"why is the sky blue?" 🅱 🅸 🅾 🅶

One of the first questions a curious child often asks about the natural world is "why is the sky blue?" 

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Yet despite how widespread this question is, there are many misconceptions and incorrect answers bandied about because it reflects the ocean; because oxygen is a blue-colored gas; because sunlight has a blue tint while the right answer is often thoroughly overlooked.

In truth, the reason the sky is blue is because of three simple factors put together: that sunlight is made out of light of many different wavelengths, that Earth's atmosphere is made out of molecules that scatter different-wavelength light in different amounts, and the sensitivity of our eyes. Put these three things together, and a blue sky is inevitable. Here's how it all comes together.

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Scientist John Tyndall:

In 1859 Irish scientist John Tyndall used this information to make the next step, becoming the first to explain why the sky is blue. He shone a beam of white light through a fluid speckled with floating particles. 

When he peered from the side the fluid glowed blue. This became known as the “Tyndall effect”. Tyndall realized this meant the blue light was bouncing or “scattering” the particles in the fluid more than any other color. Since blue light has a short wavelength, he inferred that shorter wavelengths are scattered more than longer ones.

Tyndall’s experiment takes us the final step of the way, showing that blue light scatters more than other colors.

So what if we put all of this together? As sunlight passes through the atmosphere it scatters, bouncing off the molecules of nitrogen and oxygen. 

The shorter wavelengths at the blue end of the spectrum are 10 times more likely to scatter than red light, the longest wavelength. To an observer on Earth, it is like a storm of bouncing blue light waves bombarding the eyeballs; in comparison, the drizzle of red wavelengths hardly registers, so the sky looks blue. 

Combined with blue:

Why not violet which, as the shortest wavelength, scatters the most? The answer lies with our eyes. The scattered light in fact does contain violet light as well as blue. But the eye’s receptors for colored light, the cones, are not as sensitive to violet and when it is combined with blue, they register the latter.

But hang on, why do sunsets appear red? When the sun is low on the horizon, light has to travel much further through the lower atmosphere, encountering bigger particles such as smoke and pollution that make blue light scatter all the more. 

By the time this light reaches your eyes, most of the blue light waves have scattered away. This leaves predominately red light waves to create that beautiful reddish glow.

So the next time you gaze up at a beautiful blue sky, remember the giants who explained why it is so.

The sky is blue due to a phenomenon called Raleigh scattering. This scattering refers to the scattering of electromagnetic radiation in which light is a form by particles of a much smaller wavelength.

 Sunlight is scattered by the particles of the atmosphere, and what comes through down to earth is called diffuse sky radiation, and though only about 1/3rd of light is scattered, the smallest wavelengths of light tend to scatter easier.

 These shorter wavelengths correspond to blue hues, hence why when we look at the sky, we see it as blue. At sunset and sunrise, the angle at which sunlight enters the atmosphere is significantly changed, and most of the blue and green shorter wavelengths of light are scattered even before reaching the lower atmosphere, so we see more of the orange and red colours in the sky.

The ocean is not blue because it reflects the sky, though I believed that up until a few years ago. Water actually appears blue due to its absorption of red light.

When light hits water, the water's molecules absorb some of the photons from the light. Everything absorbs at a different wavelength Your green t-shirt absorbs red, and as a result, reflects the remaining colours back at a viewer that's why your t-shirt looks green.

 In shallow bodies of water like a drinking, glass light penetrates it completely, as there is not enough water to absorb enough photons, so we see the water as colourless. 

In deeper waters however, not all the wavelengths of light can fully penetrate the liquid, as there are too many water molecules in the way of the photons.

 The water molecules absorb all the red wavelengths from the light, making it reflect blue. This is also why shallower waters appear 'less' or lighter blue than deeper ones- less absorption means less reflection.

When the sun is high overhead, the bulk of its rays intercept the atmosphere at nearly vertical angles. Shorter wavelengths of light, such as violet and blue, are more easily absorbed by air molecules than light from longer wavelengths that is, from red, orange, and yellow bands in the spectrum. 

Air molecules then radiate violet and blue light in different directions, saturating the sky. However, the midday sky appears blue, rather than a combination of blue and violet, because our eyes are more sensitive to blue light than to violet light.

When the sun is near the horizon at dawn and dusk, the sun’s rays strike the atmosphere at more-oblique slanted angles, and thus these rays must travel a greater distance through the atmosphere than they would at midday. 

As a result, there are more nitrogen and oxygen molecules and other particles that can block and scatter incoming sunlight. During this long passage, incoming radiation in the shorter blue and violet wavelengths is mostly filtered out, and the influence of these wavelengths over the color of the sky diminishes.

What remains are the longer wavelengths, and some of these rays strike dust and other particles near the horizon, as well as the water droplets that make up clouds, to create the red, orange, and yellow tints we enjoy at sunrise and sunset.