UVA vs UVB: Why does UVA penetrate deeper into the skin?

[finnch]

Hello,

My name is Chris and I am trying to understand why UVA radiation, with a longer wavelength, penetrates deeper into the skin than UVB which has a shorter wavelength.

Since E=hv=hc/ λ and

,

then how does a lower energy and lower intensity UVA wave penetrate deeper into the skin than a UVB wave?

Pardoning the pun, I would appreciate it if someone could enlighten me )

Many thanks...

 

[Ygggdrasil]

In general, longer wavelength light will always pennetrate deeper into a scattering medium. This is the same reason why the sun appears red at sunset.

 

[finnch]

In general, longer wavelength light will always pennetrate deeper into a scattering medium. This is the same reason why the sun appears red at sunset.

Dear Sir,

Thank you for your reply but I disagree with your response. In our atmosphere, blue light, with a higher frequency (shorter wavelength) scatters more than red light with a longer wavelength and lower frequency. Scattering is proportional to frequency raised to the power of four...See

An X-Ray has a very high frequency, i.e short wavelength and penetrates both skin and bone and is consistent with the energy and intensity formulas cited earlier. So again, I am trying to find justification for why a longer wave, lower frequency UVA ray penetrates deeper than a shorter wave, higher frequency, higher energy, higher intensity UVB ray.

Thank you,

Chris

 

[finnch]

Dear Sir,

Thank you for your reply but I disagree with your response. In our atmosphere, blue light, with a higher frequency (shorter wavelength) scatters more than red light with a longer wavelength and lower frequency. Scattering is proportional to frequency raised to the power of four...See

An X-Ray has a very high frequency, i.e short wavelength and penetrates both skin and bone and is consistent with the energy and intensity formulas cited earlier. So again, I am trying to find justification for why a longer wave, lower frequency UVA ray penetrates deeper than a shorter wave, higher frequency, higher energy, higher intensity UVB ray.

Thank you,

Chris

Sorry Ygggdrasil,

After a little more reflection, your response does make sense and the youtube video I posted earlier actually supports your response. I was looking for an explanation considering only the energy side of the question without considering the scattering when the light actually interacts with matter. So I guess that when dermatologists tell us that UVB burns more but UVA penetrates more deeply, then it's the scattering of the UVB along the surface of our bodies that causes the surface burn while the longer UVA transmits deeper into the skin without burning the surface as much?

Thank you for your help.

Chris

 

[Ygggdrasil]

In additon to scattering, absorption may also limit the penetration depth. Tissue is composed mostly of water and water absorbs UVB more than UVA (see http://www1.lsbu.ac.uk/water/water_vibrational_spectrum.html#uv).