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Why Is The Sky Blue WORK

The sky is blue due to a phenomenon called Raleigh scattering. This scattering refers to the scattering of electromagnetic radiation (of which light is a form) by particles of a much smaller wavelength. Sunlight is scattered by the particles of the atmosphere, and what comes through down to earth is called diffuse sky radiation, and though only about 1/3rd of light is scattered, the smallest wavelengths of light tend to scatter easier. 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.

why is the sky blue

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. The water molecules absorb all the red wavelengths from the light, making it reflect blue. This is also why shallower waters appear 'less' or lighter blue than deeper ones- less absorption means less reflection.

A clear cloudless day-time sky is blue because molecules in the air scatter blue lightfrom the Sun more than they scatter red light. When we look towards the Sun atsunset, we see red and orange colours because the blue light has been scattered out andaway from the line of sight.

The white light from the Sun is a mixture of all colours of the rainbow. This wasdemonstrated by Isaac Newton, who used a prism to separate the different colours and soform a spectrum. The colours of light are distinguished by their differentwavelengths. The visible part of the spectrum ranges from red light with awavelength 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 colourreceptors in the retina of the human eye respond most strongly to red, green and bluewavelengths, giving us our colour vision.

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

Tyndall and Rayleigh thought that the blue colour of the sky must be due to smallparticles of dust and droplets of water vapour in the atmosphere. Even today, peoplesometimes incorrectly say that this is the case. Later scientists realised that ifthis were true, there would be more variation of sky colour with humidity or hazeconditions than was actually observed, so they supposed correctly that the molecules ofoxygen and nitrogen in the air are sufficient to account for the scattering. Thecase was finally settled by Einstein in 1911, who calculated the detailed formula for thescattering of light from molecules; and this was found to be in agreement withexperiment. He was even able to use the calculation as a further verification ofAvogadro's number when compared with observation. The molecules are able to scatterlight because the electromagnetic field of the light waves induces electric dipole momentsin the molecules.

If shorter wavelengths are scattered most strongly, then there is a puzzle as to whythe sky does not appear violet, the colour with the shortest visible wavelength. Thespectrum of light emission from the Sun is not constant at all wavelengths, andadditionally is absorbed by the high atmosphere, so there is less violet in thelight. 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 colouredindigo and violet beyond the blue. The rest of the answer to this puzzle lies in theway our vision works. We have three types of colour receptors, or cones, in ourretina. They are called red, blue and green because they respond most strongly tolight at those wavelengths. As they are stimulated in different proportions, ourvisual system constructs the colours we see.

When we look up at the sky, the red cones respond to the small amount of scattered redlight, but also less strongly to orange and yellow wavelengths. The green conesrespond 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 stronglyscattered. If there were no indigo and violet in the spectrum, the sky would appearblue with a slight green tinge. But the most strongly scattered indigo andviolet wavelengths stimulate the red cones slightly as well as the blue, which is whythese colours appear blue with an added red tinge. The net effect is that the redand green cones are stimulated about equally by the light from the sky, while the blue isstimulated more strongly. This combination accounts for the pale sky bluecolour. It may not be a coincidence that our vision is adjusted to see the sky as apure hue. We have evolved to fit in with our environment; and the ability toseparate natural colours most clearly is probably a survival advantage.

The Tyndall effect is responsible for some other blue colorations in nature: such asblue eyes, the opalescence of some gem stones, and the colour in the blue jay'swing. The colours can vary according to the size of the scattering particles. When a fluid is near its critical temperature and pressure, tiny density fluctuations areresponsible for a blue coloration known as critical opalescence. People have alsocopied these natural effects by making ornamental glasses impregnated with particles, togive the glass a blue sheen. But not all blue colouring in nature is caused byscattering. Light under the sea is blue because water absorbs longer wavelength oflight through distances over about 20 metres. When viewed from the beach, the sea isalso blue because it reflects the sky, of course. Some birds and butterflies gettheir blue colorations by diffraction effects.

Images sent back from the Viking Mars landers in 1977 and from Pathfinder in 1997showed a red sky seen from the Martian surface. This was due to red iron-rich duststhrown up in the dust storms occurring from time to time on Mars. The colour of theMartian sky will change according to weather conditions. It should be blue when therehave been no recent storms, but it will be darker than the earth's daytime sky because ofMars' thinner atmosphere.

The blueness of the sky is the result of a particular type of scattering called Rayleigh scattering, which refers to the selective scattering of light off of particles that are no bigger than one-tenth the wavelength of the light.

Importantly, Rayleigh scattering is heavily dependent on the wavelength of light, with lower wavelength light being scattered most. 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 400-nanometer light (violet) is 9.4 times greater than the scattering of 700-nm light (red).

Though the atmospheric particles scatter violet more than blue (450-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.

Let's take why the sky appears blue out of the equation for a moment and begin by looking at color. From a physics standpoint, color refers to the wavelengths of visible light leaving an object and striking a sensor, such as a human eye. These wavelengths might be reflected, or scattered, from an external source, or they might emanate from the object itself.

If we were foolish enough to look directly at the sun, we would see all wavelengths, because light would be reaching our eyes directly. That's why the sun and the area around it look white. When we look away from the sun, at the clear sky, we see light mostly from shorter, scattered wavelengths like violet, indigo and blue.

So why doesn't the sky appear violet instead of light blue? The eyes have it. Your peepers perceive color using structures called cones. Your retinas bristle with about 5 million cones each, made up of three types that specialize in seeing different colors [source: Schirber].

Unlike our auditory senses, which can recognize individual instruments in an orchestra, our eyes and brains interpret certain combinations of wavelengths as a single, discrete color. Our visual sense interprets the blue-violet light of the sky as a mixture of blue and white light, and that is why the sky is light blue.

Scattering is stronger with shorter wavelengths. Among visible colors, the blue-violet end of the color spectrum has the shortest wavelengths. So the blue-violet colors are scattered more than the red-orange colors. 041b061a72

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