# How far can we see?

Discussion in 'After Hours Lounge (Off Topic)' started by ChrisHeflen, Apr 4, 2005.

1. ### ChrisHeflen Supporting Actor

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I have a long commute in the morning (50 min) and I sometimes think during it.

I was wondering how far can we see. Un-aided. If we have the standard 20/20 vision.
I mean I can look to the East and see Mt Hood. I know it's an hour and a half away, probably 50 miles the way the crow flies. So can we see 50 miles? Could I see a person 50 miles away (across a flat surface)? But then we can see stars which are millions of miles away. Are we only seeing light and only because light travels? I mean were not seeing the physical star are we? Only it's light?
Is it dependent on how big the object is? If so, how far can we see a giant object? If we can see "that" far, then how come we can't see the smaller object at say 20 miles if we can see 50?
What is the limit?
Is the reason we can see light from a long way off, because the light travels out from it?

Anyone?

2. ### John Spencer Supporting Actor

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I'm pretty sure we can only see to the horizon, but I'm no expert.

3. ### MarkHastings Executive Producer

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You are definitely seeing the light that has traveled from the particular object. So it's not as if you are able to see 3 million light years away, you are seeing the light that has reached the Earth after a 3 million light year travel. This is why they say that the stars we see might not even exist. Since it takes so long to get here, the star may have burned out before the light even reaches your eyes.

4. ### george kaplan Executive Producer

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Well, yes, but...

All you're ever seeing is light bouncing off any object, no matter how close. So when you look at your computer, you're not seeing your computer, you're seeing light which bounced off it. Now, that light left your computer much more recently than the light that left that star, and that light had to travel much less distance than the light that left the star, but you're only seeing the light in both cases.

So either you can't see even an inch away from your face, or you can see 3 million light years and more. It's all perception and how you define things. Basically you can't see anything except light that is immediately at your eye. Then there's your brain that perceives the world in a particular way depending on how it makes sense of the the light hitting your photoreceptors by interpreting the neuronal impulses sent from your eye to your brain.

What's more to the point is your ability to make a meaningful perception or not of objects depending on how much light from those objects hits you, which is where size and distance comes in, as well as how good your eyesight is.

5. ### Kyle McKnight Cinematographer

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Nicely put George.

6. ### Danny Tse Producer

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It takes 8 minutes for the light from the sun to reach earth. So when you look up in the sky and see the sun, that image is from 8 minutes ago.

7. ### MarkHastings Executive Producer

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That's true. You never really think about light speed on Earth because it's too fast to make much of a difference.

8. ### Ron-P Producer

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If that light you see is at the end of a tunnel, run, very, very fast.

Sorry :b

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I thought the reason for the difference in visibility between a person and a mountain would be easily understood at an intuitive level.

The key is the ratio between the distance to the object and it's size.

Put two dots close to each other (distance between them is l) on a flat surface, move back until he dots are no longer distincts (become one). At that distance (L), you've reached the limit of your ability to resolve the distance between the two dots from where you're standing. Which means you cannot see anything smaller than l while standing L units away.

There is a techinical term for that ratio that I can't remember for the life of me.

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H

10. ### DaveF Moderator Moderator

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Angular resolution is what you're wondering about (and is what Holadem describes). It's not about distance per se, but about the angle subtended by the object viewed. Normal (color) vision provides about 5 arc-minutes angular resolution; that's 5 / 60 degrees.

There are other issues about the intensity required (number of photons), color variation, etc., but basically if the triangle formed by the width of the object and the viewers eye has an angle greater than 5/60 deg, it can be seen.

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Way to blow my clumsy explanation out of the water with yer fancy words, Dr Dave. Next time send me a bloody PM so I could sound sharp for once .

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H

12. ### NickSo Producer

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[keanu]WHOA[/keanu]

13. ### ChrisHeflen Supporting Actor

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Ok neat, but how far is the limit?
I mean we can see the sun or the moon, but is only because it is lit? And we're not really seeing it, just the light or are we seeing "it".

We can't see a person at 50 miles, but we can see a mountain, so we can see 50 miles just not all things?
I'm not looking for a lesson in trig. or nurological fragmentational ionic feesability, I'm looking for HOW FAR CAN WE SEE.
I mean I can't see Mt. Everest (biggest thing I could think of) right now, but if the world was flat could I see it from here?

I mean do we stop seeing at a 100 miles? 1000 miles?

14. ### Jay Taylor Supporting Actor

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To infinity and beyond!

(As long as it's angular width is at least an unobscured 5 arc minutes)

15. ### Jeff Cooper Screenwriter

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You are only seeing the light, not 'it'. Need proof? Go into a windowless room and turn out the lights. Hold your hand in front of your face, and try to see your fingers. Without light, you can't see 2 inches.

16. ### Christ Reynolds Producer

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[the who]i can see for miles and miles and miles[/the who]

CJ

17. ### DaveF Moderator Moderator

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I've got an advanced degree in optics; I better be able to to explain visual limits. You, however, did a bang-up job with a layman's knowledge of optics and geometry. You should be pleased with your analytical skills

18. ### BrianW Cinematographer

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Reread George's (#4), Holadem's (#9), and Dave's (#10) posts again. The answer is that it depends. As long as the light is bright enough and the object producing/reflecting the light is sufficiently large (at whatever distance it is) to cover a spot on your field of vision about one-twelfth of a degree in diameter, you can see any distance, even infinitely far.

19. ### Garrett Lundy Producer

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I can see Mars on a good night, but I can't read newsprint more than 4 feet away without glasses.

So I can see really far, or not far enough

20. ### Joseph DeMartino Lead Actor

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We don't "stop seeing". There is no "limit" of so many miles. That simply isn't how vision works. Several people have attempted to explain this to you, but you ignore the explanations and keep repeating your (meaningless) question. This is not communication, and I don't know where to go with this.

In theory if an object is big enough and there is enough light falling on it (and that light can travel far enough without becoming too dispersed to register on the eye) you can see it - as, indeed, you do see stars and galaxies many billions of miles from here every night. On Earth one limit to vision is the atmosphere. The scattering of light caused by the atmosphere causes the edges of objects to appear less distinct, and the scattering of blue light in particular makes more distant objects look "bluer".

To ask "How far can we see?" is akin to asking "How far can we hear?" The question doesn't really apply to the subject. We can see any object whose light can distinctly reach our eyes - which depends on mostly on the properties of the object, not our eyes and brains. Similarly we can hear any sound loud enough to reach our ears. If it is the sound of a pencil hitting the floor, we probably have to be fairly close to it. But if it is a volcanic explosion we can be hundreds or even thousands of miles away and still hear it, although we might not recognize the sound. (The shockwave from the Kratkatoa eruption bounced around the world several times and was registered on scientific instruments, if not human ears, as far away as London and Rome.)

Regards,

Joe