If you look up, the sky appears blue due to Earth’s atmospheric gases and particles scattering blue light more than any other color, an effect known as Rayleigh Scattering.
Violet light has longer wavelengths than blue light, so it doesn’t disperse as quickly, creating reddish-orange tones during sunset and sunrise.
Light Scattering
Why the sky is blue is tied to light scattering. As sunlight passes through the atmosphere it is reflected and scattered by air molecules; as its wavelength decreases so does its intensity of scattering; light with shorter wavelengths (blue) being scattered more than longer wavelengths (red).
Rayleigh scattering occurs when particles in a medium are significantly smaller than the wavelength of light, causing its light to have a distinct blue tint and leaving other colors of its spectrum more scattered than expected – only sunlight will reach us directly as its result of Rayleigh scattering.
Mie scattering occurs when large particles such as dust, smoke, or water droplets exceed air molecules in size and consequently scatter all colors of the rainbow with less red than expected resulting in yellow or green hues being observed.
At its heart, Rayleigh scattering is what gives clear skies their distinctive blue hue. Blue light is scattered more heavily than other wavelengths which allows it to reach us and be absorbed by the retina of our eyeballs; all other wavelengths reach retinas at different levels without being absorbed as readily.
If we were outside Earth’s atmosphere, then the Sun would appear black. Without any light refraction through it, only blue and violet wavelengths from its spectrum would reach our eyes; any other colors emitted at higher angles would appear as reds and yellows.
Reason why our sky appears bluer is because our eyes are more sensitive to blue wavelengths from the Sun than red ones; this helps our bodies regulate temperature and ensure we get enough oxygen, while red wavelengths serve to alert the brain of danger signals so we can respond quickly.
Rayleigh Scattering
As sunlight enters Earth’s atmosphere, it becomes scattered by small air molecules into all directions. The scattering effect is most evident for light with shorter wavelengths – such as blue and violet hues that make up most of sunlight’s spectrum – because these wavelengths scatter more strongly from these air molecules than longer-wavelength lights like red and orange ones do. As such, blue skies result from this effect.
Rayleigh scattering, the process by which light gets scattered by small particles, was first quantified by 19th-century British physicist Lord Rayleigh who first described this process quantitatively. Rayleigh scattering is the dominant form of electromagnetic scattering found in gases like our atmosphere; light bounces off atoms and molecules comprising our environment such as Nitrogen and Oxygen to cause light-wave scattering that causes Rayleigh scattering.
Our atmosphere contains many other elements and particles, such as dust, sand, water vapor and pollen. When light strikes these substances in the atmosphere, it is reflected and scattered back out again, some being absorbed by molecules while some passing through without being absorbed completely. As it scatters around with intensity inversely proportional to fourth power of wavelength; light with shorter wavelengths like blue-violet hues make our skies appear bluer as their intensity is amplified more strongly.
As light passes through our atmosphere, it becomes polarized as molecules can rotate to an extent depending on where they’re situated and their energy resources. If an atmosphere is very cold and dense, its molecules will move inward more often, absorbing more sunlight; conversely if warm temperatures provide plenty of open spaces, more of its light rays will reflect back into space than enter through.
As light travels through the atmosphere, its intensity can become polarized by both direction of travel and wavelength, giving rise to different color hues during sunset and sunrise than it does during daylight hours. When the Sun begins its descent below the horizon, its short blue wavelengths become even more scattered due to Rayleigh scattering; longer wavelengths such as red and orange light do not get scattered as much and thus reach your eyes more readily.
Rayleigh’s Law
On a sunny day, the color of the sky can be explained by Rayleigh Scattering; this process involves scattering sunlight by tiny particles in the atmosphere and is known by its acronym. Blue hued skies result from short wavelengths of blue light being scattered more than longer wavelengths of red light due to being larger in wavelength. These tiny particles in turn cause Rayleigh Scattering effects.
Rayleigh scattering occurs when light passes through air particles and each is absorbed or dispersed by its own electromagnetic field, whose intensity varies with respect to polarization of incident light.
When light strikes a molecule in air, its electromagnetic field changes polarization, making an atom or molecule vibrate and emit radiation. The intensity of vibration depends on wavelength of light; Rayleigh’s law states that intensity of scattered light proportionally relates to fourth power of wavelength – shorter wavelength blue lights have higher intensities and are scattered more.
Other colors of the visible spectrum also become scattered by atmospheric particles, but at lower intensities than blue light. Therefore, only blue and violet wavelengths reach our eyes.
At dawn and dusk, when the Sun is low near the horizon during sunrise or sunset, its light has to travel further through the atmosphere, forcing more blue and violet wavelengths than red or yellow wavelengths to compete for scattering into visible space. As such, its vibrant overhead hue fades as it nears its destination on the horizon.
If our planet were to lose its atmosphere, sunlight would no longer scatter across the sky and appear entirely black – similar to what astronauts experience during their trip from Earth to Moon and back again.
Rayleigh’s Experiment
The blue hue of the sky can be explained by particles in our atmosphere scattering light – specifically blue wavelengths more than longer red ones – more effectively than any longer wavelengths do. Without an atmosphere, our world would look very different.
As Sun rays pass through our atmosphere, they encounter gas molecules which scatter light in all directions, with blue wavelengths scattered more widely than red or orange wavelengths due to refraction or bend by these molecules. When light finally reaches our eyes and we view a blue sky we also experience absorption and scattering changes as altitude does as we move up or down through our atmosphere.
Lord Rayleigh devised an experiment known as the ‘Rayleigh limit’ to demonstrate the effects of light scattering on light. According to this rule, it states that the minimum distance that can be distinguished with naked eye detection is around 0.1 micrometers – this represents an enormous limitation as bacteria typically measure 2 micrometers across.
Lord Rayleigh conducted his Experiment
In 1872, Lord Rayleigh, an English businessman with considerable wealth, conducted experiments at his country estate to explore different physical phenomena. He was particularly intrigued by alternating current’s effect on galvanometers as well as light properties; his studies were inspired by Helmholtz’s book On the Sensations of Tone.
Rayleigh conducted his experiment using two Nicol prisms and a rotating platform, hoping that its rotation would cause incoming light polarization to rotate with it. Although successful in measuring this effect, Rayleigh could not explain it fully.
He could never be sure whether his results were due to rotation of the platform or birefringence in the prisms themselves, since light polarization in Nicol prisms depended on their direction of travel.