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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.

Protecting the skin from photodamage is the single most important measure an esthetician can emphasize in order to help prevent skin cancer and slow down the aging process in clients. To use sunscreens effectively requires a basic understanding of light energy, how light interacts with skin, and how to select and use sunscreens. Don’t panic, throw up your hands and run screaming because these concepts are quite simple and enjoyable to learn, and you will use them for the rest of your professional life in a variety of situations, from sunbathing to using laser equipment.

The basic physics of light

Many children have asked parents, “What is light?” only to receive the answer, “It is just light,” or “Nobody really knows.” In most cases, the child stopped there and waited for someone to come along and provide a better answer. At present, people are still in the dark when it comes to understanding light. Keeping that thought in mind, take a look at what is known about it, and don’t worry, because when all of this has been explained, you will understand a lot more about the physics of light and will be able see a small bit of the incredible beauty of the universe.

Light is energy. Light can be made only by using some form of energy to produce it. The sun is the major source of light during the day, and electricity is the major source at night, since moonlight is reflected sunlight. So, how is light actually made? Photons need to be produced. You may ask, “What in the world are photons?” A photon is the minimum bundle or capsule of energy needed to sustain the electromagnetic phenomenon at a particular frequency. Because a photon is a small packet of pure energy, it’s a tiny, tiny particle. Light is made up of these photon packets, and there are gazillions of them in a single ray of light.

Notice the word “electromagnetic” in the definition; it means that light has both an electric component and a magnetic component—that is why it is known as electromagnetic energy. Light travels in a straight line, but it moves up and down and in and out while it travels. A very interesting aspect of electromagnetic energy is that it travels as a wave with the components at right angles to each other. Figure 1 displays this relationship. Try to visualize this relationship by picturing a person walking with one arm straight up and the other arm held straight out to one side. The arm that is straight out is the electrical component, and the one that is straight up is the magnetic component.

Is the photon traveling in this wave pattern, carrying energy, or is energy shifting between electric andmagnetic forces in very discrete quantum of energy? This is the single most important bit of information about the photon: The energy of the photon is an exact amount known as a quantum, and is measured in electronvolts (eV). The energy in a single photon may be calculated by using the following expression, which is included not only for the budding scientists among estheticians, but also for those who have had the good fortune to study physics. For the rest, you can easily learn this and will then know how all these pieces of science relate.