Page 306 - Blog Posts v1

Spatial coherence - in the above image, incoherent light is generated from the element of the light bulb because there are multiple sources where multi-wavelength light can be created. These light waves interfere with each other creating an interference pattern. When that light passes through the 'pinhole aperture' the waves become spatially coherent (as long as the hole is small enough). Using a filter, coherent waves of the same wavelength will emerge.
The high level of coherence in laser light comes from the fact that the source is essentially a point source inside a ‘mirror tunnel’.
[There are two types of coherence – spatial and temporal. I’ve only described spatial coherent here. You can find out about temporal coherence here.]
Ordinary light, such as that from light bulbs, fire or IPL systems, generate multiple wavelengths, are maximally divergent and are very incoherent.
What happens in the skin?
The skin is essentially a matrix of atoms and molecules. As soon as photons of light enter the skin, they begin to interact with those atoms. What happens next is down to probability – if a photon interacts with an atom which has a high probability of absorbing it, then the likelihood is that that photon will be absorbed and its energy ‘taken’ by the atom. The photon will no longer exist. It is an ex-photon...
If, however, the probability of scattering that photon is high, then it will likely be ‘rejected’ by that atom and sent on its way, to the next atom, where it will be either absorbed or scattered.
There is nothing else which can happen to these photons – they must be either scattered or absorbed. There is no ‘third path’.
I posted about ‘light in the skin’ earlier this year -
https://mikemurphyblog.com/2023/04/26/what-are-the-differences-between-lasers-and- ipls/
Mike’s Blog Posts 306