Science · Middle school

Light’s path through the air

Lower the lamp: air scatters blue light far more than red, giving a blue sky at noon and a red Sun at dusk.

In the learning section below the figure, predict, try, complete the tasks and check yourself with short questions.

Fig. 1Light’s path through the air

Drag the lamp along the rail or use the slider

60° · path 1.15×

Height of the Sun60°

Your own value

Short path: blue scatters sideways and paints the sky; the rest of the beam stays nearly white.

Path through the air1.15×1.15 times the overhead path

Colour of the SunSky overhead

Blue (450 nm) arriving78%
Red (700 nm) arriving96%
How much more blue scatters than red (700/450)⁴, the same at every height≈ 5.9×

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

As the Sun gets close to the horizon, what happens to the colour of the light reaching the observer directly?

Bluish haze: light the air scatters sidewaysSpectrum bars: light reaching the observer, wavelength by wavelengthThe numbers come from one model: Rayleigh scattering only (1/λ⁴), a sea-level optical depth of 0.097 at 550 nm, air thinning with an 8 km scale height, the Kasten–Young air-mass formula and a 5778 K solar spectrum. Dust, ozone and clouds are left out. The drawing makes the atmosphere far too thick: most of the air lies within 15 km of the ground, while the Earth’s radius is 6,371 km.

Why is the sky blue? Rayleigh scattering made simpleRead the article