Blue

Everyone knows why the sky is blue right?  Rayleigh scattering is where the light of our Star hits the atmosphere and the shorter wavelengths get scattered it all directions while the longer wavelengths pass through relatively untouched.  The thicker the atmosphere the more the scattering occurs so at dawn and dusk, more light gets scattered and less gets through, so the sun appears red or orange and the sky more yellow and green and even orange and red at these times and less blue.

The net effect of this scattering is that the longer wavelengths appear to be coming from the Sun and e shorter wavelengths from everywhere.  The blue light appears to come from everywhere so the sky is blue.  A month ago I started another post, which I have yet to finish, and a thought occurred to me, the more subtle implications of Rayleigh Scattering, shadows and water.

Right this moment I am looking at some shadows and they appear black compared to the fully lit areas, but they are not.  The trouble is that it is in contrast.  The second problem is that most things are made of different colours and this hides the effect I am going to talk about.  I look at the shadow and it is not black.

The best shadow to look at is a shadow on a white substance, because white reflects all the light that hits it, that is why it is white.  A shadow is lacking of light from the Sun, at least a shadow during the day, but it is not lacking the light that is not coming from the Sun or the light that was scattered.  You see what I am talking about on fresh snow as fresh snow reflects 90% of all light, but you can see it on white paper too.  

Shadows are blue.  The light from the sky, from Rayleigh Scatttering is not affected.  If the substance in the shadow absorbs blue light it will appear darker, so ground, out side of snow, appears quite dark.  Shadows that cover up much of the sky also appear very dark too.

Then I had another thought, this was actually the first thought, but since it is positioned second, it sounds better this way, water appears blue, but is known to be colourless.  water is a very good absorber of light.  Water will absorb almost all the light that hits it as long sat the angle that the water hits the water is 70° or more from the zenith.  Everyone looks at sunsets, and sunsets over water are the best, because you can see forever and the sun is unobstructed, and due to Rayleigh Scattering it is such a nice , but also because the light from the setting Sun is reflected on the water turning it red.  The Sun is low so most of the light that hits the water is reflected off it.  Before the sun turns into those romantic colours, most of the light is reflected off the water, blinding the casual observer, because the sun is only twenty degrees of the horizon and most of the sunlight is hitting the water and coming to your eyes.  When the sun is high in the sky, you only get blinded from the waves, which reflect the light that is 20° of that portion of the wave, but in any case that reflected light is mostly going into the water anyways.

Rayleigh Scattering is sending the light in all directions and from all angles.  A portion of the scattered blue light hits water at 20° or less to its surface, just like the water at sunset the clear colourless water appears to have a colour, blue.  

Why are tropical seas so blue? The sand is white and more blue light is reflected with white light, so it appears brighter.  Why does water look green when you look into it?  The longer wavelengths are absorbed more easily, the yellows and the reds, the greens and blues are reflected back off the bottom and green dominates.  

When people say that the water reflects the sky they are right but not for the reasons they think.