Why is there a lack of consistency in portrayals of the visible light spectrum?

[shiva999]

is that true that the colour of sky is blue or is it just the reflection of sea water

 

[vhbelvadi]

The sky is blue because of scattering of light.
Here's a webpage you can read more about this: http://www.sciencemadesimple.com/sky_blue.html

 

[jewbinson]

I've just read that article and it's very easy to understand.

I think the sea is blue because salt water is light blue and the size of the sea causes an accumulaton of light blue, which is dark blue. This is just a guess though.

The sea is often much darker than the sky so I don't think the sea is a reflection of the sky

 

[ZapperZ]

Salt water is light blue? Since when?

Try this at home!

Zz.

 

[jewbinson]

You can't see it in just a few cm^3 or m^3 of water. But perhaps in 10^6 or 10^8 m^3 of water you will begin to notice a pale blue. As I say, it is very, very pale.

Aha! Look what I found

http://en.wikipedia.org/wiki/Color_of_water

 

[jewbinson]

So not just salt water, pure water is blue

 

[ZapperZ]

You have to be careful and understand what you are reading. There's a difference between the color of water seen through a transmission of light versus the reflection of that light. The FACT that the ocean's "color" can CHANGE, depending the time of day, the color of the sky, etc.. etc. (try sitting by the ocean for a long time and under different circumstances) means that it has very little to do with the color of the water... unless you think the color of the water itself changes that often.

Zz.

 

[vhbelvadi]

Right, so water is blue. But that's still not why the sky is blue.
Also, we can't entirely disregard the fact that part of the reason why large masses of water such as seas look blue is because of the reflection of the sky's colour.

 

[jewbinson]

But the sky's blue colour tends to be lighter than the colour of the sea (apart from seas in exotic Islands which can be very light blue).

When the sky is cloudy, the sea still tends to be quite dark blue.

 

[vhbelvadi]

I said 'part' of the reason, not the main reason! :-)
The question here was why the sky is blue, so I think we've got that solved well.

 

[thebiggerbang]

To debunk the OP, you can just assume that the guy observing the sky is sitting in an African dessert. Miles away from water! But the sky is still blue there! It's all about scattering!

 

[jewbinson]

What does there being no sea have to do with the sky being blue? The converse clearly has some relevance, because the sea reflects light. But the sky does not. Unless you were thinking about the sea "omitting blue light and the sky trapping the blue light". But that's got quite a few flaws to it

 

[ZapperZ]

But the sky's blue colour tends to be lighter than the colour of the sea (apart from seas in exotic Islands which can be very light blue).

When the sky is cloudy, the sea still tends to be quite dark blue.

But that already debunks even YOUR suggestion. I mean, how come the color of "salt water" changes when the sky is cloudy? Yet, one can easily argue that a "cloudy" sky causes the reflection to darkens.

At some point, responses to posts on PF must be based on more than just guess work. Re-read the https://www.physicsforums.com/showthread.php?t=414380" that everyone has agreed to, especially our policy on speculative post. If not, this thread is in danger of being locked.

Zz.

 

[klimatos]

is that true that the colour of sky is blue or is it just the reflection of sea water

Obviously the color of the sky varies from day to night and also with changes in the weather.

That said, the blue color of clear daytime sky is a result of the shorter wavelengths of sunlight being scattered by the molecules of the air itself (Rayleigh scattering). The scattering takes place all over the sunlit hemisphere of the atmosphere; hence the blue light appears to come from all parts of the sky.

Air molecules are too small to reflect other light wavelengths, but larger atmospheric particulates can do this. This gives us the common reddish sunsets.

 

[Dr_Morbius]

Sunsets are red for the same reason that the sky is blue. When you look at the sun near the horizon the atmosphere preferentially scatters the blue light away from your eye more than the red light. That makes the sun look redder than usual. The affect is increased by the fact that when the sun is near the horizon you're looking through more atmosphere than when it's overhead.

 

[LostConjugate]

I can't take it any longer! Someone just tell jewbinson that the ocean is blue because the sky is blue.

 

[Drakkith]

I can't take it any longer! Someone just tell jewbinson that the ocean is blue because the sky is blue.

I don't think that's true though.

 

[LostConjugate]

I don't think that's true though.

Well yea at sunset the ocean is red.

 

[jewbinson]

I can't take it any longer! Someone just tell jewbinson that the ocean is blue because the sky is blue.

?

This simply isn't true. When the sky is cloudy, is the sea white?

At night, there is nothing to light up the ocean, and water does not emit light, so obviously you see black (apart from the reflection of the moon)

 

[LostConjugate]

?

This simply isn't true. When the sky is cloudy, is the sea white?

At night, there is nothing to light up the ocean, and water does not emit light, so obviously you see black (apart from the reflection of the moon)

The sea should be less blue if your part of the sky is covered in thick clouds, yes. Some blue still scatters from where the sky is blue perhaps.

 

[Drakkith]

Well yea at sunset the ocean is red.

If so that is only because there is no blue light to see anyways, so of course it wouldn't appear blue. You wouldn't call a white sheet of paper red simply because it is sunset and the only light hitting it is on the red end of the spectrum would you? The fact is that water IS blue. The color that it can APPEAR to be varies depending on the impurities in the water and of course the ambient light.

See here: http://en.wikipedia.org/wiki/Color_of_water

Edit: Let us not mix up the COLOR of an object with the wavelengths that it absorbs or passes. The color can vary with the ambient light but the latter cannot. Water will lightly absorb the Red end of the spectrum, giving it a Blue color.

 

[LostConjugate]

If so that is only because there is no blue light to see anyways, so of course it wouldn't appear blue. You wouldn't call a white sheet of paper red simply because it is sunset and the only light hitting it is on the red end of the spectrum would you? The fact is that water IS blue. The color that it can APPEAR to be varies depending on the impurities in the water and of course the ambient light.

See here: http://en.wikipedia.org/wiki/Color_of_water

Edit: Let us not mix up the COLOR of an object with the wavelengths that it absorbs or passes. The color can vary with the ambient light but the latter cannot.

Are you sure?

It says here:

The blue tint of water is an intrinsic property and is caused by selective absorption and scattering of white light.

Edit: Oh I guess that is the definition for the color of a molecule. Does that mean that the sky is blue as well then?

 

[Drakkith]

Are you sure?

It says here:



Edit: Oh I guess that is the definition for the color of a molecule. Does that mean that the sky is blue as well then?

In my opinion that question is a bit more complicated than one might think. The atmosphere passes the entire visible range of the spectrum. HOWEVER, it causes differing amounts of refraction along the visible spectrum. When I look through my telescope at a white star high in the sky, it appears white. However, when it is low on the horizon the star shifts around and wavers, and it changes colors between white and reddish and back and forth several times a second. This is because of the increased amount of atmosphere that is between the star and myself and because of the angle that the light is entering at.

I would say that the sky looks blue, but it only has a color if you consider a prism to have a color simply because you are looking into one of the refracted colors.

EDIT: Also, looking at the absorbtion spectrum of the atmosphere, it does absorb some light in all wavelengths. It looks like it absorbs a little more red light than other colors, so if it does have a color to it, I would say that it would be blue. However one would never be able to notice it unless you could look at a light source through many miles of air without it refracting. So we would never be able to tell with the naked eye. So, with regards to the NOTICEABLE reason why the sky looks blue it is because of scattering and refraction, not because the sky IS blue.

 

[LostConjugate]

In my opinion that question is a bit more complicated than one might think. The atmosphere passes the entire visible range of the spectrum. HOWEVER, it causes differing amounts of refraction along the visible spectrum. When I look through my telescope at a white star high in the sky, it appears white. However, when it is low on the horizon the star shifts around and wavers, and it changes colors between white and reddish and back and forth several times a second. This is because of the increased amount of atmosphere that is between the star and myself and because of the angle that the light is entering at.

I would say that the sky looks blue, but it only has a color if you consider a prism to have a color simply because you are looking into one of the refracted colors.

I guess it does come down to the difference between scattering and absorption / emission. The latter being the color of an object, the former being an illusion.

Smoke being carbon is black by absorption / emission and blue by scattering if white light is present.

 

[Drakkith]

Pretty much LC.

 

[klimatos]

Sunsets are red for the same reason that the sky is blue. When you look at the sun near the horizon the atmosphere preferentially scatters the blue light away from your eye more than the red light. That makes the sun look redder than usual. The affect is increased by the fact that when the sun is near the horizon you're looking through more atmosphere than when it's overhead.

I disagree. If it were simply a matter of the distance traveled, then red sunsets would not be more common after volcanic eruptions, forest fires, and other sources of aerosols; and less common in clean air. Moreover, reflection and scattering is just as likely to be toward the viewer as away. Finally, in the years I've spent at sea I have seen many sunsets when the sky is blue right down to the horizon.

Your interpretation seems to be shared by a number of online sites, but all of my meteorology texts attribute red sunsets to aerosols. That's what I used to teach, and I'll stand by it.

 

[Drakkith]

I disagree. If it were simply a matter of the distance traveled, then red sunsets would not be more common after volcanic eruptions, forest fires, and other sources of aerosols; and less common in clean air. Moreover, reflection and scattering is just as likely to be toward the viewer as away. Finally, in the years I've spent at sea I have seen many sunsets when the sky is blue right down to the horizon.

Your interpretation seems to be shared by a number of online sites, but all of my meteorology texts attribute red sunsets to aerosols. That's what I used to teach, and I'll stand by it.

A large part of the color of the sun and moon is due to the air quality. I'm sure it is possible to see blue sky right down onto the horizon, especially out on the sea where the air is most likely clearer than it is near the shore and on land. More atmosphere also means more impurities for the light to travel through and greater chance that the light is scattered. After an eruption or fire there is much more impurities in the air than normal, so much more blue light is absorbed than normal. Both refractive effects AND absorption through impurities changes the color you will see.

My post above explains the effect of refraction in the atmosphere. The changing properties of the air cause a constantly varying level of refraction, which is magnified by the angle and greater distance the light has to travel when something is low on the horizon.

 

[vhbelvadi]

To sum it up:
Water is by itself blue, but also partly as a result of reflection of the sky.
The sky is not blue, but looks so because of scattering. The sunset is also because of scattering but, given the distance and thickness of the atmosphere, only longer wavelengths reach the observer, making it appear to be of a colour more towards the red-end of the spectrum. As for aerosols, they are also responsible for an increased shift of red colouration creating an actual 'red' sky rather than a yellow or orange one as would have been in the absence of aerosols.

(I found more on aerosols, in this regard, here: http://www.webexhibits.org/causesofcolor/14B.html )

 

[Drakkith]

I cannot agree with any meaningful portion of the color of water being because of blue scattered light from the sky. The amount of light NOT scattered that does hit the water vastly outdoes the small amount of scattered blue light that does.

 

[vhbelvadi]

I cannot agree with any meaningful portion of the color of water being because of blue scattered light from the sky. The amount of light NOT scattered that does hit the water vastly outdoes the small amount of scattered blue light that does.

Agreed.
But the light that is not scattered wouldn't affect the sea's colour in the way you state simply because it is concentrated along the line from the observer to the sun (i.e. where the sun rays were most intense, because, unlike the blue-end of the spectrum, they can't spread out.) Therefore, the colour striking a larger area of the sea is still the scattered blue, making reflection partly the reason for the sea's blue colour.

 

[Drakkith]

Every point in the water is in direct line of sight with the sun. A MUCH greater proportion of light comes directly from the sun that it does from the scattered light. And if you think about it, if the light coming down gets scattered slightly, then the added scattered light plus the non scattered light should equal out anyways.

 

[vhbelvadi]

And if you think about it, if the light coming down gets scattered slightly, then the added scattered light plus the non scattered light should equal out anyways.

So reflection is a plausible explanation.

 

[jewbinson]

But when the sky is cloudy (on a bright day, i.e. NOT when there are thick, dark clouds), the sea is still blue.

I would say reflection is a partial explanation.

The water in swimming pools I think are pale blue even when the floor of the swimming pool is not blue (most swimming pools do have blue floors from memory)

 

[vhbelvadi]

I would say reflection is a partial explanation.

That's all we've been saying all along!
Reflection is only a partial explanation, but it's an explanation nonetheless; so it can't be entirely disregarded.

 

[jewbinson]

Yeah I think if the sky were brown or bright green or yellow or pink the sea would be a slightly different colour.