Light in nature

Sunsets, rainbows, shades varying from blue to green in the sea and the immense range of colours of plants and animals are just a few examples of the rich variety of optical phenomena in nature. Our first experiences of light and colour are through what we see in the natural world; the wonder and beauty of natural optics is everywhere. Let’s take a look at some examples of natural phenomena that exist thanks to light…
Chlorophyll photosynthesis
“Life is air woven with light” – Jacob Maleschott
The process that converts the energy of solar radiation into chemical energy from carbon dioxide and water is called chlorophyll photosynthesis. Through the process of photosynthesis, green plants synthesise the organic substances (carbohydrates) they need for their survival. They are therefore called autotrophic organisms. All other organisms that are unable to complete the process of photosynthesis, such as animals and humans, are called heterotrophic organisms. Heterotrophic organisms can only synthesise the molecules that make up their bodies from the organic molecules contained in food (plant or animal). Plant species are therefore a fundamental element in the natural balance and in the life of other living species. A seemingly simple process, photosynthesis is actually very complex and is the basis from which all our food and many important resources we use, such as fossil fuels, come. Suffice it to say that 97% of biomass is represented by plants, which can support the remaining 3%, made up of all other living beings, including humans.
Sunrises and sunsets
At sunrise and sunset, solar radiation arrives at the ground at a very low angle of incidence, so the sun’s rays have to pass through a thick layer of the atmosphere, particularly its lower layers, which are richer in dust and moisture: these scatter longer wavelength radiation in greater quantities, and so the sky appears red or orange. The greater the amount of atmospheric dust, the more ‘colourful’ the sunsets and sunrises will be.
A rainbow is a phenomenon that has always fascinated mankind, on the one hand because it generally marks the imminent end of a storm, and on the other hand because of the spectacle offered by its colours. Myths and legends have flourished about rainbows: while for the Greeks it was the visible manifestation of the messenger of the gods, according to Nordic legends a magic pot containing a fabulous treasure is buried at the end of the rainbow.
In reality, this is a simple optical phenomenon, due to the refraction that white light undergoes as it passes through water droplets: similar to light passing through a prism, sunlight passing through a water droplet is refracted and broken down into its different wavelengths. Unlike a prism, through which we can see all the wavelengths of the visible spectrum at once, we can only see one colour, one wavelength per drop, depending on the height above us and the angle at which the light refracted by the drop hits our eye. Sunlight hitting millions of microscopic water droplets at the same time therefore undergoes refraction through each of them (in fact, the light undergoes a double refraction, entering the droplet and leaving it). The result is that we see a series of concentric arcs drawn in the sky in bands of different colours. The colours are those that derive from the breakdown of the visible spectrum into different wavelengths and always appear in a precise order (depending on wavelength) from violet in the innermost arc, to blue, green, yellow, orange and red in the outermost arc.
The Sun emits a stream of charged particles into space, which can be attracted to the Earth’s magnetic poles. These charged particles interact with molecules in the Earth’s upper atmosphere, creating the wonderful spectacle of the aurora. Those observed at the North Pole are called northern lights, while those visible towards the South Pole are called southern lights.

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