Estimating Compound Color in White Light: Confused?

[atomicpedals]

I have a question which asks me to estimate the color of a compound in white light. I can easily get the wavelength (hc/dE) as 460nm, which agrees with the book. Looking at a spectrum list that would seem to fall in the blue range, but the book is saying it would be orange because blue is subtracted. What's going on here? I clearly am not understanding something fundamental about light.

 

[DrClaude]

I can easily get the wavelength (hc/dE) as 460nm

The wavelength of what? The wavelength at which the compound absorbs light?

 

[atomicpedals]

That's part of my comprehension problem; I'm not sure. All the example I'm working through states is "The wavelength 460nm corresponds to blue light; so the molecule is likely to appear orange in white light (since blue is subtracted)." I understand the first half of that sentence, but not the last half.

 

[DrClaude]

My question was a genuine question (not one to make you think). I get the numerator hc, but what is dE?

My guess is that it corresponds to the energy of a transition in the compound. Therefore, if you shine white light (i.e., the full visible spectrum), what happens to light at 460 nm?

 

[atomicpedals]

The dE is a delta-E (la-tex issues using a tablet computer); which is a change in energy.

I think I follow you now though, if the molecule absorbs blue light it would reflect all other light and appear orange.

Thanks!

 

[atomicpedals]

Absorbs might not be the correct word to use there, the frequency would cancel out between the compound and the white-light.

 

[DrClaude]

Absorb is the right word. From of the point of view of the light, "blue is subtracted", as light in that part of the spectrum is absorbed.