by Brian Denison

Some time ago Judi (Barton) asked me why plants look different at Twilight, so here is my definitive answer which I thought I would share with you. It’s all about the composition of light and light scattering.

Light is simply a collection of electromagnetic waves in the visible spectrum. Violet/blue light have the shortest wavelengths and red light the longest. Green, yellow, and orange are in the middle. These rainbow colours are shown in Fig 1.

When a ray of light travels through the atmosphere some of the colours are scattered out of the beam by air molecules and airborne particles, changing the final colour of the beam. At sunset when the path through the atmosphere is much longer, the violet, blue and green components are removed almost completely leaving the longer wavelength orange and red hues (fig 2).

In other words, light may be white or yellow at midday, gradually turning redder as it approaches sunset due to light scattering.

Evening sun is lovely, but twilight turns a garden into a place of enchantment. Colours that dazzled disappear and paler tones come to life. This is the time when white, pale yellow and light pink create really pleasing effects.

We also owe some more of our most spectacular phenomena to light scattering – not just the red of sunset, but also the blue of the sky, the white of clouds and not forgetting rainbows.

The sky is blue: we all know that, but why? Air molecules, like oxygen and nitrogen for example, are small and thus more effective at scattering shorter wavelengths of light (blue and violet). The selective scattering by air molecules is responsible for producing our blue skies on a clear sunny day (Fig 3).

But why is the sky not indigo or violet (shorter wavelengths than blue). This is because the sun emits a higher concentration of blue light waves in comparison to violet or indigo. Furthermore, our eyes are more sensitive to blue rather than violet hence to us the sky appears blue.

So why are clouds white? Cloud droplets have diameters of 20 microns or so (larger than gas molecules) and are able to scatter all visible wavelengths more or less equally. This means that almost all the light which enters clouds will be scattered. Because all wavelengths are scattered, clouds appear to be white. When clouds become very deep, less and less of the incoming light makes it through to the bottom of the cloud, giving these clouds a darker appearance.

Lastly rainbows (Fig 4), but refraction rather than scattering is the mechanism. Most raindrops are spherical, and it is this spherical shape and their size that provides the conditions for a rainbow to be seen.  Light passing from air to a raindrop at an angle slows and changes direction, in a process called refraction (fig 5). Refraction splits the light into the colours of a rainbow just like a prism. The sun’s position (angle) and that of the observer are particularly important, which is why rainbows are not seen so often.

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I hope you have enjoyed the science!

Brian Denison