Episode 1: Why Is the Sky Blue (and Sunsets Red)? ☀️

We see the sky every single day, but most of us don’t really think about it. Why is it blue? Why not green, purple, or even colorless, since air is mostly invisible? And why does that same sky sometimes turn into breathtaking shades of red, pink, and orange when the sun sets?

The answers lie in physics, hidden in the way sunlight interacts with the air we breathe.

Sunlight Isn’t Just “White”

What looks like plain white sunlight is actually a mix of all colors of the rainbow. Each color has a different wavelength:

• Red light has the longest wavelength (around 700 nanometers).

• Green light is in the middle (around 550 nanometers).

• Blue and violet light have the shortest wavelengths (around 400- 450 nanometers).

  
  

Our eyes combine all these wavelengths together, and to us, the mixture appears white. But when sunlight hits the atmosphere, those wavelengths don’t all behave the same way.

Scattering: Why Blue Wins

The air around us is filled with molecules of nitrogen and oxygen, much smaller than the wavelength of visible light. When light bumps into these tiny particles, it gets scattered, bouncing around in different directions.

Here’s the trick: short wavelengths (blue and violet) scatter much more strongly than long wavelengths (red and yellow). This is known as Rayleigh scattering. In fact, blue light scatters about 10 times more than red light.

So as sunlight pours through the sky, blue light is scattered everywhere. No matter which direction you look, some of that scattered blue light is reaching your eyes. That’s why the sky appears blue during the day.

But Wait… Why Not Violet?

If violet light scatters even more than blue, shouldn’t the sky look violet instead? There are three reasons it doesn’t:

1. Your eyes aren’t great at seeing violet. Our eyes have photoreceptor cells called cones that are less sensitive to violet wavelengths than to blue.

2. The sun produces less violet light than it does blue.

3. The ozone layer absorbs a chunk of violet light before it even reaches us.

   
   

Together, these factors make blue the clear winner.

The Magic of Sunsets

As the sun sinks lower on the horizon, its light has to travel through much more atmosphere to reach your eyes. By this point, the shorter wavelengths (blue and violet) have been scattered out of your line of sight.

What’s left? The longer wavelengths: glowing reds, oranges, and golds. That’s why sunsets and sunrises are fiery-- you’re seeing the sunlight that survived the long journey through the thickest part of the atmosphere.

And when there are more particles in the air, like dust after a volcanic eruption or pollution in a city, sunsets can become even redder and more dramatic, because more blue light is scattered away.

The Bigger Picture

So, to sum it up:

• The sky is blue because molecules in the atmosphere scatter short wavelengths (blue) more strongly than long ones (red).

• It doesn’t look violet because of how our eyes work, how the sun shines, and how the ozone layer filters light.

• Sunsets turn red because sunlight has to cross more air, removing the blues and letting the reds shine through.

  
  

Now you finally know the science behind the colours every time you look up at the sky.