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I see many websites claiming oils amplify UV waves. This would only seem possible if thicknesses caused no reflection as in a quarter-wave coating, but that seems hard to believe.

So, is it a myth that oil increases UV absorption by skin? If not, please explain the chemical/physical mechanism in detail. I suspect that total UV exposure is unchanged, but the oil has some chemical reaction with the skin, reducing scattering at the skin's surface.

bobuhito
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    I don't think that the person who wrote the claim on the website you linked knows anything about physics with their talk about baby oil being a "reflector" that "absorbs and attracts" light and "amplifies" UVA and UVB light. IMO, it's just advertising nonsense. As for whether baby oil is good or bad for the skin when tanning, you would have to ask that question on some health forum website. –  Sep 14 '19 at 17:24
  • @SamuelWeir its all about angles of incidence and refractive index of the material, see my answer below. – PhysicsDave Sep 14 '19 at 21:08

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There is no physical mechanism by which tanning oil increases the intensity of UV light. the true problem is as follows:

"Tanning oils" generally do NOT contain any UV-absorbing component to protect your skin from burning. They are there to make you look all moist & tasty while roasting under the sun's rays and to prevent heat and tropical breezes from drying out your skin. However, many people who use these products do so in the belief that they do offer some UV protection and they then tend to stay out in the sun too long for their own good and then get burned badly by the end of the day.

To them, it appears as though the tanning oil caused them to burn, when in fact it had nothing to do with the burn except for the nonphysical effect noted above.

niels nielsen
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Artists put a final coat of varnish on their paintings which provides a surface of higher index of refraction material. This makes the colours appear brighter mainly by increasing the absorption of the incident light as the incident light is actually made of light from many angles. The higher index material accepts light from more angles where normally some of it would be reflected.

Oil on the body likely has a similar effect.

PhysicsDave
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  • But, in normal suntanning conditions, rays come from one angle (perpendicular), so I think there must be more going on. – bobuhito Sep 15 '19 at 17:12
  • No reflected rays come from the water, the sand and more importantly blue and UV is scattered by the atmosphere. – PhysicsDave Sep 15 '19 at 18:29
  • I know that the sun scatters and reflects as you said, but my intuition is that rays directly from the sun alone would cause more skin damage with oil. I might be wrong. There probably are some published experiments to check this, but I can't find any with Google. – bobuhito Sep 15 '19 at 18:58
  • A ray (photon) has just as much energy reflected from the white sand or directly from the sun. Try googling anti-reflection coating. – PhysicsDave Sep 15 '19 at 21:47
  • You seem to get it backwards: the colors will be brighter (I guess you actually mean "more saturated") if the coating has lower refractive index than that of the surface coated. See also: Why do wet objects become darker? – Ruslan Aug 09 '20 at 20:30
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As far as I am aware there is no study in a reputable peer-reviewed dermatology journal that makes the claims of tanning oils evidence based. It should be noted that:

  1. Tanning oils generally do have a low SPF (in the range of 3 to 10) typically. In theory this can be effective but application of sunscreen is never uniform nor thick enough (think mass per unit area) for protection equivalent to the SPF advertised.

  2. Tanning oils may have UVA protection but no UVB protection, thus still allowing for a burn to easily occur, as well of course as tanning, which is just skin injury after all.

  3. Chemical sunscreens like avobenzone really have to be stabilised and not all brands do this, especially in tanning oil products, which means even if there is SPF it may wane.

These factors mean tanning oils don't provide adequate sun protection even if brands manage to justify sticking an SPF label on the product and thus there is a perception by the consumer one is somehow protecting the skin yet still - in contradiction - allowing for a tan.

JamalS
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