In the same manner that the frequency of visible light plays a role in the color we perceive, or in the way a broadcasting radio frequency characterizes a particular radio station, the specific frequency of ultraviolet light(UV) affects its behavior towards living cells.
Before elaborating on that point, let’s look at some characteristics and origin of UV. Ultraviolet radiation has a frequency in the range of 7.5 x 1014 to 3 x 1016 Hz (cycles per second), corresponding to wavelengths of 400 to 10 nanometers. Converting from frequency to wavelength is just a matter of dividing the speed of light in m/s by the frequency in Hz and then multiplying by a billion to get the answer in a value equivalent to a billionth of a meter.
In all atoms, electrons can get promoted to higher energy levels. In those of the sun’s most abundant element, hydrogen, when a previously excited electron falls to the second level from either the 3rd, 4th 5th or 6th levels, photons of red, blue-green, blue or violet frequencies are emitted. But because there is a much bigger gap between the 1st and second levels, the photons released are of the ultraviolet variety. Ultraviolet will also result, and will do so at progressively shorter wavelengths, if an electron calls directly from the mentioned excited levels to the ground state. What gives electrons the jolt of energy in the first place? It comes from the heat at the sun’s surface. The temperature is nowhere near as intense as it at the core where fusion releases gamma, but at about 5800 oC, it’s enough to generate the excitations leading to ultraviolet emissions. UV is invisible to our eyes but can be seen not only by insects but by at least some species of hummingbirds, pigeons, rats, cyprinid fishes and goldfish.
The UV emitted by the sun constitutes about 10% of all the electromagnetic radiation that reaches our planet from our star. The UVC-type rarely reaches our planet’s surface thanks to the absorption by oxygen(O2) which generates atomic oxygen, which in turn bonds to an undissociated molecule, yielding ozone, O3. The latter absorbs other frequencies of UVC and some lower-frequency of ultraviolet dubbed UVB. This is a good thing because UVC is the most deleterious biologically and would probably prevent most forms of land life from evolving. Having said that, I should point out that UVB is energetic enough to cause skin cancer. UVA, my namesake, can’t directly affect DNA molecules, but it’s still a threat because it is the most abundant form of ultraviolet(around 95%) at the earth’s surface and because it causes other molecules to produce free radicals. These reactive atoms that contain unpaired electrons can then move on to attack genetic material. According to Consumer Reports (July 2016), 28 of 65 sunscreen products they tested failed to meet SPF(sun-protection-factor) claims on the label. Nine products protected either fairly or poorly against UVB.
The ozone layer has been compromised by atomic chlorine and other halogens from our use of certain coolants. As a result more UVB reaches the surface of Earth. A series of protocols(see chart) has controlled the emission of ozone-attacking catalysts and has curtailed the depletion of the vital stratospheric UV-filter. The long residence time of atomic chlorine, however, means that overall there is still significantly less ozone over the Antarctic and a little less globally
From the latest report (2014)on the status of global ozone levels:
Depletion of the global ozone layer increased gradually in the 1980s and reached a maximum of about 5% in the early 1990s. The depletion has lessened since then and now is about 3% averaged over the globe. The average depletion exceeds the natural year-to-year variations of global total ozone.
Notice what we have done in describing ultraviolet light. We have mentioned some associated metrics, its origins and its interactions with oxygen, ozone and biomolecules, all of which are important and practical for us. But what do we know about its essence? Some philosophers such as Philip Goff argue that there are aspects of matter that play a key role determining the nature of consciousness. According to this modern form of panpsychism, although those aspects can be scientifically investigated, it will take a revolution comparable to that of relativity in physics in order to understand consciousness in all living organisms.
Of course ultraviolet light is a form of energy and not matter. But the two are intimately associated and in special circumstances even interconvertible. So if our understanding of matter may some day be radically rethought, the same may be true of ultraviolet and of all energy forms.What we’ve learned so far is experimentally verified, but the shield of our current mind-frames, theories and instrumentation constantly filter other truths.
American Zoologist. Ultraviolet Vision in Vertebrates 1., 32:544-554 (1992)http://0-az.oxfordjournals.org.mercury.concordia.ca/content/amzoo/32/4/544.full.pdf
Cone-based vision of rats for ultraviolet and visible lights. Experimental Biology. http://jeb.biologists.org/content/204/14/2439
Methods Enzymol. 2000; 319:359-66.
Photocarcinogenesis: UVA vs UVB.
Philosophy of Mind: An Overview