Rayleigh scattering why is the sky blue

At the risk of disillusioning poets, it is not a pure blue. All other colours are scattered as well but progressively more weakly towards red.

Why is the sky whiter near the horizon? Overhead there are only a few miles of dense atmosphere and sunlight photons are scattered once - if at all. Near the horizon the air path is 10X or more longer. Along it, photons are scattered several times and the reds and greens eventually become as strong as blues to yield white.

The near horizon sky cannot become arbitrarily bright with the extra scattering because beyond a certain atmospheric path length it effectively becomes opaque. In fact the sky often darkens slightly very close to the horizon for this reason. Rayleigh scattering: When scatterering particles are much smaller than the wavelength of light the process is known as Rayleigh scattering after Lord Rayleigh, John William Strutt, - who first described it mathematically.

The scattering is inversely proportional to the fourth power of the wavelength. For example, blue light of nm wavelength is scattered 4.

The wavelength dependence come from the extent of coupling between the frequencies associated with bound electrons within the atoms and the oscillating electric field of the light waves. Coupling increases as the oscillation frequencies get more similar. Rayleigh scattering requires that there be no coherence between the individual scatterers. In dense gases when molecules are closer together this condition is not satisfied and light is predominantly scattered forwards rather than in all directions.

In dense gases and liquids another process can operate, Einstein-Smoluchowski scattering. In the lower atmosphere, tiny oxygen and nitrogen molecules scatter short-wavelength light, such blue and violet light, to a far greater degree than than long-wavelength light, such as red and yellow. In fact, the scattering of nanometer light violet is 9.

Though the atmospheric particles scatter violet more than blue nm light , the sky appears blue, because our eyes are more sensitive to blue light and because some of the violet light is absorbed in the upper atmosphere.

During sunrise or sunset, the sun's light has to pass through more of the atmosphere to reach your eyes. Even more of the blue and violet light gets scattered, allowing the reds and yellows to shine through. These shorter wavelengths correspond to blue hues, hence why when we look at the sky, we see it as blue. At sunset and sunrise, the angle at which sunlight enters the atmosphere is significantly changed, and most of the blue and green shorter wavelengths of light are scattered even before reaching the lower atmosphere, so we see more of the orange and red colours in the sky.

The ocean is not blue because it reflects the sky, though I believed that up until a few years ago. Water actually appears blue due to its absorption of red light.

When light hits water, the water's molecules absorb some of the photons from the light. Everything absorbs at a different wavelength Your green t-shirt absorbs red , and as a result reflects the remaining colours back at a viewer that's why your t-shirt looks green. In shallow bodies of water like a drinking glass light penetrates it completely, as there is not enough water to absorb enough photons, so we see the water as colourless.

In deeper waters however, not all the wavelengths of light can fully penetrate the liquid, as there are too many water molecules in the way of the photons.