Numerous readers have asked why the sky is blue; contrary to popular belief, it’s not because water reflects it, but due to air molecules.
As sunlight passes through the atmosphere it gets scattered by oxygen and nitrogen molecules, creating a condition in which shorter wavelengths (blues) are scattered more than longer wavelengths (reds), creating what we perceive to be a blue sky.
Sky
Myth: the ocean and sky appear blue due to reflection, when in reality they’re blue because of light scattering. Sunlight reaching Earth’s atmosphere is scattered by gases and particles in its atmosphere, known as Rayleigh scattering; shorter wavelengths of blue-violet light are scattered more than longer wavelengths such as red or yellow light causing it to look blue during daylight and orange or red at sunset.
The sky’s blue hue comes from all of the different hues created when sunlight interacts with our Earth’s atmosphere, mixing various hues together into an ever-evolving mosaic. Human eyes can easily detect this range of colors thanks to our three types of cones and monochromatic rods; when sent signals to our brains they interpret as hues that we see.
Sunlight reaching Earth’s atmosphere consists of all of the colors of the rainbow. When this light hits molecules of oxygen and nitrogen in our atmosphere, however, it becomes dispersed through numerous particles like oxygen molecules and nitrogen atoms and scattered further by these tiny particles; typically more frequently taking on blue hues than reds.
As the Sun moves lower into the sky during evening and morning hours, more atmosphere gets in its path, scattering light more widely allowing more red and yellow wavelengths of light to reach our eyes resulting in redder sunsets and oranger sunrises.
Planets like Mars feature blue skies because their atmospheres contain gases and particles similar to our own that scatter light; as an example, Mars features an extremely thin carbon dioxide atmosphere which does not disperse it as easily; hence its sky isn’t blue because of oceans or air; rather it’s due to lack of sufficient air within its layers that allows light scattering.
Ocean
Many people assume the ocean appears blue because its reflection reflects off of the sky, but that isn’t accurate; rather it’s due to water molecules themselves absorbing different wavelengths at various frequencies when exposed to sunlight. When sunlight shines upon it first absorbs red and orange parts of its spectrum before blue violet and ultraviolet parts get scattered by water molecules.
Before understanding why the ocean is blue, it’s crucial to comprehend why the sky is. Sunlight doesn’t contain one single color – rather, it contains all seven hues from the spectrum – red, orange, yellow, green, blue, indigo and violet. Furthermore, sunlight consists of millions of tiny particles with different shapes and sizes which scatter light in different ways; when light passes from air into another medium with differing density such as water it bends due to refraction; thus giving blue light more freedom than any other hues creating what gives sky its signature hue.
Similar processes occur with ocean water, although less obviously. Molecule in its molecules absorb red and orange wavelengths before scattering blue wavelengths that return back to our eyes as “blueness”, making the ocean appear blue to our eyes.
However, the ocean can take on different hues due to microscopic algae growing near its surface and changing color due to cloudiness or sediment accumulation in its depths. If the ocean becomes green due to microscopic blooms growing near its surface or brown if there’s too much sediment present, or even its hue can even change completely!
At sunset and sunrise, as the Sun moves lower in the sky, its light must pass through additional layers of atmosphere before reaching our eyes. As a result, blue and violet wavelengths of light scatter more readily than they would during daytime, leaving room for red and orange wavelengths to shine through – this explains why sunset/rise sky appears redder than daytime skies.
Rain
If you have ever looked up at a clear sky, you will probably have noticed its striking hue – blue! Our atmosphere acts as a prism and makes the sky appear blue; shorter wavelengths like blue are scattered out while longer ones such as red pass straight through. This phenomenon is known as Rayleigh scattering.
Light that passes through the atmosphere also gets bent by its density; this happens because more mass than space exists within it and so denser atmospheric layers become. This causes sunlight from the Sun to travel farther to reach our eyes – meaning when its position on the horizon drops lower it passes closer to Earth surface where atmosphere has more time to scatter away shorter blue wavelengths leaving only longer red and yellow ones unimpeded on their journey to reach you.
The same process explains why the sky is blue during daylight but not sunrise or sunset: as the Sun enters our atmosphere at an increasingly shallower angle, its Rayleigh scattering has less of an effect and more red and orange wavelengths reach our eyes.
Clouds contain small water droplets which serve as effective prisms due to being smaller than air molecules that make up its composition, dispersing all colors equally across their surface and thus appearing white to us.
Water is unique in that it doesn’t absorb or reflect light efficiently; rather, it scatters it effectively, giving us a vivid blue sky during the day and darkening to gray just prior to rainstorms.
Air
There is no logical connection between blue ocean water and blue sky skies; in fact, quite the contrary is true! Ocean colors are determined primarily by temperature and salinity rather than interactions with air molecules; by contrast, light traveling through it tends to scatter short wavelengths (blues) more than long wavelengths (reds); this phenomenon is known as Rayleigh scattering, and occurs regardless of how high up or low you are within its atmosphere.
Looking through a prism or glass of water against a clear blue background makes it easy to observe all the colors present in sunlight entering it, since sun rays contain all colors in their spectrum; when interacted with matter such as glass or water it gets bent, scattered, and reflected back at us in many different ways – thus explaining why skies appear deep blue when viewing through polarised sunglasses.
Sky blue hue is more a result of light scattering than absorption; although our atmosphere absorbs quite a lot of ultraviolet radiation which explains why you won’t witness many sunrise or sunset events on Mars. Mars’ thin atmosphere is similar to our own; as sunlight passes through it, its shorter wavelengths (blues) tend to scatter more easily than their longer red counterparts.
The same phenomenon also explains why water looks blue, whether in a tranquil lake or the stormy sea. When sunlight passes through water molecules, its wavelengths get scattered and this gives it its hue. A similar thing occurs with air containing particles made up of water molecules; such as in a tank of milky water with soap added, since particles much smaller than wavelengths of light scatter blue hues more strongly than red ones; making a clear tank look bluish compared to one without milk added; similarly hazy skies such as those you might see after forest fires also appears blue-toned!