In order to truly understand the tanning process of the skin you need to have at least a basic understanding of the properties and function of light. Although light has played a central role in the histories of religion, art and science and is so common to our everyday existance, it can actually be quite elusive.

When light passes through an opening it is observed to spread out.  This is known as diffraction and becomes more pronounced with narrower openings.

Electromagnetic waves all travel at the same constant speed as light, 186,000 miles per second in a vacuum. All electromagnetic waves have the same form and travel at the same speed, but differ in wavelength. Wavelength is the distance between two successive crests in the wave. The number of crests or cycles per second is the frequency of the wave. The unit of frequency is hertz or 1 cycle per second. Therefore, if the wavelength is decreased, then the frequency is increased. Frequency and wavelength have an inverse relationship which is calculated with one of two equations where the velocity of radiation is 186,000 miles per second.

Wave Theory Ultraviolet rays are similar to X-rays, white (visible) light, infrared and other similar types of radiant energy. They are all electromagnetic waves, wavelike disturbances associated with vibrating electric charges. Most waves are transmitted by some medium; for example, you have all seen waves on the surface of the water, in which case the water is the transmitting material. When a stringed instrument is plucked, waves are set up in the string, so the string becomes the transmitting material. Strangely enough, no one knows what transmits electromagnetic waves, however, we have proof that they are in fact transmitted.

This principle is used to predict what kinds of patterns will occur when light passes through different diffraction gratings such as single and double slits.  And while nobody denies that it does a good job of predicting, there are many problems in believing that this represents a true model of what actually happens with light. First, why would the wavelets be hemispheres rather than full spheres?  Shouldn’t nature produce full spheres rather than half-spheres with edges? Second, what happens to the original wavefront?  Does it stop its forward motion, then generate the wavelets, and then cease to exist?  And if it doesn’t stop its motion, wouldn’t its velocity (of c) be added to the wavelets making them move at 2c?  According to special relativity theory, light moves only at c, not more or less. Third, why would the wavelets cancel each other on the side?  There is no particular reason why they should as they are not ‘out of phase’ with each other.  This cancellation is just an assumption required by the theory. Fourth, if hemispherical wavelets were being produced, this would allow the light to eventually turn back on itself as shown:

So how can diffraction work without a flexible medium, and why doesn’t light diffract in empty space, as the Huygens-Fresnel principle would suggest? There is something being overlooked here.  And that is the edges of the slit. As light hits the slit, most of it either gets absorbed into the walls or passes through the opening.  But a small amount also hits the edges.  What will happen to it? If the material is reflective, light will bounce off in some direction.  Mostly though, the slit will be made of a dark material.  In that case the light might be reradiated from that point via a different process, as described later.  This diagram shows the idea:

Where the velocity of radiation is 186,000 miles per second. Frequency is calculated using cycles per second and wavelength is calculated in meters. The wavelengths of electromagnetic radiation vary in size from a fraction of an angstrom unit (an angstrom is equal to ten billionths of a meter) to thousands of meters, commonly called the "electromagnetic spectrum." Some of the wavelengths of electromagnetic radiation from this spectrum are classified as follows:

Here we see light passing directly through the opening and a small amount reradiated from the edges.  The radiation from the edges will come out spherically and combine with the light passing straight through.  As a result it will produce interference patterns on a screen.

Therefore, the useful unit of measure for our purposes is the nanometer. Radiations shorter than 10 nanometers (i.e. gamma rays or X-rays) generally ionize molecules (remove electrons) producing positively or negatively charged ions and are, therefore, known as ionizing radiation. Ultraviolet radiation is absorbed by molecules and is known as nonionizing radiation.

UVC is found in the region between approximately 200-280 nm and is called germicidal UV because of its proven effectiveness in killing single-cell organisms. Solar radiation in the UVC range is absorbed almost entirely by the atmosphere and that is fortunate considering that even a short overexposure to UVC is very harmful to the eyes and causes severe erythema (sunburn). UVC is emitted by High Intensity Discharge (HID) lamps; therefore these lamps require special filter glass, to contain the output of the UVC spectrum (this will be discussed later). Another place where radiation in the UVC range can be found is in the arc of a welding torch. For that reason, optical damage referred to as �welders eye� is caused by UVC light.

The ‘screen’ shows only a large-scale interference pattern with no secondary small-scale patterns on top.  Given the shape of the water emerging through the slits, there’s no conceivable way that a secondary pattern could appear. So how can light produce a pattern within a pattern?  The reason is likely that we are dealing with four edges that generate four radiation points as shown:

UVB is found in the region between 280 and 320 nm. It comprises the wavelengths primarily associated with erythema (sunburn), is also necessary for the production of vitamin D in the skin and is primarily responsible for stimulating increased melanin production. UVB wavelengths (at 305 nm) have 1,000 times more erythemal power than UVA wavelengths.

The way light diffracts is inconsistent with how other waveforms such as water waves diffract.  The reason why light produces interference patterns is not due to the Huygens-Fresnel principle but more likely has to do with how light interacts with the electric fields of atoms on the edges of diffraction gratings.

Let’s look now into the details of how the light-bending might occur.  Earlier it was suggested that light was being absorbed and reradiated from the edges.  But this seems unlikely because the amount of radiation would depend on how dark the material was, with fully dark materials (zero albedo) producing almost no diffraction – and this is not what we observe.   Understanding how light interacts with edges requires understanding how light can pass though solid materials like glass.  We don’t exactly understand this process, other than to say that light is somehow being absorbed by atoms on one side and reemitted on the other.  This absorption/reemission process is going to require interaction with the electrons surrounding the atoms. When the atoms are within a solid material, the electric fields from the electrons are going to be heavily overlapping each other on all sides.  If the material is transparent, this allows light to move directly though it in straight lines.

When those semicircles overlap they will produce interference but the gap between interference peaks won’t be much different to the wavelength of the original wave. A double slit water wave pattern looks like this:

The explanation of how light can spread is given by the Huygens-Fresnel principle.  It states that every point on a wavefront can be considered a source for another wavefront which will then become a source for further wavefronts as shown:

The fact that this happens is to be expected.  Light is known to be a wave and waves, such as water waves and sound waves, are also known to expand through openings. Water and sound waves are disturbances in flexible mediums.  Such waves spread out because they contain areas of pressure within the medium, and regions of high pressure (wave-peaks) always expand into regions of lesser pressure.  But as far as we know, light doesn’t travel though a medium so how can it spread out?

Quantum (Particle) Theory Another theory used in reference to the electromagnetic spectrum is the quantum theory. In order to explain energy transfer, a bit of energy called a photon was theorized. Photons have no mass and when absorbed this energy is passed on to the absorbing molecule (such as skin cells) and the photon no longer exists in its same state. The amount of energy in a photon is directly proportional to the frequency of the radiation. The energy of a photon increases as the frequency increases. The more cycles per second (frequency) of any given photon, the more energy the photon has. The energy of any given photon decreases as the wavelength increases. The longer the wavelength, the less the frequency.

The above diagram shows light skimming across the surface of a material.  The light is moving within the material, but though the ‘cloud’ of electrons belonging to the atoms in the top-most layer of the material.  We will assume light is able do this even if the material is opaque as it is not actually passing though the material, i.e. it is not passing between the nuclei in the material. Since the electron fields overlap each other, the light jumps from one atom to the next.  For the atom on the far right however, which corresponds to the edge of the material, its field overlaps only on its left side.  How would light behave when it emerges on the other side of that atom?  We don’t know, but it’s possible that when light encounters this situation it radiates spherically as shown:

This pattern also fits with a prediction from the Huygens-Fresnel principle. But there’s a major problem: a single slit shouldn’t produce an interference pattern. When a wave passes through a single slit, all that should happen is that the sides of the flat wavefront that has been cut at either end will expand in quarter circles as shown:

Chapter 2 Understanding Ultraviolet Radiation - Electromagnetic spectrum - UVA, UVB and UVC - Wave theory - Quantum theory In order to truly understand the tanning process of the skin you need to have at least a basic understanding of the properties and function of light. Although light has played a central role in the histories of religion, art and science and is so common to our everyday existance, it can actually be quite elusive. Understanding Ultraviolet Radiation To understand ultraviolet radiation (UV) one needs to know UV�s placement in the electromagnetic spectrum. Ultraviolet light is located between X-radiation and visible light. UV has a higher frequency and shorter wavelength than visible light, and it has a lower frequency and longer wavelength than X-radiation. UV with its longer wavelength and less energy is less penetrating than X-ray and is sometimes absorbed by matter. Photobiology studies the interaction of nonionizing radiation between the electromagnetic spectrum and biologic systems. Nonionizing radiation represents the ultraviolet, visible and near infrared regions of the spectrum. Tanning occurs as a result of exposure to ultraviolet radiation. To fully understand this reaction, you must familiarize yourself with the electromagnetic spectrum. Electromagnetic Spectrum The electromagnetic spectrum is a way of visualizing the frequency and wavelength proportions of different forms of energy. Electromagnetic radiation has properties of both waves and particles. We divide the electromagnetic spectrum in the UV range for medical purposes. UVA is found in the region between 320 and 400 nm (nm = Nanometer = 1 billionth of a meter) and is the least powerful wavelength band of UV radiation. UVA acts primarily to cause the melanin pigments in the skin to oxidize (darken) creating the cosmetic tan and has limited power to cause erythema (sunburn). UVB is found in the region between 280 and 320 nm. It comprises the wavelengths primarily associated with erythema (sunburn), is also necessary for the production of vitamin D in the skin and is primarily responsible for stimulating increased melanin production. UVB wavelengths (at 305 nm) have 1,000 times more erythemal power than UVA wavelengths. UVC is found in the region between approximately 200-280 nm and is called germicidal UV because of its proven effectiveness in killing single-cell organisms. Solar radiation in the UVC range is absorbed almost entirely by the atmosphere and that is fortunate considering that even a short overexposure to UVC is very harmful to the eyes and causes severe erythema (sunburn). UVC is emitted by High Intensity Discharge (HID) lamps; therefore these lamps require special filter glass, to contain the output of the UVC spectrum (this will be discussed later). Another place where radiation in the UVC range can be found is in the arc of a welding torch. For that reason, optical damage referred to as �welders eye� is caused by UVC light. Wave Theory Ultraviolet rays are similar to X-rays, white (visible) light, infrared and other similar types of radiant energy. They are all electromagnetic waves, wavelike disturbances associated with vibrating electric charges. Most waves are transmitted by some medium; for example, you have all seen waves on the surface of the water, in which case the water is the transmitting material. When a stringed instrument is plucked, waves are set up in the string, so the string becomes the transmitting material. Strangely enough, no one knows what transmits electromagnetic waves, however, we have proof that they are in fact transmitted. Electromagnetic waves all travel at the same constant speed as light, 186,000 miles per second in a vacuum. All electromagnetic waves have the same form and travel at the same speed, but differ in wavelength. Wavelength is the distance between two successive crests in the wave. The number of crests or cycles per second is the frequency of the wave. The unit of frequency is hertz or 1 cycle per second. Therefore, if the wavelength is decreased, then the frequency is increased. Frequency and wavelength have an inverse relationship which is calculated with one of two equations where the velocity of radiation is 186,000 miles per second.

Since no waves crossover and interfere with each other, there’s no reason for any interference pattern to emerge. This photograph shows what happens when water waves pass through a single slit:

When the atoms are on the surface of a solid material, the fields from the electrons are going to be heavily overlapping each other on all sides but one.  If the material is transparent and the light was coming from within at a low angle, this would allow the light to be internally reflected, i.e. total internal reflection.

Here we see a spherical wavefront generating a series of wavelets.  Each wavelet sits like a soap bubble atop a primary soap bubble.  The side parts of the wavelets are said to combine and cancel each other, leaving behind only the front part of the wave.

Light energy is expressed differently. We often express radiant energy in terms of watts per square meter or milliwatts per square centimeter. Skin exposure is usually expressed in joules per square centimeter. A Joule, is a unit of measurement and is equivalent to the electrical work done in one second by an electrical current of one ampere through the resistance of one ohm; named for its inventor, British Physicist, J.P. Joule (1818-1889).

This was taken in a ripple tank.  Plane waves come in from the left, pass through an opening, and then emerge in a shape similar to the previous diagram. Notice there’s no interference pattern! That fact that light produces an interference pattern indicates we are dealing with multiple ‘sources’ and the additional light sources are likely to be the slit’s edges.

Understanding Ultraviolet Radiation To understand ultraviolet radiation (UV) one needs to know UV�s placement in the electromagnetic spectrum. Ultraviolet light is located between X-radiation and visible light. UV has a higher frequency and shorter wavelength than visible light, and it has a lower frequency and longer wavelength than X-radiation. UV with its longer wavelength and less energy is less penetrating than X-ray and is sometimes absorbed by matter. Photobiology studies the interaction of nonionizing radiation between the electromagnetic spectrum and biologic systems. Nonionizing radiation represents the ultraviolet, visible and near infrared regions of the spectrum. Tanning occurs as a result of exposure to ultraviolet radiation. To fully understand this reaction, you must familiarize yourself with the electromagnetic spectrum.

The pair of points belonging to each slit are close together and these are what would be responsible for the small-scale pattern within the large-scale pattern.

We have what looks like a large-scale pattern overlayed with a small-scale pattern.  This again fits with a prediction from the Huygens-Fresnel principle, but again doesn’t match a ‘normal’ wave. A wave passing through two slits should produce two widened semicircles like this:

We don’t understand how light can be reflected in that manner but presumably it has something to do with how the fields overlap differently at the surface. Now what would happen if a group of atoms on the surface of a material suddenly came to an end?

UVA is found in the region between 320 and 400 nm (nm = Nanometer = 1 billionth of a meter) and is the least powerful wavelength band of UV radiation. UVA acts primarily to cause the melanin pigments in the skin to oxidize (darken) creating the cosmetic tan and has limited power to cause erythema (sunburn).

Electromagnetic Spectrum The electromagnetic spectrum is a way of visualizing the frequency and wavelength proportions of different forms of energy. Electromagnetic radiation has properties of both waves and particles. We divide the electromagnetic spectrum in the UV range for medical purposes.

Here we see a primary wavefront, which generates a secondary wavelet, which generates a third/fourth/fifth, etc.  Until finally we have a wave that flows backward.  All light passing through empty space should diffract and spread in all directions.  Instead it diffracts only when interacting with an opaque material.