Sunscreen & the Electromagnetic Spectrum
How sunscreen protects skin exposure from radiation.
Electromagnetic Radiation
The nature of light has been a subject of inquiry since antiquity. In the seventeenth century, Isaac Newton performed experiments with lenses and prisms and was able to demonstrate that white light consists of the individual colors of the rainbow combined together. Newton explained his optics findings in terms of a “corpuscular” view of light, in which light was composed of streams of extremely tiny particles travelling at high speeds according to Newton’s laws of motion. Others in the seventeenth century, such as Christiaan Huygens, had shown that optical phenomena such as reflection and refraction could be equally well explained in terms of light as waves travelling at high speed through a medium called “luminiferous aether” that was thought to permeate all space. Early in the nineteenth century, Thomas Young demonstrated that light passing through narrow, closely spaced slits produced interference patterns that could not be explained in terms of Newtonian particles but could be easily explained in terms of waves. Later in the nineteenth century, after James Clerk Maxwell developed his theory of electromagnetic radiation and showed that light was the visible part of a vast spectrum of electromagnetic waves, the particle view of light became thoroughly discredited. By the end of the nineteenth century, scientists viewed the physical universe as roughly comprising two separate domains: matter composed of particles moving according to Newton’s laws of motion, and electromagnetic radiation consisting of waves governed by Maxwell’s equations. Today, these domains are referred to as classical mechanics and classical electrodynamics (or classical electromagnetism).
Visible light and other forms of electromagnetic radiation play important roles in chemistry, since they can be used to infer the energies of electrons within atoms and molecules. Much of modern technology is based on electromagnetic radiation. For example, radio waves from a mobile phone, X-rays used by dentists, the energy used to cook food in your microwave, the radiant heat from red-hot objects, and the light from your television screen are forms of electromagnetic radiation that all exhibit wavelike behavior.
Waves
A wave is an oscillation or periodic movement that can transport energy from one point in space to another. Common examples of waves are all around us. Shaking the end of a rope transfers energy from your hand to the other end of the rope, dropping a pebble into a pond causes waves to ripple outward along the water’s surface, and the expansion of air that accompanies a lightning strike generates sound waves (thunder) that can travel outward for several miles. In each of these cases, kinetic energy is transferred through matter (the rope, water, or air) while the matter remains essentially in place. An insightful example of a wave occurs in sports stadiums when fans in a narrow region of seats rise simultaneously and stand with their arms raised up for a few seconds before sitting down again while the fans in neighboring sections likewise stand up and sit down in sequence. While this wave can quickly encircle a large stadium in a few seconds, none of the fans actually travel with the wave-they all stay in or above their seats.
All waves, including forms of electromagnetic radiation, are characterized by, a wavelength (denoted by λ, the lowercase Greek letter lambda), a frequency (denoted by ν, the lowercase Greek letter nu), and an amplitude. The wavelength is the distance between two consecutive peaks or troughs in a wave (measured in meters in the SI system). Electromagnetic waves have wavelengths that fall within an enormous range-wavelengths of kilometers (103 m) to picometers (10−12 m) have been observed. The frequency is the number of wave cycles that pass a specified point in space in a specified amount of time (in the SI system, this is measured in seconds). A cycle corresponds to one complete wavelength. The unit for frequency, expressed as cycles per second [s−1], is the hertz (Hz). The amplitude corresponds to the magnitude of the wave’s displacement and so this corresponds to one-half the height between the peaks and troughs. The amplitude is related to the intensity of the wave, which for light is the brightness, and for sound is the loudness.
The Electromagnetic Spectrum
The figure below shows the electromagnetic spectrum, the range of all types of electromagnetic radiation. Each of the various colors of visible light has specific frequencies and wavelengths associated with them, and you can see that visible light makes up only a small portion of the electromagnetic spectrum. Because the technologies developed to work in various parts of the electromagnetic spectrum are different, for reasons of convenience and historical legacies, different units are typically used for different parts of the spectrum. For example, radio waves are usually specified as frequencies (typically in units of MHz), while the visible region is usually specified in wavelengths (typically in units of nm or angstroms).
Sunscreen
A skin pigment, known as melanin, protects our skin from the sun’s radiation (UV radiation). However, when our skin is exposed to too much harmful UV radiation, skin cells may be damaged. Our body’s natural response to this damage is to produce more melanin, increasing the amount of radiation that can be reflected from the body. This is why our skin tans! Therefore, a tan is the visible evidence of the body’s attempt to protect itself from harm. If the damage cannot be repaired, the DNA within the exposed cells may be damaged causing genetic mutations that can lead to the development of skin cancer. Continuous exposure to this harmful radiation without protect can be detrimental to the health of our skin.
There are two major types of radiation emitted by the sun: UV-A and UV-B. UV-A ranges from 320-400 nm on the electromagnetic spectrum. This type of radiation is less harmful UV-B and produces a slower-appearing tan. UV-B is more powerful (290-320 nm). Exposure to this radiation causes redness and blistering. Long exposure can lead to skin cancers.
Sunscreen attempts to protect our skin from harmful radiation by scattering and reflecting UV radiation away from the body. Most importantly, sunscreen blocks UV-B radiation, which can be incredibly harmful. The Sun Protection Factor (SPF) refers to the amount of exposure your skin will be protected from damage compared to your skin without the sunscreen. For example, if SPF 6 is applied to your skin, the amount of radiation exposed to your skin normally would be reduced by a factor of 6. Remember that during different times of the day, the sun’s radiation is stronger. This is why it’s incredibly important to check the UV index (1~10+). UV intensity also effects the amount of radiation exposure. Therefore, SPF is a relative factor rather than an exact amount of protection for a specific amount of time. For example, SPF 30 is more protective than SPF 15.
