Sky Colors

Grecia P. Butler Pérez

Since, I was a little girl, I question myself, why the sky is blue?  I pass a lot of time looking the sky and all the beautiful colors in it.  It has an intense yellow and a shiny orange when the sun comes down and a baby blue and a dark blue when a sunny day is or the rain is coming.  This situation was interesting for me and I decide to investigate to found an answer for my question.  The important part is that the colors of the sky have a physical explanation.  The sky is like a prism; it reflects colors.

The beauty of the sky is the result of the interaction between the sun light and the atmosphere.  It’s necessary the damp and the particles of dust to have an impressive fest of colors in the sky.  The most seen color is the blue.  The presence of this color is like when a sun ray passes through a prism.  When we have this situation, the prism refracts the sun ray in five colors: purple, blue, green, yellow and red.  The color rays with less wave length are purple and blue.  The other three rays have long wave length.  The short wave length colors have more deviation.   These two colors with more deviation collide with air particles and it trajectory changes, when this happen the two colors collide with another air particle.  The collisions of the purple and the blue rays with the air particles make it travel all the sky and then reach our eyes.  When the rays reach our eyes we see the blue color in the sky.  This is a cyclic process; it is happening all the time.  This energy diffusion process is called Rayleigh diffusion.  The blue sky color is a result of the diffusion of the wave with short length

The color with less wave length is the purple color.  If we analyze this situation, the sky is supposed to be purple.  The sky isn’t blue for two reasons.  The first one is, the sun rays have more blue than purple and the second one is the human eye is more sensible to blue light.  For these two reasons the color of the sky is the blue.  If the particles are big the Rayleigh diffusion doesn’t work.  In this case the process needed is the Mie diffusion.  For this diffusion the sky isn’t blue.  When the clouds are too thick the sky turns gray.  This is the situation in rainy days.  When we see the sun coming down the sky turns orange and red.  This situation is because the rays of light moves more and the colors with short wave length are capture in the particles.  Only the red rays survive because it travels in rectilinear movement.  If the earth haven’t had atmosphere the sun light would reach our eyes directly without diffusion and we would see the sky black.  The astronauts watch this effect in space.  With this effect they can see the planets, stars and the moon, because there is no atmosphere.

Now I understand the color and appearance of the sky, because it has a physics explanation.  Isn’t a simple act of being blue, it is a relation between particles and light rays.