White light contains all of the colors of the rainbow, yet appears blue due to atmospheric scattering of its wavelengths. This phenomenon also accounts for why sunsets and sunrises tend to look red as longer wavelengths travel further through our atmosphere before reaching our eyes.
The Sun’s Light
An astounding blue sky is truly spectacular to behold, yet the mystery behind its creation remains unclear. One theory suggests it could be related to sunlight passing through Earth’s atmosphere: light comes in various wavelengths or colors that get scattered by gas molecules as it travels. Short wavelengths (such as blue) tend to get scattered more than longer wavelengths ( such as red).
As sunlight passes through the atmosphere, its beams spread more and more widely as it travels, giving off a blue tint in the sky. This happens because blue light’s frequency resonates closely with that of electrons found within molecules in air; pushing on these electrons causes them to scatter widely while red light does not have this same effect; hence continuing on its original path.
Other wavelengths of light are absorbed by gases in the atmosphere or bounce off dust particles and water droplets, giving off different shades depending on weather conditions, such as hazy or cloudy days. As a result, skies appear different shades depending on these weather factors.
When the sun is high overhead during the day, its rays must pass through a smaller fraction of atmosphere before reaching your eyes, meaning they are less likely to be scattered and create a bluer sky. Conversely, when low on the horizon at certain times during the day it must pass through more layers before reaching you – increasing its likelihood of scattering and taking on a redder hue than it otherwise might.
Human eyes contain three types of color receptors (or cones), called red, green and blue cones. Each cone responds to different wavelengths of light – red and green receptors respond to yellow/green parts of visible spectrum light while blue/violet receptors react more strongly against shorter wavelengths of blue/violet light – with our brains then combining all these signals into perceiving single colors, usually white or blue in hue.
Rayleigh Scattering occurs when sunlight hits the atmosphere around you, as molecules in our atmosphere scatter and redirect light in many different directions – this gives the sky its characteristic blue hue.
But not all wavelengths scatter equally: shorter ones like blue are scattered more frequently in comparison with red lights; thus more blue light pings around the atmosphere and thus increases your odds of seeing it reach your eyes.
All other light that reaches your eyes indirectly, meaning it has passed through the atmosphere before reaching you, is indirect light that was refracted by atmospheric particles before reaching you and its color is dependent on how strongly this was done – generally, more refracted light tends to appear brighter.
So for instance, when viewing a leaf in the shade it will appear white due to most of the light hitting it being scattered away by shadows; while when seen under sunlight the same leaf will appear green as most of its light has not been scattered away by shadows.
As sunlight passes through the atmosphere, its path bends as its density varies with distance from the sun. A dense atmosphere bends light more strongly as you get further away; giving the sky its characteristic blue hue – particularly in mountainous regions with high air density where dust and droplets exist.
One key reason the sky appears blue is due to our eyes having three types of photosensitive cells (rods and cones) that respond differently to colors: red-yellow photosensitive cells respond more strongly to less scattered wavelengths of visible light; green and blue photosensitive cells respond to those that have been scattered more strongly and therefore stimulate more. All of this combined makes the sky appear blue for us!
The color of the sky can also change due to other atmospheric phenomena. A volcanic eruption might produce ash particles with wavelengths comparable to blue wavelengths of light; as such they could alter its hue to red. More typically though, our atmosphere reflects and refracts light to make our sky appear blue.
Dust and Water Droplets
Light passes through air or water and scatters, altering its path through it. This causes objects to appear different colors from what they would without scattering; for instance, shining a flashlight through milk causes it to appear blue because shorter wavelengths (violet and blue) scatter more strongly than longer ones such as red.
John Tyndall suggested in 1859 that the sky’s hue was caused by dust particles and droplets of water vapor floating through the atmosphere, but later evidence proved otherwise; today it is known that atmospheric gases, specifically oxygen and nitrogen molecules sized perfectly to allow most colors of sunlight through – except blue!
Nitrogen and oxygen molecules have the ability to disperse short wavelengths of blue light more effectively than long wavelengths of red or yellow, meaning blue light gets scattered more than any other hue, giving it an appearance of rich saturation.
Violet light scatters less in the atmosphere due to it being absorbed by molecules higher up, leaving sufficient blue light for skyscape appearance.
As you approach the horizon, the sky becomes paler as less blue light reaches your eyes due to an increase in atmospheric conditions nearer to where you stand – more air, more dust particles in the atmosphere which reduce the efficiency with which blue light scatters across its spectrum than it does other colors of the spectrum.
Have you noticed how during a solar eclipse the sky appears red even though the Sun remains unseen? This is caused by direct sunlight not reaching large regions near you directly due to Moon shadow, turning these regions red instead.
The Human Eye
Human eyes are highly attuned to wavelengths of light and can distinguish a range of hues. When sunlight hits an object, its light can be scattered off in all different directions, creating different reflections and colours. When light reflects off something so as to appear blue-tinged it indicates more wavelengths were scattered than absorbed during absorption.
As light enters Earth’s atmosphere, its molecules scatter it all directions but certain wavelengths more strongly than others – this process is known as Rayleigh Scattering and results in blue skies.
This occurs because shorter wavelengths such as blue and violet scatter more easily than longer red and orange wavelengths, so that if the sky were violet instead of blue, sunlight would appear much brighter because less of its path has to travel through atmospheric layers to reach us.
The sky’s blue hue can also be traced back to how our eyes perceive it. Each eye contains three types of cones that detect light, and these combine together to detect specific wavelengths that create its colors – red and green receptors stimulate blue wavelengths while yellow and indigo receptors have less impact, giving the sky its signature blue with an almost greenish tint.
As the Sun rises and sets, its light must travel through even more atmosphere than it does during the day, which scatters blue wavelengths even more widely across its path than usual. By doing this, long wavelengths of red and orange light reach our eyes more directly resulting in vivid orange-red skies at sunrise and sunset.