Page 267 - Blog Posts v1

The ‘speed of light’ depends entirely on what is it travelling through. It is fastest in what is known as ‘free space’ – that is deep space where there is virtually nothing – no atoms or molecules!!
But, when light enters other medium, such as air or skin, the local environment changes. In particular, the permittivity and the permeability both change. You will recall that light is an electromagnetic wave (made up from a magnetic wave component and an electric wave component), so, any change in the local electrical and magnetic environment will alter its properties. (Incidentally, it was the great Scottish scientist James Clerk Maxwell who first figured this stuff out).
The main property change is a slowing of its speed. Normally, this is discussed as a change in refractive index – this change is due to the changes in both the permittivity and permeability.
So, we slow down as we enter the skin. Now, if we could ‘see’ what wavelength (colour) we are, we would also see a change in that too. When light slows, its wavelength also shortens. This is because its frequency cannot change – that is determined by the source of the light.
So, if we entered the skin as a ‘red’ photon, we might now see that the colour is closer to green or yellow!
The first layer of skin we will encounter is the stratum corneum. This is very thin (typically around 10 to 40 microns) so we can imagine we will pass through it unaffected (apart from slowing and changing colour!!). However, we will begin to encounter reflections here known as ‘Fresnel Reflections’. This is when light is simply reflected back out of the skin and can account for between 4 and 8% of all light falling onto the skin. This also occurs with laser
Mike’s Blog Posts 267