[bbrain]

The other day our teacher taught us about why chemicals possess colour. He mentioned HOMO-LUMO transition, and that red light gets absorbed first, if the energy jump is relatively small. My question is why does red light get absorbed, rather than reflected?

Note: change in energy = Planck's constant x frequency,
so it makes sense why red light is at all involved, because we're talking about a small energy jump here, and red light has the lowest frequency. But why isn't it reflected then?

[fledarmus]

The other day our teacher taught us about why chemicals possess colour. He mentioned HOMO-LUMO transition, and that red light gets absorbed first, if the energy jump is relatively small. My question is why does red light get absorbed, rather than reflected?

Note: change in energy = Planck's constant x frequency,
so it makes sense why red light is at all involved, because we're talking about a small energy jump here, and red light has the lowest frequency. But why isn't it reflected then?

What do you mean by "reflected"? If the light simply bounces off the molecule, then the molecule absorbs no energy and there is no transition.

Think of the incoming light as a small packet of energy - it hits the molecule and drives an electron up from the highest-occupied orbital energy state to the lowest unoccupied orbital energy state. The light is now gone - its energy has been absorbed by the molecule, which is now in a higher energy state. One of the pathways by which the molecule can return to its ground state is for the electron to simply drop back to its precious energy state, re-emitting the energy as red light with the same energy it originally absorbed.

So it is possible for red light to be absorbed by the molecule, and then re-emitted by the molecule, but although the light is coming in and going out, it isn't "reflected" as such.