Why is the Sky Blue and Not Violet?

The sky appears blue due to light photons with wavelengths other than violet and indigo being scattered by air molecules, creating the blue hue we recognize today.

Rayleigh Scattering refers to this phenomenon, named for Lord Rayleigh who lived during the 19th-century in Britain and studied physics.

Rayleigh Scattering

Light passing through molecules of air is scattered by their interactions, with the amount that scatters varying depending on its wavelength. This phenomenon, known as Rayleigh Scattering, was first described by Lord Rayleigh who demonstrated that its magnitude is directly proportional to fourth power of wavelength; so blue light scatters more strongly than red.

The sky appears blue because shorter wavelengths of blue light scatter more effectively than longer wavelengths, known as strong wavelength dependence of scattering (l-4), thus giving it its characteristic hue of blue instead of violet or indigo.

Another factor contributing to why the sky appears blue rather than violet is our eyes’ greater sensitivity to blue light versus violet, meaning more of its sunlight reaches us and is absorbed before being reflected back through the atmosphere and back out again.

Scattering happens regularly and contributes to why the sky looks blue. Other contributing factors are clouds and dust haze – both created by particles larger than wavelengths of light that scatter all colors equally (Mie scattering).

Sometimes these smaller particles are so small that they only scatter blue and violet lights; for example if there is a forest fire or volcanic eruption nearby, their fine particles of smoke and ash would scatter blue light instead.

As atmospheric path lengths cause photons to scatter more as they travel through the atmosphere, sky tends to appear most vibrant overhead and gradually fade to pale nearer the horizon.

Note that atmospheric pathways are significantly longer compared to their distance from Earth to sky, due to being comprised of hydrogen and oxygen molecules in the air.

Rayleigh Absorption

As you step outside at night or on a bright sunny day, you might notice that the sky looks blue. This hue comes from sunlight being scattered into individual colors by small particles in the air – this process was named Rayleigh Absorption by British physicist Lord Rayleigh (John William Strutt) back in 1899.

Light found in our atmosphere consists of many colors including violet, blue, green, yellow and red light. Violet and blue light have shorter wavelengths that tend to get scattered more by microscopic air molecules than their longer-wavelength cousins – yellow and red light are more stable against this effect.

Scattering occurs because air molecules are 1,000x smaller than visible light wavelengths and thus possess very small repulsion and interaction angles, scattering shorter wavelengths more effectively than longer ones.

As a result, shorter wavelengths of light scatter much more than their longer counterparts and so the sky appears bluer instead of redder. Indeed, its intense blueness can even cause sunsets to look red!

There are two primary causes for the shift. First, blue light wavelengths shift downward. Furthermore, redness from the Sun’s spectrum also decreases.

Repulsion between air molecules and sunlight results in blue and violet light being scattered more than red or yellow light, mostly as a result of its interactions with randomly located air molecules.

Light is also reflected by Earth’s surface and clouds; clouds being composed of water droplets with larger repulsion/interaction angles than air molecules and hence scatter sunlight very effectively at all visible wavelengths resulting in brighter skies overhead while gradually becoming paler nearer the horizon.

Scattering of light waves is also directly proportional to their fourth power of wavelength; therefore, blue and violet waves scatter more than longer wavelengths from red and yellow lights, thus shifting down their sun’s spectral curve in these parts of the spectrum more so. Thus creating the appearance of blue skies!

Cones

Consider what is around you – Christmas trees, carrots, party hats, ice-cream cones and traffic cones (used as road-dividers). Cones are one of the few three-dimensional shapes with only one flat face – and boast one vertex or point of interest at their tops!

Rayleigh scattering, which occurs when light passes through air, gives rise to blue skies. This process involves different wavelengths of light being scattered by particles in the atmosphere in various ways causing them to bend into one another resulting in what we perceive as blue light being made up of multiple colors of the rainbow spectrum.

Skywatchers may also see something called refractive refraction taking place, in which light bounces off molecules in the air before being bent more as it travels from air to water or back again. Refraction occurs when light from outside sources is reflected off water molecules before it’s absorbed back by air molecules and eventually returns back through.

Utilizing the Rayleigh scattering model, physicists have shown that blue light rays reflect off more air molecules than its red and green counterparts; this additional bending or refraction contributes to keeping our sky from looking purple!

Conversely, if violet light rays were to reflect off of more air molecules than red and green light waves do, we would perceive the sky as purple due to increased refraction allowing more of its rays into our eyes more readily.

Human retinas contain many photoreceptors that detect different colors; each photoreceptor specializes in sensing different hues. Rods are the most sensitive photoreceptors, though they cannot see details very well; therefore they’re better for general vision than detail-focused sight.

Human eyes contain both rods and cones – more specialized photoreceptors which detect green, red and blue wavelengths – but also possess more sensitive photoreceptors that detect additional hues like violet. Their signals reach our brains which in turn interpret those messages into color perception.

Physiology

Physiology is the study of how your body works, from its basic functions and metabolism to adaptations that enable us to meet physical challenges. Doctors and physicists use tests such as urine or blood tests or electrocardiograms to investigate how organs and systems in your body operate.

Human physiology encompasses many subdisciplines, each with their own area of specialization. Some physiologists specialize in studying single proteins or cells while others conduct research into how different cell types work together to form tissues, organs and systems in the body. Furthermore, physiologists explore how organisms have developed over time as they adapt to their environment.

A degree in physiology can open doors to various career opportunities, from medical research to lab technician and research assistant positions in the field. Students graduating with a bachelor’s in physiology possess the foundation needed for further academic study – including master’s or doctoral programs.

Most physiologists begin their careers by earning their bachelor’s in physiology, biology, physics or math before enrolling in graduate school to earn their master’s. Graduate studies provide invaluable experience for faculty positions as well as publishing their research in academic journals.

In the 19th century, modern physiology first made its debut, employing chemical, physical and anatomical methods for study. France was home to many pioneers including Claude Bernard. However, Johannes Muller, Justus von Liebig and Carl Ludwig were prominent players from Germany who contributed significantly to this field of knowledge.

Physiologists today still follow an evidence-based approach, using experimental and observational methods to study body processes. They often combine analytical with integrative methodologies in order to better discover new problems or answer fundamental questions about organism functioning.

Some physiologists specialize in one system or part thereof, such as the respiratory or nervous systems, studying their workings and how organs differ between functions; and also how bodies behave during normal and abnormal states.

Exercise physiology is another area of physiology with significant applications to sports medicine, where understanding how the body reacts to physical challenges can help elite athletes improve performance, avoid injuries, and recover more quickly. According to the Bureau of Labor Statistics’ estimates of demand for exercise physiologists in the US alone, you could find work as either an assistant coach or researcher working directly with athletes on understanding how their bodies can best function when competing.