Why Human Eye is most sensitive in green ?????

"George Orwell" wrote in message
...






i dont get it . some body claim the sun isnt yellow ? i
never heard
any thing other then like a " yellow sun " .
if it isnt yellow whta is color like the brite sun ?

White.

"Shawn Grant" wrote in message
...
"Color" is closely related to human perception. The color of the Sun is
white.
It is not yellow at all, although it may appear very slightly so in some
cases
from the bottom of our atmosphere due to the loss of shorter wavelengths
to
scattering.

If I colored the sun white my art teacher would give me an F.



Your science teacher wouldn't.

On Mon, 12 Jan 2004 09:09:09 +0000, Martin Brown
wrote:

I think it may be your air that is unusually clear. In many places the
air does scatter enough yellow and even orange light to look greenish.
The "blue" sky is considerably desaturated so there is plenty of green
light there. I would agree that above 30 degree it is pale blue.

I am presently looking out on a cold winters day and the sky near the N
horizon is an interesting shade of pale green bordering on orange lower
down. I expect NOx trapped in an inversion layer is playing a part in
this.

There is quite often a thin layer of orange NOx sat over some industrial
areas at this time of year if the air is cold and still.

Yes, our air here is very clear. That is unusual only in a relatively modern
context. While there are plenty of natural agents that can increase scatter,
there are also plenty of artificial ones.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Mon, 12 Jan 2004 08:45:24 GMT, wrote:

How can you say that when the color of sunlight is a function of the
amount of air through which the Sun is observed? Rayleigh scattering
gradually makes the Sun redder as it gets closer to the horizon.
Theoretically, the Sun would also be redder at sea level than at
14,000 feet elevation.

I think you missed the point of my earlier discussion. I agree that direct
sunlight is slightly colored by having shorter wavelengths scattered out. But in
clear air and at reasonable altitudes the effect is very slight. If I look at
the Sun through a neutral density filter, it still looks white, not yellow. But
looking at the Sun is a poor way to judge its color- we are talking also about
the apparent color of sunlight. My point was that objects illuminated by the Sun
don't generally appear to be lit by "yellow" or "golden" light, but by white
light. There are two reasons for that, on physical, the other physiological.
Physically, the light is indeed white because it consists of a mix of the direct
sunlight, which is slightly blue-deficient, and scattered blue light.
Physiologically, it is white because we quickly adapt to the dominant light
source and see it as white even if it is not.

I certainly am not saying that there aren't plenty of conditions under which
sunlight can appear rather strongly colored, just that under standard conditions
of clear air and reasonable altitude it does not.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris L Peterson writes:

How can you say that when the color of sunlight is a function of the
amount of air through which the Sun is observed? Rayleigh scattering
gradually makes the Sun redder as it gets closer to the horizon.
Theoretically, the Sun would also be redder at sea level than at
14,000 feet elevation.

I think you missed the point of my earlier discussion.

What you think I missed is irrelevant.

I agree that direct
sunlight is slightly colored by having shorter wavelengths scattered out. But in
clear air and at reasonable altitudes the effect is very slight.

Could you be more quantitative? Have you ever looked at a plot of
extinction coefficient versus wavelength? At an elevation of 8000
feet (above which very few people live), the extinction in yellow-
green light is about 15 percent, and in blue light, about 25 percent.
At lower elevations, the numbers are higher. Not exactly my idea of
"very slight". And that's for straight up. Put the Sun 30 degrees
above the horizon and you can double the numbers. Put the Sun on the
horizon and you can understand why it looks orange or even red.

If I look at
the Sun through a neutral density filter, it still looks white, not yellow.

How it looks to you is not how white light is defined. The fact that
you can't tell a difference with the Sun at various zenith distances
from 0 to 60 degrees shows how relatively insensitive the human eye
is to such color changes. In other words, there is a rather large
range of colors that would be perceived as white. I can look at one
sheet of paper in isolation and call it white. I can look at another
one in isolation and also call it white. Put them side-by-side and
suddenly I can tell that they're not quite the same, one being
whiter than the other. But perhaps neither one is truly white,
because they're being illuminated by light from an incandescent
light bulb. Use a fluorescent light bulb, and the situation is
different.

But
looking at the Sun is a poor way to judge its color- we are talking also about
the apparent color of sunlight. My point was that objects illuminated by the Sun
don't generally appear to be lit by "yellow" or "golden" light, but by white
light. There are two reasons for that, on physical, the other physiological.
Physically, the light is indeed white because it consists of a mix of the direct
sunlight, which is slightly blue-deficient, and scattered blue light.

Certain objects illuminated by the Sun, such as the Moon, for example,
don't have the source of illumination passing through the Earth's
atmosphere first, except on the rare occasion of a lunar eclipse,
during which time the effect of the Earth's atmosphere has a more
readily discernible effect.

Physiologically, it is white because we quickly adapt to the dominant light
source and see it as white even if it is not.

You've just ruled out using human perception for defining white light.
We're back to using something like "all colors (or the primary colors)
in equal amounts". The Sun definitely does not produce all colors (or
the primary colors) in equal amounts. To use an analogy, perhaps you
have heard of white noise and pink noise.

I certainly am not saying that there aren't plenty of conditions under which
sunlight can appear rather strongly colored, just that under standard conditions
of clear air and reasonable altitude it does not.

That doesn't mean sunlight is defined to be white.

John Steinberg writes:

How can you say that when the color of sunlight is a function of the
amount of air through which the Sun is observed?

He said nothing, he wrote it.

Irrelevant.

He can write whatever he wants because he
understands the issue.

How can you say that when someone claims that sunlight is white but
fails to note how the color of sunlight depends on the amount of
air through which the sunlight has passed?

The color of sunlight has nothing whatsoever to
do with the amount "air through which the Sun is observed."

On the contrary, the color of sunlight input to a prism has everything
to with the amount of air through which the sunlight has passed, and
the prism experiment is what was being used to support his argument.
The apparatus was not located on the Moon.

Would you put on a pair of rose colored glasses and exclaim the sky is
pink?

No, so why would someone put on an atmosphere and exclaim the Sun
is white?

Would a blind man accurately describe the color of sunlight as
black?

Depends on whether he understands his qualifications to make such
a statement.

Would an animal with monochromatic vision need or perceive color?

If that animal was being asked to provide a definition for white
light that is useful to humans, yes.

Would the color of sunlight be the same on earth as it is the moon?

Not at the bottom of the Earth's atmosphere, given that the sunlight
doesn't have to pass through an atmopshere to reach the surface of
the Moon, except during an eclipse.

The color of sunlight is a fixed and measurable entity.

Not at the surface of the Earth.

Like a kilometer, an angstrom, or the camshaft of a 440 Hemi V8.

On the contrary, the length of a kilometer is indeed fixed (though
it is based on the adopted values for the speed of light and the
atomic second), whereas the color of sunlight is not fixed at the
surface of the Earth.

Rayleigh scattering gradually makes the Sun redder as it gets closer
to the horizon.

Irrelevant. See above.

On the contrary, it's quite relevant; see above.

Theoretically, the Sun would also be redder at sea level than at
14,000 feet elevation.

Irrelevant. Non sequitur. See above.

On the contrary, it's quite relevant, and it's definitely sequitur;
see above.

Theoretically, a Ph.D. would know
what the color of sunlight actually is.

On what basis do you make that claim? Suppose the Ph.D. is in
music performance.

Classic case of failing to understand the posited question.

On your part.

Now, as to the color of the blue sky. Since it's currently cloudy here
in NY I will provide several exemplars based upon current seasonal
disposition:

The issue isn't the color of the sky, but rather whether sunlight
is white.

"Classic case of failing to understand the posited question."
--John Steinberg

All exemplars reflect local NY conditions and utilize actual physical
swatches. [Never trust RGB to accurately display PMS, even on a
perfectly calibrated monitor. Additive, subtractive, you're asking for
trouble on press, mi amigo.]

PMS 2925 U at 11:17AM EST
PMS 293 U at 5:33PM EST
PMS 2965 U at 7:12PM EST

For a later lesson we will examine process color sky values and mix in
some of Zane's cerulean blue for more of those delightful happy
accidents.

Of course, none of this has any relevance to the color of sunlight

Then why did you bring it up?

which
was empirically demonstrated to be white several hundred years ago, back
when Dr. Swanson "Howchee" Lester-Strehl, CLU, MP3, RMS, was still a
freshman at Genoa Highschool.

Define "white".

BTW, great fight song at Genoa High:

"We play football, we play soccer, we keep slami's in our lockers!"

Irrelevant.

Plants tend to be quite green - I think it's something to do with that.
Our visual spectrum merely expanded to either side of green as humans
evolved.

You could say that clorophyl is the sole explaination for why we see
the colours we do.

Maybe.
Paul.