What's happening up there
- Sunlight holds every colorWhite light is a mix of red through violet
- Air molecules are tinyMuch smaller than the waves of light
- Short waves bounce moreBlue light scatters several times more than red
- Blue comes from every directionSo the whole sky glows blue
The name for it
Rayleigh scattering, after the British physicist Lord Rayleigh, who worked it out in the 1870s.
Why not violet?
The sun sends less violet
Sunlight is weaker at the violet end
Our eyes favor blue
We're far less sensitive to violet
Some violet is absorbed
High in the atmosphere
The mix looks sky blue
Our brain blends what's left
Sunsets and other skies
Red sunsets on Earth
- Low sun shines through much more air
- Most blue is scattered away on the trip
- Reds and oranges are what's left
- Dust and smoke make them brighter
Upside-down skies on Mars
- Fine red dust fills the thin air
- Daytime sky looks butterscotch
- Sunsets on Mars look blue
- Dust scatters differently than air
Remember
Blue sky is sunlight bouncing off the air itself.
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Answer key
- What makes the sky blue?Air molecules scatter blue light more than redTiny air molecules scatter short blue waves of light across the whole sky.
- What is this scattering called?Rayleigh scatteringIt's named for Lord Rayleigh, who explained it in the 1870s.
- Why isn't the sky violet?The sun sends less violet and our eyes favor blueLess violet arrives, some is absorbed and our eyes are less sensitive to it.
- Why are sunsets red and orange?Low sunlight passes through more air, scattering away the blueThe long path through the air removes most blue, leaving reds and oranges.
- What color are sunsets on Mars?BlueFine Martian dust scatters light differently, giving blue sunsets and a butterscotch day sky.