Page 307 - Blog Posts v1

You can view the above video at
https://www.youtube.com/watch?v=fnFDgEVUQXc&t=251s
So, when laser light enters the skin, it also undergoes these two processes. Now, this is very interesting because something quite fundamental occurs.
Firstly, when photons are scattered, they usually take a new direction compared with their initial direction. As a result, a laser beam rapidly becomes highly divergent, just like ordinary light. This is purely due to the many, many scattering events.
Secondly, each scattering event is essentially a new source of photons (in reality, photons do not ‘bounce’ off atoms. They are absorbed and a new photon is generated, which emerges in a new direction). So, the coherence is lost rapidly too since each scattered photon comes from a new source.
Thirdly, the skin has a higher refractive index compared with air. This induces a change in the velocity of the light – it slows down (fractionally). But the light’s frequency is set at its source (inside the laser cavity) and cannot be changed. As a result, the wavelength must change – this is the principle of refraction. If the light is slower, then the wavelength must decrease. Assuming an average refractive index of around 1.36 to 1.41 for the dermis, it’s easy to show that the Nd:YAG fundamental wavelength of 1064nm will change to around 768nm in the skin, while a diode wavelength of 808nm will drop to about 583nm.
(I’ve written a wee paper on this issue which I really must get published...)
Mike’s Blog Posts 307