Why the sky is blue
Show on a big screenThe sky glows blue from every direction. Nothing up there looks like a lamp. Light travels as waves. The length of one wave is its wavelength. Blue light has short waves. Red light has long waves.
- Sunlight crosses the air in a straight line.
- Short waves scatter sideways, every way at once.
Loading lab…
Tap a place. Watch what the air scatters.
The rule
To scatter is to send light off its path in many directions. Air molecules and fine dust are smaller than the wavelength of light. They scatter short waves best.
Air scatters short waves like blue more than long waves like red. The scattered blue light makes the sky look blue. With no air, the sky would look dark.
An astronaut high above the air sees a dark sky. There is no air up there to scatter light. Air and fine smoke scatter red light the least. So red light passes through them most easily.
Fine particles scatter light and make a beam visible. This is the Tyndall effect. A torch beam in a dusty room shows it.
The air also bends light
A star is a tiny point of light. The air keeps moving, so the bending of starlight keeps changing. The star seems to move and flicker. That is twinkling.
A planet is closer than a star. It shows a small disc, not a point. A disc is many points together. The flickers of those points cancel each other. A planet usually shines steady.
Watch out!
The sky is not blue because of the sea. It is blue far from any sea. Air itself scatters the blue light.
Try the lab to finish this lesson.