Blacklight reflection

[5614]

So I recently got a 600mm (2ft) 20W UV blacklight which is quite cool... anyway, the thing with UV light is that it only reflects off (generally, there are some exceptions,) white surfaces, and only some white surfaces. So for example it reflects of most normal A4 paper but not the white paint on my ceiling.

 

What I was wondering is that when the UV light reflects off a white surface you see white light... how is that a single-high-frequency UV can be reflected as multiple-lower-frequency white?

 

I understand how different materials reflect different frequency light which is affected by the electrons in the material. Also, whenever it comes to visible light we say that white surfaces reflect everything... but that doesn't work for UV light because then it'd reflect UV (which we can't see) light. Instead the white surface splits the UV beam into all of the visible spectrum (+ possibly more that I can't see)... hows that work?

[swansont]

There are several things that could be going on. One is that it could be broadband fluorescence, like in a fluorescent bulb or white LED. They use transitions in the UV that are absorbed by the fluorescent material, which down-converts it into the visible, but there are a lot of transitions available, so you get light throughout the whole spectrum, so it's white. I think that's what some whiteners do as well - the "whiter than white" detergents add a material that gives you strong fluorescence. The paper may contain such an ingredient. If that is the case it's not reflection, but absorption and emission at a longer wavelength (aka fluorescence).

[5614]

Yes, I've read that's how the "whiter than white" detergents work...

 

So there's this down-convert thing, which is what I assumed was happening and asked how?

 

If that is the case it's not reflection, but [u']absorption and emission at a longer wavelength[/u]

1) Isn't that the same as a down-converter?

2) Would it be reflection if you got UV light bouncing off and absorb/emit if you got a converstion (up or down)?

[YT2095]

you have to respect the fact that A4 paper (manufacturer dependant) is a composite also, and thus the reflectivity and absorbtion ratio will differ, as it will with any given grade of paper taken appart particle by particle

[DQW]

There are two broad questions here, that I'll address briefly :

 

1. How is reflection any different from fluorescence (absorbtion and re-emission) ?

 

Fluorescence, is an atomic phenomenon, while reflection is not. By that, I mean that (i) fluorescence involves excitations and decays within the electronic levels of individual atoms, while

(ii) reflection involves collective excitations of either an electron gas (in metals, known as plasmons) or of phonon modes (in insulators and semiconductors, these are the optical phonon modes).

 

Simplistically speaking, (in the case of reflection) the incident photon excites a collective oscillation in the free electron gas/lattice. These oscillations are unstable and decay rapidly, re-emitting the photon that excited them. These excitations conserve momentum, making reflection conserve angle.

 

2. Why does the emission spectrum of a fluorescent material always look "down-shifted" from the excitation spectrum ?

 

This is simply due to the large number of energy levels in any real material. An excitation between a pair of levels allows for a large number of possible decay modes (see pic below for an example showing 3 such modes). The emission spectrum is simply the sum over all these decay spectra (weighted by their probabilities). Clearly the maximum energy of a single transition to the ground state will be the excitation energy itself. In addition to this, several other intermediate energies (and hence photon frequncies) are sampled through the different decay modes.

 

It is this breaking up that causes the "down-shift" in the emission (aka decay) spectrum, compared to the excitation (aka - though loosely - absorption) spectrum.

[swansont]

2) Would it be reflection if you got UV light bouncing off and absorb/emit if you got a converstion (up or down)?

 

 

As DQW said, it's not reflection if it involves a real absortion in an atomic state. To get up-conversion you need energy from somewhere else. A phonon mode possibly, or two-photon absorption.