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Robert Andrew
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With Sunlight, What You See is Not What You Get

12 February 2014, 10:25

Sometimes there is a tendency for us to take the Sun for granted: it’s just there and provides us with light.

Light, or more correctly, visible light, is actually a very small part of the electromagnetic spectrum that emanates from the Sun. Physicists define light as a stream of photons, which can behave as either particles or waves. All of the electromagnetic radiation emitted by the Sun travels at a speed of about 300,000 km per second and can circle the globe more than 7 times per second. With this incredibly fast speed, one might suppose that sunlight reaches us almost instantaneously. This is not true. Because of the large distance from the Sun to the Earth, it actually takes light 8 minutes to travel from the surface of the Sun to the Earth.

Visible light is made up of different frequencies (often referred to as the ‘colours of the spectrum’) from violet to red, in a small band of wavelengths, ranging from 400 billionths of a metre to 700 billionths of a metre. We cannot see the other parts of the electromagnetic spectrum, like Gamma rays, X-rays and ultraviolet rays, which have much smaller wavelengths than visible light and infrared, microwave, short radio waves and long radio waves with wavelengths up to hundreds and thousands of metres long.

Although we can only see visible light, we are aware of the effect of ultraviolet radiation from the heat we feel, and the tans we get from sitting in the sun.  Microwaves at high intensity can be sensed as warmth as well, since like food, our skin can be heated by microwaves, but the effect on us is very small. Fortunately, highly damaging X-ray and gamma rays from the Sun are blocked by the atmosphere.

It’s somewhat surprising that while the visible region of the electromagnetic spectrum spans a very narrow range of wavelengths, it represents the peak of the Sun's energy output. We are indeed very fortunate that sunlight should be brightest in the range that our eyes are capable of seeing. Imagine that our species had evolved on a planet orbiting a star that gave off most of its energy in the ultraviolet region of the spectrum. Presumably, we would have evolved eyes that could see UV ‘light’, for that sort of light is what would have been required to illuminate our planet's landscapes. The same sort of reasoning would apply to species that evolved on planets orbiting stars that emit most of their energy in the infrared; they would most likely evolve to have IR sensitive eyes. So it seems that our eyes are tuned to the radiation that our star most abundantly emits.

Sunlight can have quite a few health effects.  Sunburn can have mild to severe inflammation effects on skin, although this can be avoided by using a proper sunscreen cream or lotion or by gradually building up melanocytes with increasing exposure. Another detrimental effect of UV exposure is accelerated skin aging which produces a cosmetic effect that is difficult to treat and is known to be a cause of skin cancer. Some scientists are concerned that ozone depletion is increasing the incidence of such health hazards. A 10% decrease in ozone could cause a 25% increase in skin cancer[1].

A lack of sunlight, on the other hand, is considered one of the primary causes of seasonal affective disorder (SAD), a serious form of the ‘winter blues’. With our sunny climate this is not much of a problem in South Africa, but SAD occurrence can be a problem in locations further from the tropics, and most of the treatments (other than prescription drugs) involve light therapy, replicating sunlight via lamps tuned to specific wavelengths of visible light, or full-spectrum bulbs.

Apart from electromagnetic radiation from the Sun, Cosmic rays are a type of radiation that comes from space. Cosmic rays are particles with very high energies. Cosmic rays come from various places, including the Sun, but also from distant sources in far off galaxies. Because of their high energy, this type of particle radiation can be dangerous to people and to machines. Fortunately on Earth, we are mostly shielded from them by our planet's magnetic field and atmosphere.

Cosmic rays are made up a wide range of particles smaller than atoms. One of the strangest of these particles is the neutrino, which is extremely light and has no electric charge and accordingly has very few interactions with any other particles. It is estimated that billions of neutrinos pass through the Earth without a single interaction (they could be passing through you while you are reading this). Only large and very sensitive detectors are actually able to detect the occasional neutrino although about 65 billion neutrinos per second pass through every square centimetre of the Earth’s surface perpendicular to the direction of the Sun.[2] If they are so difficult to detect, why bother? The main reason is that can provide valuable information about the interior working of the Sun.

So next time you admire a lovely sunny day, think about all the science that has gone into finding out how the Sun works and the effect on our planet. Many cultures around the world have interesting myths about the Sun, reflecting its importance in all our lives. For example the Aztecs believed that four suns had been created in four previous ages, and all of them had died at the end of each cosmic era. We are perhaps fortunate that we have only one sun to be proud of and know only one cosmic era.



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