How far can we see? (1 Viewer)

[Mike Voigt]

Just to add a bit more confusion : what we see is a very limited part of the electromagnetic spectrum. If you could see longer waves (as in lower frequency, or larger wavelength), you would see infrared. If you could see shorter waves, you could see ultraviolet, even X-rays or gamma-rays.

Distance does not matter EXCEPT for the distance between dots. If it is bright enough, and has a direct line to your eyes, you can see it.

And almost EVERYTHING you see is reflection. Light reflects off the rocks on Mt. Hood, for example. Depending on the material, it has preferential properties for certain frequencies, hence colors.

As for radiation in general, it has 3 things it can do when it hits an object - be absorbed and retained, like heat from infrared, transmitted and (at least partially) emitted, like the heat coming FROM the rock once heated, or reflected - which is what makes some items visible and colorful. Direct sources, like lamps or the sun, are merely very powerful emitters.

Incidentally, the spectrum we see in is almost perfectly aligned to the spectrum emitted by the sun.

See attached URLs for more info.

Mike

Electromagnetic Spectrum

NASA Description

 

[Joseph DeMartino]

Now you see, I didn't even want to go there...

Regards,

Joe

 

[Jon_Gregory]

So is light simply the absence of Darkness



Sorry just had to put some kind of funny in there.

 

[DaveF]

I need to amend my earlier comment. The 5 arcmin subtense is for resolution -- our ability to distinguish two separate objects or features.

But that's not wholly what's asked. We can see an object smaller than 5 arcmin (or whatever your resolving limit is). If enough photons get from it to your retina, you can see it, but perhaps as nothing more than a pinpoint. And if a second something was really close to it, you could see it as well, but you wouldn't be able to tell it apart from the first object.

Waves are the tool of the devil! My light propogates by luminiferous aether, and that's the way I likes it! ( http://www.fortunecity.com/lavendar/...224/metric.wav )

(And to pre-empt Mike's rebuttal: http://www.fortunecity.com/lavendar/...135/thermo.wav )

 

[Jon_Gregory]

That's funny

 

[BrianW]

Thanks for the clarification, Dave.

 

[MarkHastings]

I was gonna say, if it's not a direct light source (i.e. a lightbulb, a fire, a monitor, or the sun) it's 'reflective' light, which leads me to something that maybe someone can confirm.

As was explained to me in art class, you technically aren't even seeing the actual object, what you are seeing is the absence of particular frequencies of light.

For example, if you see something red, what is happening is, the light is hitting the red object...the red light is absorbed into the object and every color BUT red is reflected back to your eye. This creates the illusion of 'red'.

 

[RobertR]

The most distant object visible to the naked eye is the Andromeda galaxy, about two million light years away. This is NOT an absolute limit on "how far" we can see unaided. A sufficiently large, bright object could be seen from still farther away.

Your question about "only seeing light" vs. "seeing the physical object" isn't a very good one, since all we can EVER see are light waves coming from an object, either reflected or generated.

 

[DaveF]

You see red because red reaches your eye.

Color is determined by the wavelenghts reflected and/or transmitted. In reflection, the colors we see are reflected, and the others are obsorbed. A red ball reflects reds and absorbs greens, blues, etc. In transmission, the colors we see pass through the object and the others are reflected or absorbed. Red glass transmits red to us and the greens, blues, etc. were reflected or absorbed.

To further muddy the waters, you might be perplexed by a color filter for a camera. An red/IR filter is designed to block long-wavelength reds from passing through while allowing all other wavelengths. But a good design will accomplish this by reflecting, not absorbing, the reds. So looking through it towards a white light, you would see a blue tint, as the reds are removed. But if you look at it in reflection, seeing the white light reflected from the front surface, you will see a redder tint. The reds are selectively reflected (and the rest of the spectrum passes through, not reflected back towards you).

There is also subtractive and additive color, which I think may be what confused you. Additive color means color creation by adding multiple wavelengths of light. Shine red, blue, and green lights onto a surface and it looks white. All three colors added to give white. This is the basics of a monitor or TV: the RGB colors are added to go from black to white (by phosphors being excited or the DLP chip reflecting the light). More, varying light moves the image towards white.

Subtractive color is used to create color by removing wavelengths. Put our hypothetical red glass in front of a whitelight. We've subtracted greens and blues to transmit red. Put a blue in front of that, and subtract out the remaining blues from the red light, and you get black (nothing comes through). This is the principle used for printed materials. Placing inks on a paper controls what is absorbed and also what is reflected. A mix of inks will absorb a greater range of wavelengths, reflecting less, and give a darker color. Mix all your inks together, little is reflected and you get black. You can try this with your watercolors or fingerpaints.

 

[Holadem]

I will never forget the Jay Leno show where he went around the streets asking people whether or not the lenses on their glasses "wore out" faster when they looked thru them... yes, a lot of people thought so.

I guess intuitively people think of their eyes as active devices are rather than the sensors they are. And frankly, why wouldn't they.

I remember very clearly the day we were told in second grade that the earth was round, the sun was indeed a star like all others, that it did not move, but the earth did. It was the most amazing thing I had ever heard and the feeling of wonder has rarely been toped since.

--
H

 

[ChrisHeflen]

This simple answer is all I was mainly looking for, but thanks to all for the principles on reflective light.
Now I can finish that thesis...


Next up how far can we pee....