Can someone please help me with LIGHT WAVELENGTHS??! BIOLOGY/CHEMISTRY

Question

Hello. I am struggling a bit with understanding the concepts of transmission, absorption, and reflection, and was wondering if my thought process is correct for these questions.

 

1. What can be concluded about a dye that makes liquid look green?

 

I think that the dye contains electrons in molecular orbitals that correspond to different energies. A dye that makes liquid look green is absorbing the wavelengths of light that correspond to blue and red? Is THIS BECAUSE the wavelenghts for blue and red match the gap in energy differences between orbitals?  The dye is able to make the liquid look green because it is reflecting back the wavelength for green? The dye has energy ‘gaps’ between its molecular orbitals that have the same wavelengths of visible light colors except for green.

 

One of the things that really stumps me is whether or not the color we are seeing, green, is being reflected or transmitted. Could it be either? If so, what dictates whether the wavelength that corresponds to visible green light is reflected or transmitted back for our eyes to see?

2. What is happening to a leaf that looks green to your eye? Remember, in this case you’re not looking through the leaf (more correctly, light is not passing through the leaf from the back).

I am guessing that a leaf that looks green to the eye is not actually ‘green’. Is it that the cells of leaves have chlorohyll pigments that are absorbing high amounts of wavelengths for blue and red and REFLECTING/TRANSMITTING the green wavelength?

 

Once again, I am just stuck on understanding these concepts and whether or not we are seeing the reflected or transmitted light. I am assuming the hint is saying that it might not be transmission, but I don't really know.

Thank you for all the help!!!!

Answer 1

Very few molecules have visible emission spectra. In general these molecules must be composed of an extended conjugated chain (aromatics and polyenes) or contain elements for which decay can occur via spin cross-over, where flipping of the electron spin occurs leading to a spin forbidden transition of lower energy (e.g. lanthanides).

 

In general, the energy of the photon emitted will be lower than that of the photon absorbed due to various modes of decay that can occur in a molecule (rotational, vibrational, etc.). What this means is that if a molecule emits visible wavelengths of light (green --> red) it must absorb at the very least violet -->green photons in order to emit light. If this is the case then you have both absorbance and emission contributing to the overall color of a substance.

 

On the other hand, if the compound requires higher energy photons to emit light then it will not absorb visible light and any color associated with the compound will be due to emission.

 

As a rule of thumb, most compounds you'll encounter in live will yield color because of absorbance and not emission. The obvious exception being those where high inputs of energy are supplied to provide color: TV and computer monitors and neon lamps, to name a few.

Answer 2

To be honest, I don't remember much about the energy gaps, but as far as transmissions vs reflections I have this:  

 

For something to be transmitted it must pass through an object, such as the light passing through the liquid.  The colors that you do not see are being absorbed by the liquid (think filtering out the other lights so that only green can be transmitted) because the other absorbed wavelengths have energy that matches the difference between the chemical's initial state and its's excited state.

 

In the case of the green leaf, the other colors are being absorbed by the chloroplasts in the leaf(for the exact same reasons which you stated), and the green light is being reflected to your eyes.  Think about a mirror.  Light can not pass through a mirror, but it is reflected off of the mirror, hence "looking at your reflection."

 

Hope this helps