Interactive Primary Newsletter 34 Nothin' but blue sky....

Why is the sky blue? Sunlight is made up of all the colours of the rainbow (Fig. 1 – picture of rainbow) plus a few that we can’t see (infrared and ultraviolet). We can sense infrared when we feel the warmth of the sun and experience the burning or tanning effect of ultraviolet. Light can be described as being like waves – like those you see on water but much, much smaller. So why is the sky blue? When sunlight shines through the Earth’s atmosphere the blue light is scattered sideways by air particles. When you look up at the sky, you see this blue light everywhere. Therefore the sky is blue. Figure 1 - Rainbow over Fife The scattering of light can be demonstrated in the classroom by shining white light from a torch at a sheet of greaseproof paper. From the front or the back the greaseproof paper emits light (Figs. 2 & 3).            Figure 2 - Demonstration of scattering of light                          Figure 3 - Demonstration of scattering of light (light emitted from front)                                                                 (light emitted from back) This happens because the rough surface of the paper scatters light in all directions. In nature, the scattering of light can be observed when a shaft of sunlight breaks through clouds (crepuscular rays). We can see these shafts of light because a column of air is lit by the sun and a small portion of the sun’s light is scattered towards us by air particles (Fig. 4). Figure 4 - The sun’s light is scattered towards us by air particles Why is the sun red at sunset?