Science of Skin

Light and skin interaction

Light movement

Figure 1. Electromagnetic radiation undergoing refraction, reflection, absorption, and scattering
Figure 1. Electromagnetic radiation undergoing refraction, reflection, absorption, and scattering

Light can be thought of as energy moving through a space. It is transmitted as particles, photons, from a radiating light source, such as the sun. Photons are packets of mass-less and charge-less energy, and cannot be detected by the human eye. These energy parcels move in a continuous wave-like motion, and are referred to as having wave-particle duality. The amount of energy contained in each photon is directly related to the frequency and subsequent wavelength at which the light is traveling.

The wavelength of a ray of light is the distance from peak to peak (nm), while frequency is the number of wave cycles that pass a point in space in a given time interval (Hertz). Light consists of energy in the form of electric and magnetic fields, and is referred to as a form of electromagnetic radiation. Various wavelengths exist along the electromagnetic spectrum, where the invisible part lies between 200 and 400 nm and the visible part between 400-700 nanometer (nm).

All levels of electromagnetic radiation travel at the equal speed (velocity) in a vacuum, approximately 299800 km/sec (186282 miles/sec), whereby the velocity depends on the medium through which the light is traveling. As light travels through the atmosphere, refraction, reflection, absorption, and scattering will affect the velocity.

Both reflection and refraction are variations of scattering. Scattering can be viewed as the alteration of the path of an electromagnetic ray in any direction. If the radiation passes through an object and its path is altered forward of the object, then refraction has occurred. Reflection describes a process where incoming radiation appears to bounce off a surface, where the angle of incident light is equal to the angle of refraction. If radiation is said to be absorbed, the energy is transferred to either chemical or thermal energy within the object.

UV radiation and skin

Ultraviolet radiation (UVR) is emitted in large quantities from the sun as light source. Contained within 200nm and 400nm, UVR is known to be able to cause damage to organisms and tissues. Long-term research into the effects of ultraviolet radiation on human skin shows the potential to cause irreversible damage to human skin cells (keratinocytes, melanocytes).

Our skin serves many purposes: to provide protection from the environment (milieu exterieur), to help regulate our body temperature, to maintain an equilibrium of electrolytes and water. Receptors located in the skin provide us with the sense and ability to experience tactile functions (touch), and are important for the distinction of pain and pressure, all part of the human defense mechanism.

The most common skin condition resulting from overexposure to sun is sunburn. Sunburn occurs when the skin lacks sufficient capacity to absorb the incident ultraviolet radiation. The best prevention to sunburn is to avoid the sun when the UV risk is relatively high, when the sun is at zenith (highest point normally between 10am and 4pm), to wear protective clothing and to always wear protective sunscreen when exposed outdoors. Sunscreens have been shown to provide up to 40% protection against skin damage seen as wrinkles, loss of pigmentation or hyperpigmentation, freckles and loss of elasticity. Protection from sunscreens is achieved much the same way as when absorptive chromophores (natural absorptive substances) protect the skin; one of the main chromophores, melanin possesses the ability to scatter, reflect and absorb (harmful) radiation.

DNA held within the cells (keratinocytes, melanocytes) of the top skin layer (epidermis) can be permanently damaged by UVR. This can lead to precancerous transformation of the dermal layers. To protect against actinic (sun) damage, one of the mechanisms of the skin is to react by the production of melanin, an absorptive pigment of radiation to protect from both sunburn and long term damage.

The sun's UVR is thought to play a fundamental role in the process of skin aging. As the process progresses, the epidermis thins out, while the number of skin layers remains constant. The changes in the skin’s connective tissue cause loss in capacity in the form of decreased elasticity and strength of the dermis. This results in the skin to become more fragile affecting various sensory functions (such as tactile sense) and a higher risk of injury. Skin can easily be torn (lesion), may take more time to heal, blood vessels are easily bruised (hematoma), and the thinner skin (atrophy)leads to a greater chance of becoming undercooled (hypothermic) in cold environments.

Loss of water (dehydration) of the skin occurs when moisture evaporates through the skin, this can be a result of environmental factors, such as exposure to sunlight, a change in weather seasons, and prolonged exposure to water. It can leave the affected area looking and feeling scaly, dry, red, rough and cracked.

Skin types

Figure 2. Fitzpatrick classification system
Figure 2. Fitzpatrick classification system

The amount of melanin in one’s skin can be used to determine how likely you are to become sunburnt. The Fitzpatrick Classification Scale is a system that classifies ones skin type as either skin type I to VI, and provides tanning/sun burning tendencies. Developed in 1975 by Harvard Medical School dermatologist Thomas Fitzpatrick, this scale has been adopted by various health organisations, such as the World Health Organisation, and since has been extended to include a scale of the amount of melanin in the skin (Figure 2).

Reflection, Refraction and Scattering

Electromagnetic radiation moves as a wave, but can demonstrate particulate properties. Because of this physical phenomenon, its movement is expressed as having wave-particle duality. Whilst moving through space, the direction and speed of radiation can be altered by being reflected, refracted, scattered, or absorbed.

Figure 3. Scattering of light
Figure 3. Scattering of light

Reflection of radiation occurs when the frequency of the incident ray doesn’t match the natural frequency of the atoms electrons. The electrons absorb the energy, but instead of vibrating continuously with a large amplitude, they quickly return to their previous state, and the energy is reemitted (reflected ray). If the material is dense, then the energy isn’t passed to other atoms, but backs out from the direction it came.

If the material is transparent, then the energy is passed through the bulk of the material, from atom to atom, and is then re-emitted on the other side. This is called refraction and this process takes time, resulting in an obscured view of the objects on the other side of the material.

Absorption occurs when the vibrational energy of the electrons is used to increase the movement of the surrounding atoms, resulting in thermal energy. This occurs in materials that aren’t completely transparent

Scattering is a term used to describe a general process whereby forms of radiation deviate from their trajectory as a result of non-uniformities. These non-uniformities could be almost anything, from dust particles and bubbles, to fluctuations in density, and the light can be scattered in all directions. It is basically reflection of a rough surface, and because it is rough, the radiation can be reemitted in any direction.

Figure 4. Rayleigh scattering
Figure 4. Rayleigh scattering
Figure 5. Mie scattering
Figure 5. Mie scattering

There are two types of radiation scattering, Rayleigh scattering and Mie scattering.

Rayleigh scattering occurs as a result of radiation being scattered by a molecule, which is smaller than the wavelength at which it is traveling. Nitrogen and oxygen are smaller than the wavelengths of UV and visible light, causing Rayleigh scattering.

Mie scattering is radiation being scattered by molecules larger than their wavelength, causing a predominantly forward scattering. Aerosols, water droplets in clouds, and dust particles are much larger than the wavelengths of UV and visible light, therefore cause Mie scattering.

Wave-particle duality

During the 1600’s, debate arose about the nature and composition of light. Three different theories of light were proposed: Pierre Gassendi proposed a particle theory of light, René Descartes proposed that light was a disturbance of the plenum (the continuous substance of which the universe was composed), and Robert Hooke published a wave theory of light. The particle theory and wave theory where preferred over the plenum theory, and these where continually expanded and changed until Max Planc’s new theory of black-body radiation, which was published in 1900. This new theory was updated regularly by Planc and eventually led Einstein to the discovery of the photoelectric effect.

The photoelectric effect occurs when light emitted on a metal surface supplies enough energy to produce an electric current. This only takes place if the incoming light exceeds a specific frequency, independent on the brightness of the light. The wave theory suggests that the brightness of a lightsource is indicative of the energy it possesses; using this to explain the photoelectric effect, bright light, irrespective of its frequency should be able to provide sufficient energy to evoke an electric current. This phenomenon highlights an obvious flaw in the wave theory, as the photoelectric effect demonstrates that brightness has no relation to the energy of the incoming radiation.

If, however, light is thought to consist of particles, brightness becomes proportional to the number of photons passing a point in space in a given time interval. Each photon then contains its own frequency and energy, and if this frequency is less than the required amount, irrespective of the number of photons passing to the metal, these would not dislodge any electrons, and no current would be produced.

The wave-particle theory explains how light travels as particles, photons, and these can behave as in a wavelike manner, each with their own wavelength. This theory also allows us to disregard either the particle or wave portion, to explain a certain phenomena, such as diffraction.

Skin effects due to sun radiation

Human skin inevitably ages as we pass through time. By taking preventative measures, we are able to slow down the formation of wrinkles and loosening skin. The sun (UVR) is known to cause premature aging of the skin, as the ultraviolet radiation that isn’t absorbed by chrompohores in the skin can pass through to the dermis. Here it can cause collagen and elastin fibres to break down prematurely. Collagen and elastin fibres are both major fibrous proteins, collagen is a main component of the extracellular matrix and provides extracellular structure. Collagen is present in dental material, bones, cartilage and skin, where it provides strength and elasticity. Elastin fibres are also a major fibrous protein, but only provide elasticity. When these proteins break, strength and elasticity are lost within the skin, wrinkles appear, and the skin can become loose and sag.

Dehydration occurs when there is an imbalance between the loss and intake of fluid. Chronic loss of fluid (water) is found to result in dry scaly skin. Rapid changes in temperature, cool or dry weather may also result in dehydration.

References

  • Einstein, A (1905). ‘Zur Elektrodynamik bewegter Korper’. Annalen der Physik. (English translation: On the Electrodynamics of Moving Bodies) Vol 17, pp. 891-921.
  • Greiner, W (2001). Quantum Mechanics: An Introduction, 4th Edition. Springer.
  • Giancoli, D C (2004). Physics: Principles and Appliations, 6th ed. Prentice Hall.
  • Freudenrich, C. (2008). How Light Works. [Online] Available from: http://science.howstuffworks.com/light.htm [Accessed on 23/04/2008]
  • Strobel, N. (2001). Electromagnetic Radiation. [Online] Available from: http://www.astronomynotes.com/light/s3.htm [Accessed on 23/04/2008]

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