A Quantum of Sol

We arrived safely back in Belgium, all of us with a gentle tan (protected from the worst ravages of ultraviolet energy by bottles of various sun creams, particularly the children).  The daily ‘oiling-up’ ritual was, as it is always for me; a complete faff – not being the sort that actually enjoys lying around in the sun on holiday.  However, the lying around in the Sicilian sunshine led me to thinking about the very expression, “Sicilian sunshine”.

Sunshine is just lots and lots of packets of light energy called photons, with each photon carrying a quantum of electromagnetic energy.  The energy, released in the process that keeps the sun glowing, derives from a series of nuclear reactions which, overall, result from the fusion of two hydrogen nuclei (the surrounding electrons having become separated from the atom) into one helium nucleus. The energy released represents the difference beyween the masses of the ingoing hydrogen nuclei and the mass of the outcoming helium nucleus.  The energy released is equivalent to the difference in mass multiplied by the speed of light squared as noted by Einstein’s most quoted equation.

It is a big number as the Sun, with phenomenally high temperatures and pressures at its core, fuses the equivalent of a hundred thousand tonnes of hydrogen per second!  Even at this rate it is estimated that there are still five billion years worth of hydrogen that can be converted in this way before the Sun enters the next phase of its life and starts to fuse the helium that has been produced in this first phase.  When that happens the Sun will enter a phase where its size will grow and its temperature (and hence its colour) will change and it will start to swallow up the rocky planets, including the Earth and Mars.  We should be long gone before then.

Coming back to the idea of Sicilian sunshine, the energy from the Sun reaches us, on our chosen beach, in the form of electromagnetic waves of different energies.  Some of this energy is visible to our eyes, hence we can see; some of it is detected as warmth and some of it is very powerful energy in the from of ultraviolet (UV) light.  It is the UV light that sun tan oils and creams protect us from while our own protection system is stimulated and we turn various shades of brown – although, in my case, red and peeling is the commonest intermediate phase.  The photons that arrive on Earth have been on a long journey.  The intense pressure at the centre of the Sun and the dense packing of the hydrgen nucei mean that it is hard for a photon (the unit of light energy at any given frequency) to get out of the core.  It is constantly reabsorbed and re-emitted as it ‘bounces around’ following a more of less random, although gently outward, path until it can break free of the Sun and head out into space.  Calculations by astrophysicists show that this may take a photon one hundred thousand years.

Imagine that for a moment, the photons that are causing your body to tan were originally produced one hundred thousand years ago and your eyes and your body lying there wherever you are have become their final ‘resting place’ as the absorbed energy triggers a physical sensation, warmth or a chemical response, tanning, in the outer layers of your skin.  The photons have one destination – Sicilian sunshine is exactly that – the photons don’t go anywhere else.

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