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Light Energy and Sunscreens

By: Peter T. Pugliese, MD
Posted: March 26, 2009, from the April 2009 issue of Skin Inc. magazine.

page 2 of 13

The energy E, frequency f and wavelength λ of a photon are related by: E (energy) = h (Planck’s constanta) x f (frequency of light) or E = hf.

This formula can also be used and will provide the same result: E (energy) = h (Planck’s constant) x c (speed of lightb)/λ (wavelength of light) or E=hc/λ.

This can be made easier if h is multiplied by c to get the wavelength of 1,240 nanometers (nm), then all that is left to find a specific energy in eV for a photon in a particular wavelength is to divide 1,240 by that wavelength and the answer is available in eV. Here is an example: Find the energy of a photon with a λ of 400: E = 1,240 nm/λ = 1,240 nm/400 nm = 3.101 eV.

The main fact to learn from this equation is that the energy in a photon is discrete, directly related to the frequency of the wave and inversely related to the wavelength. Before giving up, this is important because the color of light depends on the wavelength of light. Blue light has a smaller wavelength than red light and, therefore, it has more energy. In Figure 2, the full range of the electromagnetic spectrum is shown with wavelengths of visible light clearly illustrated. For this article, visible light should be of less interest than ultraviolet light (UV), which packs a tremendous wallop. When the interaction of photons with the skin is covered, this will not only become clearer, but you will also understand why it is important to have this basic knowledge.

From radio waves to gamma rays

In Figure 2, the full range of the electromagnetic spectrum can be seen, from large radio waves to very small gamma rays. Radio waves can be more than one kilometer long, which is longer than several football fields arranged end to end, while gamma rays can be as small as one-millionth of a nanometer. Only a small part of the full electromagnetic spectrum can be seen; the range of wavelengths between 400–700 nm is called visible light. Below 400 nm is the range of UV light. The shorter the wavelength, the more energy the photon carries. Throughout the years, UV light was classified into separated wavelengths to denote various energy levels.