為甚麼『 水是藍色 』不成立？

比喻錯誤。也沒有提出有力的理由反對水是藍色這個理論。

http://en.wikipedia.org/wiki/Color_of_water
看一下再說好嗎

本人應發問者要求回答此問題，但以下內容是代友貼出答案，希望可以解答到發問者的問題
1. In physics, color is an immeasurable quantity. It is NOT a property of light but a property of human brain. And the term to use is a matter of taste. For example, some people like to use the word violet but other people the word purple. There is no exact definition of each color. So, violet and purple are both right as long as the color does not violate "common conception" of the color.
For the case of water, it is an undeniable fact that when in large quantity, water is blue. So, we may say
a. Water is colorless but when in large quantity, it is blue.
b. Water has a bluish tint which may be detected only in large quantity.
c. Water has a bluish tint. But when in small quantity, we cannot see it.
The above descriptions are a matter of taste, all are acceptable. But if one says "water is colorless", I regard it as an acceptable but incomplete description. It simply ignores the fact that water is blue in large quantity.
But why do so many people (and websites) describe water as colorless?
It is not a WRONG description but it is just an INCOMPLETE description.
Many people, even some scientists, just do not know water is blue in large quantity. Also, saying water is colorless is sufficient in most purposes in our daily life or in chemistry labs. So, even if one can find a million web pages saying that water is colorless, it does not disprove "water is blue in large quantity".
Most of the people think that it is due to the reflection of sky, impurity of water. But, if one considers the case more logically, it is easy to rule out these wrong explanations. In fact, it is the intrinsic property of water that makes it blue in large quantity. We can see that how a common misconception can deeply affect people's mind.
2. Although many web sites say water becomes bluer as the depth increases, actually
only the number of water molecules light encounters in water matter, which is mainly determined by the distance light travels in water. The longer the distance light travels in water, the more water molecules it encounters, hence the more absorption of red light and the bluer the water. But in many discussions, a sea or a tank of water is considered. So, the distance light travels in water is treated the same as the depth of water. The density of water molecules does increase slowly with the pressure of water and thus pressure has only minor effect on the absorption of red light.
Some people also argues that when light travels horizontal in water, water is not blue. This is also wrong because red light is absorbed no matter the direction of light. And he did not provide any evidence of his claim.
If you want to prove your argument, you should also explain (microscopically) why pressure makes water blue, not just claims the effect of pressure without explaining it.

根據實驗證明，在純水（H20）的光譜中，在約760nm(紅光)的位置有一個很強的吸收帶，在相同的實驗中，重水（D20）的吸收帶是在紅外線的位置，所以這個吸收帶不是因為散射或其他化學原因造成，而是由水份子的核子運動所引發。水吸收了紅光，我們便只看到剩下來的藍光，而重水才是真正透明的。從世界各地的海洋搜集到的海水樣本都有大致相同光譜，我們可以相信海水的藍色是從純水的顏色而來。



一般的誤解敍述及其不足之處

敍述：「泳池池底是藍色，所以池水才是藍色」

解答：有很多泳池的底部是白色，沒有一點藍色，但池水還是藍色。


敍述：「海／泳池水反射天空的藍色, 因此變成藍色 」

解答：陰天時，海或泳池水還是藍色。甚至在晚上或室內的泳池，池水都仍是藍色。


敍述：「海水本身既然是無色的，那麼大海又怎麼會是藍色的呢？這是因為大海的顏色是由海面反射的光和來自海水內部的回散射光的顏色決定的
」
敍述：「海是藍色跟天空是藍色的原因是一樣，都是因為散射」

解答：越深的海越是藍色，這就證明了從海面反射的光對海的藍色的貢獻是很微小。至於散射，他的確可以令海水看起來藍些，但並非主因。從實驗知道，藍光比紅光散射多5倍，但是水對紅光的吸收是比藍光多100倍。

敍述：「因為海水對不同波長的光有不同的散射與吸收效果。當陽光照到海面時，海水會吸收紅光至黃光，散射藍光，因此海洋呈現藍色」
敍述：「一般情況下，紅色和黃色等色光的波長比較長，最容易被海水吸收，當他們射入海水後，大部分光會被海水吸收，只有極少部分被水分子及海水中的懸浮顆粒反射和散射。而波長較短的藍光和綠光的穿透能力強，當它們射入海水後，只有少部分被海水吸收，大部分光遇到水分子或其他懸浮顆粒便向四周反射和散射，這樣一來，海水對藍光吸收得少而反射得多，而且越往海水深處越有更多的藍光被折回到水面上來。當這些被反射的藍色光射入到我們眼睛裏時，我們看到的大海便是藍藍的一片。 」

解答：不錯，水吸收了紅色的光，但不是因為波長比較長，而是因為這是水的特性。再者，在 Rayleigh 散射中，波長越長（即紅光）越能穿透，而波長越短（即藍光）越能被散射（即是被吸收再反射），藍光的穿透能力是比紅光弱，所以這個說法自相矛盾。或者，以上所說的「吸收」其實並非真正的水份子對紅光的吸收，而是指紅光穿透到更深的海裏，但是，正如之前所說，散射對水的藍色貢獻不大。

其實湖是藍色，是因為在正常情況下，光的藍光會被反射，所以是這樣，其實所有的水都基本是無色，所以水不能說是藍色。

It is because that the color of the sea (blue) is only a reflection of the light that goes into the sea, blue is the main color of the reflection, so we only see blue color, but there are more color that is reflected from the sea.

水同海/湖唔同
海同湖係包括埋同邊既事物
例如海底/湖底 ,又或者海/湖中既生物
呢一切都可以影響到海/湖既顏色

但係水就唔同
水只係H2O
係無色既

海是藍色，是你直接觀察所得，海水反射藍光，又是天空的倒影…
湖可能唔係藍色，係綠色，都係反映其他光線…
但係，就唔會有人話「海水」係藍色，亦冇人話「湖水」係綠色…
因為，抽離了海水和湖水，沒有了反射其他光線的效果，水就是透明，無色的！

有時，我地要將描寫文，抒情文和敍述文分開理解…
不要捉住一字半句就評為「無稽」或「無理」。
有時，我們的表達，只是心中所想，眼中所見，未必全是屬實！
所以，你可以理直氣壯告訴你的朋友，海水和湖水本來就是透明無色的，只是反射了其他物事的光線而看來是藍色或者綠色而已。

肅然起敬

因為淨水事實上是無色的,而海是藍色和湖是藍色那只是反射天空的藍色

the海是藍色；湖是藍色 is because of the things under the 海 and 湖 make them be blue colour. so水是藍色不成立