[jsnm0211]

Hello,

I have a question about UV Visible Spectroscopy.
I would like to know about the wavelengths that the monochromator selects.
I've seen a chart that shows a wavelength that the monochromator can select and its corresponding colour absorbed and colour observed.
Is the colour absorbed column relating to the colour absorbed by the electrons when they become excited and move to a higher state?
When is the colour observed actually observed?
For example:
Wavelength: 380-420nm       Colour absorbed: Violet       Colour observed: Green-yellow

All suggestions and ideas are welcome!
Thanks!

jsnm0211

[syko sykes]

Is the colour absorbed column relating to the colour absorbed by the electrons when they become excited and move to a higher state?
When is the colour observed actually observed?

On the first question i think it is the opposite of what you're suggesting but i could be wrong. I think atoms/electrons absorb a color because the wavelength of that color doesn't relate to the wavelength of the valence electrons.

For the second question, when the wavelengths of observed colors hit the atom it excites certain electrons. When these electrons return to lower energy levels they emit (reflect) waves with wavelengths that corrolate to the color observed. These colors are therefore observed by the eye after the electrons have returned lower orbitals. In the monochromator, i think their is a sensor opposite the light source that serves as an analytical eye.

I hope I am remembering all this correctly and didn't just make myself sound like an idiot. If not, please enlighten me.

[mike]

This is how I have always thought of it in simple terms. Take an example of a coloured object, such as a blue notebook. Now when the white light (from the sun or the ceiling light etc) hits the book the pigment molecules in the notebook absorb all of the wavelengths of light except the blue wavelengths. So what you are seeing is basically the "left over light". Of course this is not completely black-and-white (lol excuse the pun). So coloured objects can absorb over a range of wavelengths and your eyes will detect colours which are a mixture of wavelengths.

Now on the table you are talking about you will probably notice that an object that is percieved as blue will absorb mostly orange light (580-620nm or so). So if you were to start with a light bulb that only emitted orange light (or wavelengths 580-620nm) in theory you would not see any colour for the notebook.

So here is where the spectrophotometer works. Say for example instead of a blue notebook you now have a blue solution. You put this in the spectrophotometer and can select the wavelength you require (in this case the most appropriate wavelength will be the "orange" one so maybe something like 600nm). It is most appropriate to select this wavelngth simply because you want to use the wavelength that is absorbed the best or most, this leads to better results.

Now the organge light will go through the sample and some of it will be absorbed. The amount that is absorbed can be detected by the  spectrophotometer (it know the initial amount of light and the final amount of light and tells you this in terms of absorbance). The beauty of this is that the more concentrated your solution is in the compound that is absorbing the more light of that wavelength it will absorb, this is a linear direct proportionality relationship known as Beer-Lambert Law.

[Yggdrasil]

You can use a color wheel to figure out which colors are being absorbed by a solution.  A solution of a particular color will always absorb that color's complimentary color, which can be found by looking directly across the color wheel.  For example, an blue solution absorbs orange, a green solution absorbs red, and a yellow solution absorbs violet.

[mike]

Also, I forgot to mention your other question. The colour absorbed will be realted to electrons being promoted to higher energy level and this energy level will correspond to a particular wavelength of light (you can equate wavelength with energy etc etc). I don't completely agree with syko sykes when he says that the observed colour is from the electron returing to its lower energy level. As I said before the observed colour is the light which is not absorbed (so must be reflected, refracted, transmitted, scattered etc).

As for "when is it observed"? Maybe this is like the saying: "Does a tree falling in the forest make a sound if there is no one there to hear it." LOL

[jsnm0211]

Great!

Thanks a lot for your help.

I suppose when I asked "when is this colour observed" I meant will I see the colour listed in this 'colour observed' column when the light is shone through, before or.... well... that's where I was confused.

But, thanks for your help - I have a much better understanding of this now.

jsnm0211