexy Robotnik (SKG_Scintill) {.0001/10} [--]
___
. RHYTHMS PR LAYERING
. ZOMG I HAD TO QUIT OUT TERRIBLE
.
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Old 08-18-2016, 10:15 AM   #7
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Default Re: Reflection of Light Stupidity

Something you seem to be overlooking is that the sun doesn't emit light as 1 beam, it's a sphere and light radiates out from every point on it.
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Old 08-18-2016, 10:54 AM   #8
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Default Re: Reflection of Light Stupidity

basically, light scatters in all directions when it hits different particles, including and not limited to water vapor, dust, and other gas particles.



given perfect conditions, (sun is a point source, ground is a 100% reflective mirror and smooth and uniformly flat, no other particles exist in this universe), then you'd be dead

non jokingly though, you would not see any light coming from those areas, because the light will never reflect back to your eyes. (so that much is true)

of course, given an actual atmosphere, and not walking on a perfectly flat perfect mirror, you tend to see everything just fine. The caveat being that your body will block the direct light on the ground that's opposite from the light source, which is what a shadow is

edit: on the moon, anything that you're able to see means that there's light bouncing off of it and eventually to your eyes. If there's anything that's hidden, it'll be hidden in shadow
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Old 08-18-2016, 12:31 PM   #9
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Default Re: Reflection of Light Stupidity

Ye basically you are applying the concept of reflection to light... You need to realize light is both a particle and a wave, so essentially things that light hit would be radiating the light off it in all directions, rather than a reflection like a mirror.
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Old 08-18-2016, 01:19 PM   #10
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Default Re: Reflection of Light Stupidity

At least it wasn't as obvious as I thought.
I was genuinely afraid of having forgotten high school physics.

Ye, I see it now.
Thanks!

Edit: wait what?
Quote:
Originally Posted by Dinglesberry View Post
so essentially things that light hit would be radiating the light off it in all directions, rather than a reflection like a mirror.
I... can I have a source?
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Quote:
Originally Posted by bluguerilla
So Sexy Robotnik (SKG_Scintill) {.0001/10} [--]
___
. RHYTHMS PR LAYERING
. ZOMG I HAD TO QUIT OUT TERRIBLE
.

Last edited by SKG_Scintill; 08-18-2016 at 01:29 PM..
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Old 08-18-2016, 02:55 PM   #11
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Default Re: Reflection of Light Stupidity

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Old 08-18-2016, 03:18 PM   #12
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Default Re: Reflection of Light Stupidity

With regards to your "Idea 3", this is a decent enough explanation I dug up:

Have you ever looked at your shadow when it is bright and sunny? Move your hand further away from the ground... what will happen is that the shadow will look blurry. Why? As the above stated, diffraction comes into effect BUT more importantly, the smaller the object, the smaller the shadow. The smaller the shadow, the less diffraction is needed to hide or remove the shadow. So, simply, in a pinch, you could say that Air molecules are too small and light doesn't tend to hit them. Also note that light reflects off of objects too! So that means that light hits a single spot from multiple angles. That is why you can see the ground even though there is a shadow.

Reference https://www.physicsforums.com/thread...hadows.761571/


The atmosphere thus has no real effect on shadows, however it does play a role in a number of optical phenomena (it's why the sky is blue FYI):
https://en.wikipedia.org/wiki/Atmosp...ial_refraction

Last edited by Dynam0; 08-18-2016 at 03:19 PM..
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Old 08-18-2016, 04:57 PM   #13
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Default Re: Reflection of Light Stupidity

just typical that, though light can come from any direction, it still prefers the straight path, with shadows being as relatively dark as they are

heh the idea of a lot of sunlight bending all the way to the other side of the world in a freak accident
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Quote:
Originally Posted by bluguerilla
So Sexy Robotnik (SKG_Scintill) {.0001/10} [--]
___
. RHYTHMS PR LAYERING
. ZOMG I HAD TO QUIT OUT TERRIBLE
.

Last edited by SKG_Scintill; 08-18-2016 at 04:58 PM..
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