Sky: only blue because we see it that way | Photo: Shutterstock

The sky is blue. It could be yellow, green or red. But it's blue. Blame it on the atmosphere and on our eyes.

What would happen if we woke up one day, open the bedroom window, and outside the sun shone in a brown sky? It would be quite surreal. The truth is that the sky is not brown only by chance.

The explanation for the fact that the sky is blue is quite simple. The light emitted by the sun crosses the Earth's atmosphere's filtering system (Rayleigh scattering) and is perceived blue by our vision.

If we let the sunshine pass through a simple magnifying glass or a thick glass, we can observe seven different colors, precisely the ones that are visible in a rainbow: red, orange, yellow, green, blue, purple and violet. These colors are part of the so-called Newton Disc.

Dispersion of light: different colors refract at different angles

But then, how does the sky turn blue? It is quite simple. The atmosphere plays the role of a magnifying glass and filters the sunlight. Or, to be more precise, the particles of water, dust, and gasses, all together, better diffuse the wavelength associated with blue and violet.

Actually, the color of an object is nothing more than the reflection of that object's color wavelength. Of all the colors emitted by sunlight, blue is, so to speak, the "most efficient" to reach the eyes of humans.

Interestingly, as the afternoon progresses into the "golden hour," we begin to observe a more orange or reddish sky. This phenomenon occurs because, as the sun gets lower at eye level, the blue light is cut off from our line of sight.

As a conclusion, we must admit that our retina has its limitations. The chart below shows us that we are only able to see a relatively narrow range of colors. And the ultraviolet and infrared lights get excluded or dramatically restricted.

Electromagnetic Spectrum: our retina only captures light wavelengths in the range of 400–700 nanometres (nm)