# Germicidal UV and Wavelength

1. Sep 8, 2008

### sbarton

If the primary germicidal UV wavelength is 270nm and the secondary is 254nm and I have a UV light source in air to provide germicidal effects in water, would the wavelength of the UV light source need to be 360nm and 338nm respectively assuming the index of refraction for water to be 1.33?

2. Sep 8, 2008

### Andy Resnick

Heh- now that's an interesting question! Never thought of that one.

My guess is no- the germicidal properties come from the *frequency* of the light- the energy, as opposed to the wavelength (the momentum). Since frequency does not change in different media, the lamp should still work.

Now, AFAIK, water has a deep absorption band in the UV: http://www.lsbu.ac.uk/water/images/watopt.gif, so the effect will be attenuated rapidly with depth.

Try it out- I'm curious how well it works.

Can you try it?

3. Sep 9, 2008

### sbarton

That's interesting. All of the UV light source manufacturers that I know of quote wavelength of light rather than frequency but your arguement makes sense. I suppose the wavelength of light specified for each light source is actually an indicator of the frequency? I think I will have to look into this more closely and perhaps talk to some of the UV light manufacturers.

BTW, I am going to try it - I just have to make sure my thinking is correct before purchasing the UV light source.

4. Sep 10, 2008

### Andy Resnick

I suspect wavelength is quoted rather than frequency for 'ergonomic' purposes only: I understand intuitively what 500, 300, 250 nm wavelength light... ahem.. looks like. The wavelength and frequency are relatable via c= fw (f =frequency, w = wavelength, =speed of light) easily enough.

5. Sep 10, 2008

### Andy Resnick

I suspect wavelength is quoted rather than frequency for 'ergonomic' purposes only: I understand intuitively what 500, 300, 250 nm wavelength light... ahem.. looks like. The wavelength and frequency are relatable via c = $\lambda\nu$, where $\nu$ is the frequency and $\lambda$ is the wavelength. c is the speed of light.

Edit: sorry for the double post... how odd.