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1. This is simply because melanin is there precisely to minimise the number of blue photons from entering the dermis. Individual blue photons are highly energetic – they can have much more energy than red photons.
• These highly energetic photons can cause all sorts of damage to DNA and RNA in cells. So, the melanin granules are like a ‘guard’, waiting for those nasty blue photons. They will defend the dermis, as much as they can, to stop entry. It is for this reason the melanocytes produce more melanin when bombarded with blue photons.
• Mere humans refer to this process as ‘tanning’!
• So, we’re back on our nice, low energy, red photon – it’s ‘safe’ compared with the
blue nasties.
• In the dermis, the chances are that we will be scattered many, many times. This might be off atoms of water, collagen, blood, hair or nerve tissues or many other possible scattering sites.
• Now, this is an interesting point. Because our little photon is undergoing so many scattering events, its direction is constantly changing. Sometimes, it will change so that it is heading back towards the skin surface; other times it will be heading deeper into the dermis. This is essentially determined by the anisotropy of the epidermis. As a consequence, many photons will escape the skin entirely, never to return. In the skin, it typically takes about 1 mm of travel for the photo’s new direction to be truly ‘random’ – this is due to both the scattering coefficient of the tissue and its anisotropy.
• However, many will continue their journey deep into the skin.
Photons emerge in new directions according to the anisotropy, g.
• Since the dermis is so thick (up to 3 or 4 mm in some people) most of the photons entering the skin will likely be absorbed here. But don’t forget, after only 1 mm of
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