# What penetrates deeper in water? Blue or voilet?

I was wondering about what colour of light penetrates deepest, and stumbled upon this pic while reading about related content.

The pic shows, violet is penetrated to a lesser extent than blue.

If this information is right, what could be the possible explanation?

If light with higher energy penetrates deeper, then violet should reach to greatest of depth. Or is it because water absorbs violet light?

The Wikipedia article on light absorption by water includes this graph of absorption in the visible region:

The minimum absorption, and therefore the greatest penetration is around $\lambda = 420$nm, which is on the boundary between blue and violet light.

However the spectrum of sunlight looks like this:

Note that the intensity of sunlight peaks between $\lambda = 500$ and $550$nm and it falls rapidly at smaller wavelengths. So even though water absorbs least at violet wavelengths the light intensity decreases because there isn't that much violet light in sunlight.

The maximum penetration at around $\lambda = 475$nm is a trade off between the decreased absorption and decreased intensity as the wavelength decreases.

• To speak of a sunlight-spectrum-corrected maximum penetration without regard for the distance of water to penetrate is rather arbitrary. – LLlAMnYP Mar 2 '17 at 16:55
• @LLlAMnYP: presumably the article that the original figure came from is taking some arbitrary cut-off intensity and solving $I(\lambda)=I_0(\lambda)exp(-a_\lambda d)$ to get the depths shown in the figure. – John Rennie Mar 2 '17 at 16:59
• Oh, I see. The first time I loaded the page, that figure didn't show up. My bad. – LLlAMnYP Mar 2 '17 at 17:29
• On second thoughts, isn't that figure dealing with sunlight which actually penetrates the atmosphere, but has nothing to do with penetrating water? – LLlAMnYP Mar 2 '17 at 17:32
• @John Rennie Thanks for such a beautiful explanation. So, the extent to which different wavelength of light is absorbed by water (or any media) is independent of energy possessed by the light? – Tushar Mar 3 '17 at 10:13