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Our star, the Sun, emits electromagnetic waves in the full energy range, that is, from radiation (γ) through X-ray (X) radiation, ultraviolet (UV) radiation, to the range visible to humans, called "light" (Vis), and then further infrared (IR), microwave (MW) and radio frequency (RF). The light recorded by the human eye is in the range of electromagnetic waves from about 400 to 700 nm (1 nanometer is one-billionth of a meter). Recently dying bees have a different eye structure (faceted eye) and see the world differently because their "light" is the range of radiation in the ultraviolet region, i.e. from about 350 nm. Ultraviolet (formerly called extra-violet radiation) is the range of electromagnetic waves from 10 to 400 nm. There are some characteristic sub-ranges due to the effects of their action on living organisms. Almost 97% of this solar radiation in the UV-A range (315-380 nm) reaches the Earth's surface. The ranges of UV-B (280-315 nm) and UV-C (100-280 nm) are only about 3%. UV-A radiation is radiation dangerous to living organisms, although, for example, bees partially see in this area.

On a sunny day, we often wear sunglasses to protect our eyes from UV-A. Keep in mind that the structure of the human eye, especially the cornea and lens, filters this wavelength range almost entirely. There are also other defence mechanisms, recently discovered by biophysicists from the UMCS Institute of Physics, the so-called dynamic molecular "blinds", blocking the penetration of the eye not only by UV radiation but also excessively harmful blue light. Wearing tinted glasses causes the pupil of the eye to dilate. Sudden taking them off in high sunlight may expose us to overexposure of the yellow spot in the eye, which contains the lightest receptors and suppositories and induce minor damage to it. It is known that overexposure to UV-A radiation can lead to opacification of the eye lens, despite the various mechanisms of eye protection described above. Ordinary glass glasses or ordinary glass also fully absorb UV radiation. For example, it is impossible to get a tan in a greenhouse. Scientists studying molecules have to buy very expensive quartz cuvettes that pass ultraviolet to study molecules in this area by spectroscopy.

It should be remembered that the water is fully transmitting UV-A, UV-B and partially UV-C. So you can get tanned even while swimming underwater. Too abundant sunbathing combined with swimming, for example in the sea, can harm us. It should be mentioned here that the colour of our skin depends mainly on the number of dyes it contains - melanin. Melanin synthesis is a defence process of the skin that prevents further damage by, for example, extinguishing free radicals, the formation of which is induced by UV radiation. At this point, it is worth adding that a free radical is an atom or a molecule that contains at least one unpaired electron (a pair creates a chemical bond), so it will gladly accept it from another molecule, changing its properties. We must avoid free radicals, for example, by eating a lot of green plants containing chlorophylls, carotenoids, xanthophylls and flavonoids, which have many electrons that are easy to transfer. In addition, the collagen fibres contained in the skin are also damaged by UV radiation, which accelerates the effects of skin ageing. However, UV radiation, especially from the UV-B area, positively affects the synthesis of vitamin D3 in our skin. Therefore, the golden rule when it comes to UV exposure is the best.

We often use various types of sunscreen lotions or creams containing the so-called filters. These substances contain molecules with the so-called chromophores that absorb electromagnetic waves and quench the above-mentioned free radicals by donating electrons. The question is why many substances, such as water, alcohol or sugar dissolved in water, are colourless, i.e. they do not absorb visible light. It is not enough to have adequate energy levels. There are many of these levels in complex organic molecules. Two more conditions are necessary. First, some electrons need to be ready to move to a higher energy level, and they cannot be electrons forming chemical bonds (as this would normally end up destroying the molecule). Electrons from the so-called double bonds. For example, when a C = C double bond absorbs radiation, electrons are excited. Therefore, this bond is the site responsible for the absorption (and emission) of electromagnetic waves, which is called a chromophore. Chromophores can also exist in metal complexes.

There is also a second important condition: for the light energy to be effectively absorbed, electrons are needed (relatively) free to move around in the molecule. Such quite loosely bound (excess) electrons are, e.g. in long hydrocarbon chains containing a double bond (they are called alkenes). Carotenoids, "carrot", or "tomato" pigments have chains as part of their particles. The number of conjugated double bonds introduces large changes in the spectrum regarding its position on the energy scale. While a light absorption band characterizes a single C = C bond with a maximum at 180 nm (UV-C), multiple conjugated double bond systems are characterized by an absorption maximum at higher wavelengths, even within the limits of visible light (hence the beautiful colours of carrots, tomatoes and pansies). Sunscreen filters contain molecules that absorb electromagnetic waves and have chromophores in their chemical structure. The use of such filters prevents skin burns and the serious consequences of excessive skin degradation. It should be added that UV cut-off filters in creams can be coloured or colourless. Typically yellow compounds absorb UV-A. However, many colourless compounds also absorb this radiation. It all depends on the structure of the molecule and especially on the vicinity of the chromophores with polar groups built into the structure, e.g. with the -OH group.

The light absorption mechanism discussed above is related to the so-called chemical filters. There are also the so-called physical filters such as TiO2 or ZnO, the purpose of which is to reflect or scatter radiation. Surely everyone noticed that stars are visible in the sky on a clear night, and a clear day, they are not visible. How is it possible? We owe it to the atmosphere of the Earth. Our planet is such an extraordinary, wonderful place, not too close and not too far from the Sun. Considering travelling at the speed of light, it is about 500 seconds from here. It is an oasis of life, shielded by a mantle of magnetic field force lines that protects all living creatures from deadly bombardment from space (by the way, the Earth is one of the few planets in the solar system with such a strong permanent magnetic field). Our gaseous atmosphere protects us from the sun's dangerous radiation, and it is thanks to it that we have the effect of a blue sky at noon and reddening as it sets. It is an effect called light scattering on air molecules that is responsible for this phenomenon. Blue light scatters many times more than red light on particles in the atmosphere. The above-mentioned diffusing filters work similarly.

The blue sky and scorching sun make it a good idea to take shelter under a tree on hot days. Plant leaves absorb ultraviolet light, converting solar energy, carbon dioxide and water into molecular oxygen and biomass. We call this the most important chemical reaction on our planet, photosynthesis. The chlorophyll in the leaves is green - it diffuses green light and absorbs UV, blue and red light. Chlorophyll is, therefore, also a reflection filter. The two chlorophyll absorption areas are clearly separated, and the green colour is almost completely transmitted through the leaves. Biologically, these seemingly strange areas. However, there is a reason for this structure - absorption bands are a manifestation of evolutionary adjustment. There would be so much green light in the solar spectrum that the photosynthetic apparatus could be damaged if they absorbed green light. On the other hand, the absorption of sunlight at the ends of the visible spectrum, where the relative intensities are weaker, is a defence mechanism for plants. If we used a thermometer and measured the temperature of, e.g. a concrete pavement, asphalt surface, car bonnet surface and compared with the surface temperature of a green lawn and tree leaves exposed to the same sun, it would turn out that the leaf and the lawn will be the coolest while touching the car bonnet or asphalt, we can even get burned. Thanks to the reflection of long-wave light, plants can cope with thermal radiation. At the same time, we live in concrete cities, where there are very few trees, and we are exposed to radiation that significantly raises the temperature. Considering only the protection of the environment in which we live, let us plant trees, especially honey-bearing trees, as they significantly increase the ecosystem's biodiversity.

I suggest spending a sunny, hot day under the linden tree, sipping chilled water with honey and lemon, watching the bees sipping nectar from its flowers and waiting for honey picking.

However, that is a completely different story ...

Sources:

Encyclopedia of Contemporary Physics PWN (Warsaw 1983), Editorial Committee headed by prof. Andrzej Kajetan Wróblewski.

M. Gagoś, G. Karwasz, Color and the chemical compound structure, Chemistry at school, 3/2012, 14-25. http://dydaktyka.fizyka.umk.pl/Publikacje_2012/Chemia_1strona.pdf

G. Karwasz, M. Gagoś, Once more about cabbage juice, or colours in chemistry, biology and art education, Foton 120, Spring 2013 http://dydaktyka.fizyka.umk.pl/Publikacje_2013/Foton_GK_2013.pdf

R. Luchowski et al., J. Phys. Chem. B 2021, 125, 23, 6090-6102 https://pubs.acs.org/doi/10.1021/acs.jpcb.1c01198