As soon as sunlight reaches Earth’s atmosphere, gases and particles scatter it all around in all directions – blue light being scattered more widely than other hues; hence why the sky appears blue.
Irish scientist John Tyndall first demonstrated this principle in 1859 by passing a beam of white light through a liquid with floating particles; its light turned blue, showing that shorter wavelengths scatter more readily than longer ones.
The Sun
The Sun is our source of illumination in the sky. This sunlight comes in various wavelengths (colors), which when they hit Earth’s atmosphere are scattered by gases and particles in our planet’s air – particularly shorter wavelengths such as violet and blue which get scattered more than longer ones (red). This process of scattered light known as Rayleigh scattering after Lord Rayleigh first described it in 1870’s.
At midday when the Sun is at its highest, our skies appear blue due to shorter wavelengths entering through our atmosphere and reaching our eyes. As our atmosphere (which primarily comprises nitrogen and oxygen molecules) are smaller than the wavelength of light, they “bounce off” of it when passing by, making its journey through our atmosphere even more disorienting and distorting its path. As it passes through us it also gets bent as it travels through it all the way into space. Bending of light waves occurs due to similar principles as those which cause pencil lead to bend when dropped into water from an angle. As light bends, its original path becomes scattered into other directions; since blue wavelengths scatter more heavily than their counterparts, these blue light waves tend to dominate.
At sunset and sunrise, sunlight that reaches our eyes has had to travel a much longer journey through the atmosphere than during a typical day, passing through more layers and encountering larger particles such as smoke or pollution that scatter blue light more than red. When this occurs, sunsets take on their characteristic reddish hue.
The Earth’s Atmosphere
Why does the sky appear blue? Unfortunately, this question doesn’t have a straightforward answer; there are multiple factors at work here that all combine together to produce this perception of blue. These include sunlight containing light of different wavelengths; Earth’s atmosphere scattering them differently; and our eyes being sensitive to them all contributing factors that create our blue hued skies.
As soon as sunlight hits our atmosphere, it is scattered by particles and gases present within it, and is called Rayleigh scattering after its discoverer Lord Rayleigh. Shorter wavelengths, which correspond with our eyes’ most sensitive colors, experience more dramatic scattering; longer wavelengths corresponding to reddish blood can penetrate more readily as there is less Rayleigh scattering involved.
Earth’s atmosphere also absorbs certain wavelengths of light, particularly ultraviolet radiation and some infrared radiation, creating the deep blue hue seen most prominently on clear days and from space. This phenomenon may be explained by ozone being present in its environment – known to absorb UV radiation while simultaneously scattering wavelengths near it such as violet, blue and indigo).
Combining these processes explains why our eyes perceive the sky as blue. Furthermore, light from the Sun penetrating the entire Earth’s atmosphere before reaching us adds further intensity and color to this perception of blueness.
As a result, the sky appears as an intense blue near the horizon and fades into less intense blue near its center. Color variations depend on conditions like haze and dust as well as weather events like fog or rain; furthermore, other planets don’t share our atmospheric conditions so their skies may differ significantly from ours.
The Human Eye
As sunlight reaches Earth’s atmosphere it is scattered by gases and particles in the air. Light waves with shorter wavelengths (blue ones) are more easily bent (refracted), creating what we perceive to be blue skies.
Violet light scatters less widely than blue light, which explains why the sky doesn’t appear purple – there simply isn’t enough violet light present to give the atmosphere vibrant hues.
Human eyes are built to detect short wavelengths of light, so blue hues stand out most strongly when presented to them. If you shine a beam of white light onto a tank of clear water with small particles suspended within it, shorter blue wavelengths will scatter more strongly than longer red wavelengths; this effect causes water to appear blue as well as causing it to appear as part of the sky itself.
Physics have long understood why the sky is blue, yet no clear explanation existed of how human vision works. Now physicists and visual psychologists have collaborated on a paper linking light physics with eye physiology.
It led to a novel explanation for why the sky is blue that goes beyond simply noting the Sun bends more for blue wavelengths than red ones. At its heart is human eyes having two types of colour-sensitive cones which respond differently to different forms of light we encounter; scattered blue wavelengths stimulate these cones most strongly, thus giving sky its signature hue; less-scattered red and yellow wavelengths don’t contribute as significantly and don’t influence its colour as heavily.
As the Sun moves lower in the sky at sunset and into nightfall, more of its light is reflected back to us through atmospheric reflections. This causes more blue light to be scattered out, while red and yellow wavelengths remain hidden – giving rise to a reddish orange appearance of our star.
The Moon
Rayleigh scattering gives the sky its signature blue color; when light passes through the atmosphere it gets scattered in all directions and shorter blue wavelengths scatter more easily than longer, redder wavelengths due to their greater energy and likelihood of being bent (refracted) by air molecules than longer, redder ones.
Light from the Sun takes an extended journey through our atmosphere before finally reaching us; closer to noon or sunset, when more atmosphere exists between it and us, more light has the chance of refracted and scattered causing it to appear bluer than normal.
But the blue of a sunny day isn’t solely caused by Earth’s atmosphere–it also comes from our moon. Because the moon doesn’t have an atmosphere to reflect sunlight back into space, its reflection can vary significantly compared to Earth’s. Moon light reflects almost all visible wavelengths apart from blue and violet wavelengths, explaining why daytime skies appear bluer while at night they take on more yellow or red tones.
The moon plays an integral part in creating seasons and sunrise and sunset scenes that look so lovely. At full, its surface reflects more sunlight that hits it, making its reflection brighter and more colorful; when new, less sunlight hits it directly and its color dims slightly.
If you have ever found yourself wondering why the sky appears blue, keep three things in mind. Sunlight contains all of the colors of the rainbow; our atmosphere contains molecules which scatter wavelengths differently; and finally our eyes can detect and process these different wavelengths, producing an ever-blue sky during daylight hours.