Why Is the Sky Blue?

Overview

The sky appears blue because white sunlight interacts with Earth's atmosphere. Tiny gas molecules, primarily nitrogen and oxygen, scatter the short-wavelength blue light in every direction more effectively than longer wavelengths. This phenomenon, combined with human eyes being more sensitive to blue, creates the optical illusion of a blue sky.

In this video

• The massive blue canopy of the sky is actually a massive optical illusion.
• Sunlight is pure, blinding white, but it is a combination of every color of the rainbow.
• In 1666, Isaac Newton proved that white light can be fanned out into red, orange, yellow, green, blue, and violet using a glass prism.
• Every color of light travels as a wave; red light has a very long wavelength, while blue light has a short, choppy wavelength.
• Earth's atmosphere acts as a dense, chaotic obstacle course packed with countless billions of tiny gas molecules, primarily nitrogen and oxygen.
• British scientist Lord Rayleigh discovered that short-wavelength blue light slams directly into nitrogen and oxygen molecules and scatters in every possible direction.
• The phenomenon where blue light bounces off atmospheric molecules and scatters is literally called Rayleigh scattering.
• Although violet light waves are shorter than blue, the sky does not appear purple because the sun produces more blue light, and human eyes are highly sensitive to blue but notoriously terrible at picking up violet.
• At sunset, the sky turns fiery red and orange because the sunlight travels through significantly more air, causing almost all the short blue waves to scatter away.
• The sky essentially acts as a massive prism, constantly filtering out short waves and directing them into our extremely blue-biased human eyes.

Frequently asked questions

What is the true color of sunlight?

Sunlight is pure, blinding white light. However, white light is actually a combination of every color of the rainbow, as demonstrated by passing it through a prism.

Why does blue light scatter more than other colors in the atmosphere?

Blue light has a short, choppy wavelength, which causes it to slam directly into the tiny nitrogen and oxygen molecules in Earth's atmosphere. This collision makes blue light scatter, bouncing off in every possible direction, a phenomenon known as Rayleigh scattering.

What role do nitrogen and oxygen play in the sky's color?

Nitrogen and oxygen molecules make up the majority of Earth's atmosphere, forming a dense obstacle course. When short-wavelength blue light from the sun enters the atmosphere, it collides with these tiny molecules and scatters, painting the sky blue.

Why isn't the sky violet if violet light has shorter wavelengths than blue?

While violet light waves are even shorter and scatter significantly, two factors prevent a purple sky: the sun produces more blue light than violet, and human eyes are biologically biased to be highly sensitive to blue while being poor at detecting violet.

Why does the sky appear red and orange during sunset?

At sunset, sunlight enters the atmosphere at a sharp angle and travels through significantly more air to reach our eyes. By the end of this longer journey, almost all the short blue waves have scattered away, leaving only the long, lazy red and orange waves to be seen.