AGREE or DISAGREE: Common Knowledge Does Not Equal Fact?

[Up_Tilt_390]

Wow, just wow! That is the most interesting Wikipedia article I have ever seen, so props up to you for posting it But what it seems to have missed, though I might've just missed it myself, is the colour of water. I think that ought to have a bit of coverage, so I'll do it here...

The common misconception is that water is colourless and that the ocean and other large bodies of water appear blue because of the reflection of the sky. However, this isn't true. The ocean still appears blue even on a cloudy day, and I've been on a boat recently so I've seen this myself. Also take note of indoor swimming pools: the bottom tiles are a white colour (if I'm not mistaken), so the water should appear as such, but instead it's a very light turquoise blue colour. There's no sky to reflect in this particular case, so it kind of debunks the idea that the colour of large bodies of water are determined purely by the sky, though it does have an effect given how it can appear red in the sunset, but even then when you're up close it appears blue.

"But wait" you might say, "why is my tap water, bottled water, and sink full of water transparent?". Well, to answer that question, we have to consider where water gets it's colour. It gets it's colour by selective absorbing of certain colours of the visible colour spectrum, but it scatters the blue light rather than absorb it like the infrared red light. But this only gives it a blue colour so light, that in small quantities water can appear colourless, and in fact it is so light that you need a deep body to see the true colour. The deeper the body, the bluer the water. Even in the blue ocean, you can see this in a shallow areas of the beach where small amounts of water will seem transparent, and some of the sand beneath it will still appear visible.

But as you get deeper, it gets bluer, and below depths of about 200-500 metres, the water is pitch black because of the lack of light that can penetrate the surface. If it was transparent, surely you'd still be able to see wouldn't you? Light may be the primary determinant of the colour of water, but some impurities that are both dissolved or suspended can affect it's colour. Swamps for example have a green texture to the water, while putting water in a bucket of sand can make it appear brown. There are several factors that can affect the colour of water, but in it's pure form (by that I mean just by itself) it is blue because of selective absorption of certain light on the colour spectrum.

 

[Dai Corner]

Is that your or your kids science homework?

 

[Up_Tilt_390]

Is that your or your kids science homework?

Well I don't have kids, and I don't plan to at any point in my life, so it's primarily my own knowledge that I've gathered over the years. Why do you ask?

 

[Dai Corner]

Well I don't have kids, and I don't plan to at any point in my life, so it's primarily my own knowledge that I've gathered over the years. Why do you ask?

It reads like the sort of thing my science teachers asked us to write about

 

[Up_Tilt_390]

It reads like the sort of thing my science teachers asked us to write about

I think I'll take that as a compliment

 

[AndrewE]

The common misconception is that water is colourless ... [at depth] the water is pitch black because of the lack of light that can penetrate the surface. If it was transparent, surely you'd still be able to see wouldn't you? Light may be the primary determinant of the colour of water, but some impurities that are both dissolved or suspended can affect it's colour. Swamps for example have a green texture to the water, while putting water in a bucket of sand can make it appear brown. There are several factors that can affect the colour of water, but in it's pure form (by that I mean just by itself) it is blue because of selective absorption of certain light on the colour spectrum.

The bold bit above (and most of it) is just plain wrong. The water is still COLOURLESS, you just think or imagine it is black because there is no light to reveal (i.e. to allow you to see) any colour. How do you think films are made at great depths? White light still illuminates the scene no mater how deep.

Is that your or your kids science homework?

Given the level of some contributions here recently, I'd guess that may be his homework... The problem is that GCSE (or whatever it's called now) has horrendous over-simplifications to the point that a lot of what is taught - to get the "right answer" - is simply wrong. Pure water has no colour, and a swamp is green because of the chlorophyll in the scummy stuff that lives in it.
Let's not get into the "colour" of light, or the fact that the "colour"of something depends on the frequencies that it absorbs and what frequencies were illuminating it in the first place.

 

[Up_Tilt_390]

The bold bit above (and most of it) is just plain wrong. The water is still COLOURLESS, you just think or imagine it is black because there is no light to reveal (i.e. to allow you to see) any colour. How do you think films are made at great depths? White light still illuminates the scene no mater how deep.

Given the level of some contributions here recently, I'd guess that may be his homework... The problem is that GCSE (or whatever it's called now) has horrendous over-simplifications to the point that a lot of what is taught - to get the "right answer" - is simply wrong. Pure water has no colour, and a swamp is green because of the chlorophyll in the scummy stuff that lives in it.
Let's not get into the "colour" of light, or the fact that the "colour"of something depends on the frequencies that it absorbs and what frequencies were illuminating it in the first place.

I've been out of education for years so I have no homework to do, but one of my teachers thought water was transparent also. Fair enough about the point of imagining it is black in the dark, but regardless the water will not appear as clear when a submarine illuminates it in the depths, it will still appear as blue. The idea that water has no colour is completely baseless, especially now that I have already explained why it may appear colourless

 

[Dai Corner]

I'm having an hilarious conversation with fans of surreal comedy elsewhere on t'Interweb. This thread is even funnier

 

[AndrewE]

I've been out of education for years so I have no homework to do, but one of my teachers thought water was transparent also. Fair enough about the point of imagining it is black in the dark, but regardless the water will not appear as clear when a submarine illuminates it in the depths, it will still appear as blue. The idea that water has no colour is completely baseless, especially now that I have already explained why it may appear colourless

In that case how does colourimetric chemical analysis work? You create a compound, measure the absorption at a chosen suitable frequency and deduce the concentration of the determinand. You don't have to avoid blue, as far as I know. Water may absorb a tiny fraction of the spectrum, allowing white light seen through it to appear blue, but for most purposes it is colourless.

The ocean still appears blue even on a cloudy day, and I've been on a boat recently so I've seen this myself.

is shallow and naive in the extreme. (A) You are looking at the surface (and so reflected light), and (B) take no account of suspended matter either, which is why the channel looks (and is) is milky (chalk).
You also weren't paying attention (or you teacher was pretty dim) if you can't distinguish between transparent and colourless. Blue glass is transparent but not colourless.
My dad used to say that half the work of a teacher/lecturer at degree level was undoing the errors taught in schools (or words to that effect.)

 

[Up_Tilt_390]

In that case how does colourimetric chemical analysis work? You create a compound, measure the absorption at a chosen suitable frequency and deduce the concentration of the determinand. You don't have to avoid blue, as far as I know. Water may absorb a tiny fraction of the spectrum, allowing white light seen through it to appear blue, but for most purposes it is colourless.

Except water doesn't absorb a tiny fraction, it absorbs all red, orange, yellow and green wavelengths of light, while the shorter wavelengths of the blue and violet ones are scattered. This is what primarily determines the colour of water. There is nothing to suggest to suggest that it colourless.

is shallow and juvenile in the extreme. (A) You are looking at the surface (and so reflected light), and (B) take no account of suspended matter either, which is why the channel is milky (chalk).

How is it shallow and juvenille/naive? Science is observable! The surface of the water might be affected by the light on the surface (which I have already acknowledged) but it is NOT the primary cause of it's colour. I also talked about suspended matter can change the colour of water. In this case it was a very dark bluish, almost black colour.

 

[AndrewE]

If you swim under water with your eyes open or wearing goggles you see things in their normal colours (i.e. the colours you would expect to see in air) so it clearly does NOT

"absorbs all red, orange, yellow and green wavelengths of light"

otherwise things would appear to be different colours.

There is nothing to suggest to suggest that it colourless.

So why does a falling water curtain not make things seen through it seem a different colour? or the water appear to have a colour itself?

Why do you assume that when you look at the surface of water you are seeing its colour?

 

[Up_Tilt_390]

If you swim under water with your eyes open or wearing goggles you see things in their normal colours (i.e. the colours you would expect to see in air) so it clearly does NOT otherwise things would appear to be different colours.
Why do you assume that when you look at the surface of water you are seeing its colour?

You can see things in their normal colour UP CLOSE. However, when the object gets further and further away it gradually disappears until you can no longer see it. I go swimming three times a week and swim with my head underwater wearing a pair of goggles. I don't see something until I start getting within very close range, in contrast to the air where on a clear day you can see things a great distance away. You see the colour of water on the surface because that is where the blue wavelengths penetrate the surface and give it the colour, and as you get deeper the other wavelengths have already been absorbed to the point where they are no longer present or travelling down as far, and as such the water will be a much bluer colour than at the surface because the blue wavelengths have travelled further down than they would if they were absorbed.

So why does a falling water curtain not make things seen through it seem a different colour? or the water appear to have a colour itself?

Because the falling water curtain is scattered, and as such it's quantities are minimal and cannot form a body of water itself, as well as the fact that the things seen through it aren't behind a thick or deep enough layer to not be seen. If water there is enough water to make a deep body but is scattered, it will appear colourless because of the small quantities and it's lighter hint of blue. The deeper the body, the bluer the water.

 

[ComUtoR]

Genuine question for AndrewE. If water is colourless and ice is solid water. Why is snow white ?

Cheers in advance.

 

[Dai Corner]

You can see things in their normal colour UP CLOSE. However, when the object gets further and further away it gradually disappears until you can no longer see it. I go swimming three times a week and swim with my head underwater wearing a pair of goggles. I don't see something until I start getting within very close range, in contrast to the air where on a clear day you can see things a great distance away. You see the colour of water on the surface because that is where the blue wavelengths penetrate the surface and give it the colour, and as you get deeper the other wavelengths have already been absorbed to the point where they are no longer present or travelling down as far, and as such the water will be a much bluer colour than at the surface because the blue wavelengths have travelled further down than they would if they were absorbed.

Now come it's the blue light that doesn't get absorbed? Why not red, green or something outside the visible spectrum?

 

[ComUtoR]

If water is blue and you put a blue object in it, surely that object would then be invisible.

 

[Up_Tilt_390]

Genuine question for AndrewE. If water is colourless and ice is solid water. Why is snow white ?

Cheers in advance.

This is another thing as well. From a distance it can appear white, but when you get up close it appears to be more of a lighter blue colour. You can see this if you Google an image of an ice glacier both from a distance and up close.

If water is blue and you put a blue object in it, surely that object would then be invisible.

If it goes deep enough it will appear invisible to the human eye. Someone who I go swimming with has blue goggles, and in the deep end it isn't very helpful when he drops them.

EDIT: I also just watched a video and saw someone take a red apple to a depth of 25 metres. Because the red wavelengths couldn't go down that far, the apple appears as a dark grey, and some red objects appeared a different colour too. You could only see the red of the apple when another source of light was turned on and shone upon it.

 

[AndrewE]

Genuine question for AndrewE. If water is colourless and ice is solid water. Why is snow white ?

Cheers in advance.

I guess because of the air in the gaps between adjacent ice crystals (which are miniscule bits of clear colourless solid water) or reflection of white light off their surfaces. Doubtless Wikipedia will give the correct answer if I'm wrong....

 

[Up_Tilt_390]

Now come it's the blue light that doesn't get absorbed? Why not red, green or something outside the visible spectrum?

I'm not entirely sure, but I am in the process of finding this out. I imagine the answer will be more down to the properties of the blue wavelengths, and how it might react with water. The question could be applied to many things whose colours are determined by the visible spectrum, and I'm curious to see what the answer will be.

EDIT: It appears that the answer to why it is blue in particular is because of the water molecules. They seem to absorb the longest and lowest energy wavelengths such as red first, and leave blue and violet to travel further down and scatter. I can't say why the water molecules do this, but it seems that the properties of these molecules are why the blue makes it down further. They react differently to wavelengths than other objects.

 

[AndrewE]

You can see things in their normal colour UP CLOSE. However, when the object gets further and further away it gradually disappears until you can no longer see it.

Irrelevant. By your reasoning it should change colour as the blue is absorbed, rather than just disappear. I think you are being fooled by opacity, which is why you lose sight of things, leaving only the scattered blue light for you to see.

 

[AndrewE]

If water is blue and you put a blue object in it, surely that object would then be invisible.

and colourless glass becomes invisible in [clear/colourless] water, but that's because the refractive indices are similar too!

 

[Dai Corner]

I won't claim to understand this but Wikipedia says (my bold)

In the near-infrared range liquid water has absorption bands around 1950 nm (5128 cm−1), 1450 nm (6896 cm−1), 1200 nm (8333 cm−1) and 970 nm, (10300 cm−1).[19][20][15] The regions between these bands can be used in near-infrared spectroscopy to measure the spectra of aqueous solutions, with the advantage that glass is transparent in this region, so glass cuvettes can be used. The absorption intensity is weaker than for the fundamental vibrations, but this is not important as longer path-length cuvettes are used. The absorption band at 698 nm (14300 cm−1) is a 3rd overtone (n=4). It tails off onto the visible region and is responsible for the intrinsic blue color of water. This can be observed with a standard UV/vis spectrophotometer, using a 10 cm path-length. The colour can be seen by eye by looking through a column of water about 10m in length; the water must be passed through an ultrafilter to eliminate color due to Rayleigh scattering which also can make water appear blue.[16][21][22] In both liquid water and ice cluster vibrations occur, which involve the stretching (TS) or bending (TB) of intermolecular hydrogen bonds (O–H . . . O). Bands at wavelengths λ = 50-55 μm (44 μm in ice) have been attributed to TS, intermolecular stretch, and 200 μm (166 μm in ice), to TB, intermolecular bend[11]

The spectrum of ice is similar to that of liquid water, with peak maxima at 3400 cm−1 (2.941 μm), 3220 cm−1 (3.105 μm) and 1620 cm−1(6.17 μm)[14]

 

[AndrewE]

Except water doesn't absorb a tiny fraction, it absorbs all red, orange, yellow and green wavelengths of light, while the shorter wavelengths of the blue and violet ones are scattered. This is what primarily determines the colour of water. There is nothing to suggest to suggest that it colourless

So shine white light through water in a colourless glass: if it absorbs absorbs all red, orange, yellow and green wavelengths of light how come you get white light?

Also the Wikipedia page gives you pretty feeble support. Note that you have to have super-clean water to eliminate the blue colouration due to light scattering, and even then you have to be able to see through 30 metres of the stuff to detect the colour... so it's not very blue to my mind!

 

[Up_Tilt_390]

So shine white light through water in a colourless glass: if it absorbs absorbs all red, orange, yellow and green wavelengths of light how come you get white light?

Also the Wikipedia page gives you pretty feeble support. Note that you have to have super-clean water to eliminate the blue colouration due to light scattering, and even then you have to be able to see through 30 metres of the stuff to detect the colour... so it's not very blue to my mind!

The wavelengths still travel a certain distance before they are absorbed, but regardless the source of light will still be emitting the wavelengths. When you take a red apple to a depth of 35 metres, it will lose the red colour because of how the red wavelengths no longer reach down there and become grey. However, shining a light on it from a torch brings back the colour because new wavelengths are being emitted from a new light source, hence the infrared can give the apple its colour again. These wavelengths will travel and be absorbed or scattered through the water just like any other source of light.

Because of the small quantities of shallow water in things such as glasses, there are insufficient molecules for the wavelengths to be completely absorbed, and so the low energy wavelengths will be able to travel through it all. The blue colour is still there, but it is a very, very vague hint of it, hence why you need a deep body for the colour to appear visible to the human eye. Swimming pools for example have a blue colour in the areas where people swim because of their depth, but when it washes up and goes down the drain, it is too shallow for light molecules to be absorbed or scattered before they make it out the bottom.

 

[Busaholic]

'Always steer into a skid when driving' is Common Knowledge. If I'd followed that advice thirty odd years ago I probably wouldn't have lived to tell the tale. I was a newly-qualified driver in my old Ford Cortina Mark 1, driving to work in the morning rush hour. Around the GLC/Kent boundary where the A20 morphs into the M20 I hit some black ice just as the one lane went into three. I was driving against the rush hour flow and as I pirouhetted towards the central barrier with three lanes of slow-moving traffic visible on the other side I steered against the skid and managed to go from the outside lane up on to the bank on my side into the overnight snow cover without hitting any other vehicle. A following lorry driver helped get me off the bank, without damage to car or owner, other than hurt pride, I guess. What I did was instinctive, and my instincts proved to be good. but was still 'wrong' according to CK. Personally I'd rather be wrong and alive than right and dead.

 

[Up_Tilt_390]

'Always steer into a skid when driving' is Common Knowledge. If I'd followed that advice thirty odd years ago I probably wouldn't have lived to tell the tale. I was a newly-qualified driver in my old Ford Cortina Mark 1, driving to work in the morning rush hour. Around the GLC/Kent boundary where the A20 morphs into the M20 I hit some black ice just as the one lane went into three. I was driving against the rush hour flow and as I pirouhetted towards the central barrier with three lanes of slow-moving traffic visible on the other side I steered against the skid and managed to go from the outside lane up on to the bank on my side into the overnight snow cover without hitting any other vehicle. A following lorry driver helped get me off the bank, without damage to car or owner, other than hurt pride, I guess. What I did was instinctive, and my instincts proved to be good. but was still 'wrong' according to CK. Personally I'd rather be wrong and alive than right and dead.

Good job you didn’t follow what is regarded as common knowledge. This is why I voted for the false option, cause of situations just like this.

 

[AlterEgo]

Good job you didn’t follow what is regarded as common knowledge. This is why I voted for the false option, cause of situations just like this.

But the “false” option says that common knowledge is more often right than wrong. So I don’t understand why you would vote for it in light of “situations just like this”.

 

[Up_Tilt_390]

But the “false” option says that common knowledge is more often right than wrong. So I don’t understand why you would vote for it in light of “situations just like this”.

Oh dear! I forgot what my own question was then! I meant why I chose TRUE, not false.

Unfortunately the question can’t be changed. I tried to see if a mod could after it’s poor wording was brought up beforehand, but nothing can be done I’m afraid.

 

[Bromley boy]

Oh dear! I forgot what my own question was then! I meant why I chose TRUE, not false.

Unfortunately the question can’t be changed. I tried to see if a mod could after it’s poor wording was brought up beforehand, but nothing can be done I’m afraid.

So, as I understand it, the current options are:

- common knowledge can always be assumed to be incorrect;

- common knowledge can be assumed to be correct, on the balance of probabilities.

I guess you could have added “common knowledge can always be assumed to be correct.”

Is that the gist of what you intended?

 

[Up_Tilt_390]

So, as I understand it, the current options are:

- common knowledge can always be assumed to be incorrect;

- common knowledge can be assumed to be correct, on the balance of probabilities.

I guess you could have added “common knowledge can always be assumed to be correct.”

Is that the gist of what you intended?

That actually sounds a bit like what I was trying to get at actually...