Why are the Oceans blue?  Why are the lakes blue?  Why are the swimming pools blue (Refer to figure 1 - Courtesy of Bea's Wonderland)? 

 

Be prepared, it wonnot be a short story, historically, and scientifically. Since I would label myself more as a historian than a scientist, the story has to begin with its historical facade:

 

The memory went back to an old quiz raised 10 years ago by Prof. Zhang Fubao and company, which linked the fate of many of my best friends and myself together. 

Let's start from the very beginning: it was the fall of 1994, about one hundred smart boys and girls gathered in the campus of Shanghai Jiaotong University.  A couple of months later, about 40 among us turned another brand new page of our life, and became comarades of a so-called 95TR trial class.  Definately this changed our destination totally.  Myself I got educated and influenced dramatically by the friends i met during these 3.5 years.  Albeit we tend to believe that -one can not choose the people he is going to meet, sometimes he can, in one way or another. 

Let's go back to that afternoon.  I was among the first group of students got interviewed.   "What do you think of these questions, boy, are they too easy?" after my resolving those 3 so-called physics quiz in a heartbeat, Prof. Zhang asked me, smiling.  I was young and pround enough to reply without any hesitation, "maybe you should think of some more cunning quiz, these are just, too boring."  This answer single-handed (I would dream) set my classmates during the next 3 years, since all of the rest interviewees went thru the following wierd question which is today's topic:

Why are the oceans blue?

Whether all of my college classmates in 95TR answered the quiz correctly or at least partially correct, is to my ignorance.  I am so glad to have them as my classmates.  I am so lucky to have them as my friends.

 

Now let's get to the more interesting (or boring) facade, the scientific explaination of the question:

Pourquoi le ciel est-il bleu (ou azure)?

The first half of the answer is very simple: The oceans are blue because the sky is blue.

 

The second half is a PhD thesis.  Why is the sky blue?  A long-story-in-short answer will be: A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. 

For the same reason, when we look towards the sun at sunrise/sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight.

 

That brief answer alone wonnot qualify the quiz to be a PhD thesis.  In order to earn a PhD degree, you have to learn how to make a short story long, long enough for 3 years' funding, long enough for 3 years' idle time, which you can use freely to travel around Europe, to play bridge, and to learn French.

Well, (cough), as all the physics thesis, this scientifically long story begins with a quiz, a theory, and a proved effect, and it's always better to associate it with some big names, such as Sir. Issac Newton.  As stated the world's greatest Physicist modestly, you see the oceans blue, simply because you are standing on Giants' shoulders.   

 

The white light from the sun is a mixture of all colours of the rainbow.  This was demonstrated by the Sir Isaac Newton, who used a prism to separate the different colours and so form a spectrum.  The colours of light are distinguished by their different wavelengths.  The visible part of the spectrum ranges from red light with a wavelength of about 720 nm, to violet with a wavelength of about 380 nm, with orange, yellow, green, blue and indigo between.  The three different types of colour receptors in the retina of the human eye respond most strongly to red, green and blue wavelengths, giving us our colour vision.

The first steps towards correctly explaining the colour of the sky were taken by John Tyndall in 1859.  He discovered that when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red.  This can be demonstrated by shining a beam of white light through a tank of water with a little milk or soap mixed in.  From the side, the beam can be seen by the blue light it scatters; but the light seen directly from the end is reddened after it has passed through the tank.  The scattered light can also be shown to be polarised using a filter of polarised light, just as the sky appears a deeper blue through polaroid sun glasses.

This is most correctly called the Tyndall effect, but it is more commonly known to physicists as Rayleigh scattering--after Lord Rayleigh, who studied it in more detail a few years later.  He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles.  It follows that blue light is scattered more than red light by a factor of (700/400)^4 ~= 10.

 

Now, if you are smart enough, or you are curious in physics more than enjoying sunshine at beachs of Cotes d'Azure, you might have noticed a word, another beautiful color: Violet. 

Indeed, if shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength.  The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high atmosphere, so there is less violet in the light.  Our eyes are also less sensitive to violet.  That's part of the answer; yet a rainbow shows that there remains a significant amount of visible light coloured indigo and violet beyond the blue.  The rest of the answer to this puzzle lies in the way our vision works.  We have three types of colour receptors, or cones, in our retina.  They are called red, blue and green because they respond most strongly to light at those wavelengths.  As they are stimulated in different proportions, our visual system constructs the colours we see. When we look up at the sky, the red cones respond to the small amount of scattered red light, but also less strongly to orange and yellow wavelengths.  The green cones respond to yellow and the more strongly-scattered green and green-blue wavelengths.  The blue cones are stimulated by colours near blue wavelengths which are very strongly scattered.  If there were no indigo and violet in the spectrum, the sky would appear blue with a slight green tinge.  However, the most strongly scattered indigo and violet wavelengths stimulate the red cones slightly as well as the blue, which is why these colours appear blue with an added red tinge.  The net effect is that the red and green cones are stimulated about equally by the light from the sky, while the blue is stimulated more strongly.  This combination accounts for the pale sky blue colour.  It may not be a coincidence that our vision is adjusted to see the sky as a pure hue.  We have evolved to fit in with our environment; and the ability to separate natural colours most clearly is probably a survival advantage.

 

Sunsets (refer to figure 2: Sunset - Courtesy of Bea's Wonderland)

When the air is clear the sunset will appear yellow, because the light from the sun has passed a long distance through air and some of the blue light has been scattered away.  If the air is polluted with small particles, natural or otherwise, the sunset will be more red.  Sunsets over the sea may also be orange, due to salt particles in the air, which are effective Tyndall scatterers.  The sky around the sun is seen reddened, as well as the light coming directly from the sun.  This is because all light is scattered relatively well through small angles--but blue light is then more likely to be scattered twice or more over the greater distances, leaving the yellow, red and orange colours.

 

Blue Haze and Blue Moon (Refer to figure 3: A blue haze over the mountains of Les Vosges in France)

Clouds and dust haze appear white because they consist of particles larger than the wavelengths of light, which scatter all wavelengths equally (Mie scattering).  But sometimes there might be other particles in the air that are much smaller.  Some mountainous regions are famous for their blue haze.  Aerosols of terpenes from the vegetation react with ozone in the atmosphere to form small particles about 200 nm across, and these particles scatter the blue light.  A forest fire or volcanic eruption may occasionally fill the atmosphere with fine particles of 500-800 nm across, being the right size to scatter red light.  This gives the opposite to the usual Tyndall effect, and may cause the moon to have a blue tinge since the red light has been scattered out.  This is a very rare phenomenon--occurring literally once in a blue moon.

 

OK.  Time for la soutenence de la these: A last quiz to finish this long and tedious thesis off:

Imagine that you are sitting on the Mars Exploration Rover, what color is the sky you observe from the Martian surface?