As soon as sunlight hits our atmosphere, its rays scatter, creating what is known as Rayleigh scattering, named for 19th century physicist Lord Rayleigh. Shorter wavelength light such as blue and violet scatters more than long wavelength light; hence why our sky appears bluer. This phenomenon was named for Lord Rayleigh’s 19th century experiment on this phenomena.
Nearly everyone enjoys gazing upon the breathtaking blue sky on a clear day, yet physicists do not always agree on its cause.
Sunlight
Sunlight is the main factor that gives our skies their signature hue of blue. When light enters our atmosphere it scatters and spreads out, creating less intense lighting that has various shades and tints of hue scattered by atmospheric molecules. This process gives rise to its characteristic blue tint since shorter wavelengths (blue and violet) tend to get scattered more heavily by these processes than longer ones (white light). Without all this happening our eyes would only see pure white sunlight that hits us directly from above.
Physicists refer to it as the Rayleigh effect, named after Lord Rayleigh in 1871, as being responsible for why the sky appears blue. Lord Rayleigh discovered that when molecules in air are much smaller than the wavelengths of light they scatter, shorter wavelengths such as blue and violet tend to get scattered more than longer ones that have less likelihood of scattering.
if this were all taking place simultaneously, the sky would be an ever-shifting mixture of all of the colors that make up white light – violet and blue hues would mix with green, yellow, orange, and red light; our brain will interpret these signals from our eyes as being blue tinged.
People often argue that, without our pollution of the planet, there would be beautiful blue skies every day – however this should not be used as an excuse not to reduce greenhouse gas emissions as best we can.
At sunrise and sunset, when the sun is low in the sky, its light must travel further through our atmosphere before reaching you – this means more blue and violet light may be scattered away from where it should reach, while red and yellow wavelengths continue their way directly toward your eyes – so skies tend to appear redder at these times of the day.
Humans are unique among animals in possessing three different cones (for detecting color) in their eyes; all other creatures, such as most plants and aquatic life forms, only possess two cones – with those having trouble distinguishing between blue and violet seeing all sky as being blue.
Atmosphere
An atmosphere is the thin layer of gaseous substances surrounding Earth, consisting mostly of nitrogen and oxygen molecules. Light traveling through this atmosphere interacts with these molecules and becomes scattered – with blue-violet light being scattered more than other colors, giving the sky its characteristic Carolina blue hue.
As the sun rises and sets, its brightness changes the sky depending on where you stand on earth. Closer to the horizon, lighter is the sky because light must travel further through more layers of atmosphere before being scattered by scattering particles.
Red, orange and yellow wavelengths pass straight through the atmosphere while shorter blue wavelengths are scattered more by atmospheric molecules due to them more likely colliding with blue than with red or orange wavelengths – meaning blue light becomes mixed with other hues to produce what we perceive as blue skies.
Light from the sun passes through our atmosphere and is scattered by Rayleigh scattering, similar to how billiard balls bounce off each other when they collide – except photons of light and air molecules are much smaller particles than their counterparts in billiard balls.
Rayleigh scattering is responsible for giving the sky its characteristic hue. When photons of blue light hit an atmospheric molecule and get dispersed into multiple directions, creating an optical illusion that light is emitting from every point in space.
Atmospheric conditions also cause sunlight to appear red near sunset and sunrise due to absorption and scattering of short wavelengths of blue and green light by molecules in the atmosphere, leaving longer wavelengths such as red and orange to reach earth’s surface.
At its zenith, the sky becomes darker because more atmosphere must pass before reaching its destination; higher up you go in the atmosphere, the less blue the sky appears as there are fewer molecules scattering light at these elevations compared to lower ones; should this continue, eventually the gases would dissipate into space and turn black as their molecules dispersed into space.
Human Eyes
As light passes through the atmosphere, it gets scattered by nitrogen and oxygen molecules, and this effect is most pronounced for wavelengths shorter than visible light spectrum – so blue and violet colors tend to spread more than reds or greens, creating the blue sky effect without actually having molecules of that hue in its atmosphere.
Noting the important point that when we see something, our brain quickly converts it to its corresponding color. Our eyes detect different wavelengths of visible light which strikes them to create this perception of color – for instance red paint may look black under certain lighting conditions!
Eyes work like cameras in that they can show us what would appear if we looked directly at something; in this instance, the sky would appear deep violet.
Human eyes do not contain blue pigment, yet have an overall bluish tint due to Rayleigh scattering. Over time, their color can change, though transplanting an entire eye between people may prove challenging due to how its nerve fibers attach directly to its eyeball.
Children often ask their parents why the sky is blue. A standard explanation in school is that it is due to short wavelengths of Sunlight being more easily scattered by air molecules than longer ones such as reds and yellows; additionally, our eyes are more sensitive to blue light than violet, so we perceive the sky as being predominantly blue.
As the Sun sets low on the horizon, its light has to travel further through Earth’s atmosphere before reaching our eyes, often mixing in other colors as it passes. Thus, less blue light reaches us directly.
Other Animals
The blue of the sky provides comfort to many. It brings back memories of simpler times when life was less complex and brings nature into our daily lives anew, reminding us to be present in each moment and enjoying each experience fully. Therefore, looking up into its depths has the ability to bring us out of ourselves and back into reality.
But the sky may not appear blue to all animals. Some possess only two kinds of cones instead of three (dichromatic vs trichromatic), enabling them to detect wavelengths invisible to us humans; honeybees for instance can detect ultraviolet light which reflects off certain plant surfaces and appears blue when seen by us, while insects and amphibians alike can see infrared lights which our eyes cannot detect.
The color of the sky depends on a range of factors including humidity, particulate matter and pollution in the air. Tropical countries tend to have much more humid atmospheres than Australia and thus will typically feature darker skies. Polluted cities and bushfires can have an equally profound impact; when air quality remains clean and clear however, its hue becomes profound blue.
Astronauts who have witnessed Earth from space have observed its blue hue, caused by nitrogen and oxygen molecules present in its atmosphere. By comparison, however, viewing the Moon from Earth reveals its lack of atmosphere; hence it appears black when seen from our planet. When touched by various objects it reflects their colours back onto itself – for instance water will reflect blue-green while land reflects brown or white shades.
Leonardo da Vinci attempted to explain the color of the sky in his notebooks; he wrote that its apparent Carolina Blue hue resulted from sunlight being scattered by air molecules surrounding it, a phenomenon called Rayleigh Scattering named for 19th Century British physicist Lord Rayleigh who discovered Argon, wrote books on sound still used today, and helped jump-start quantum mechanics research.