Why an Object Reflects a Specific Colour

[oreo]

We see an object coloured because of the component of light reflected by it. But what is in the object that makes other wavelengths absorbed by it and one to reflect.

 

[phinds]

We see an object coloured because of the component of light reflected by it. But what is in the object that makes other wavelengths absorbed by it and one to reflect.

The atomic composition. Gold, for example, is all one kind of atom and so has a single characteristic wavelength that is emitted by the excitation caused by impinging photons. The impinging photons have various levels of energy and when they are strong enough, they cause the electrons in the gold atoms to jump up one energy state and when they drop back they produce one "gold" photon that then impinges on our inner eyes and our brain interprets it as what we call "gold". The energy not so used by the photons hitting the gold goes into making the bar of gold slightly hotter.

 

[sophiecentaur]

The atomic composition. Gold, for example, is all one kind of atom and so has a single characteristic wavelength that is emitted by the excitation caused by impinging photons. The impinging photons have various levels of energy and when they are strong enough, they cause the electrons in the gold atoms to jump up one energy state and when they drop back they produce one "gold" photon that then impinges on our inner eyes and our brain interprets it as what we call "gold". The energy not so used by the photons hitting the gold goes into making the bar of gold slightly hotter.

In a dense medium like a metal, the electrons do not "jump up an energy state". There is a continuum of energy levels (an energy band) and transitions of all values between limits can have any value. It is important to recognise that the H Atom model is not enough to deal with solid state situations. The reflection will be coloured slightly because of the details of the metal structure where certain frequencies may lose a different amount of energy in the reflection process.

 

[phinds]

In a dense medium like a metal, the electrons do not "jump up an energy state". There is a continuum of energy levels (an energy band) and transitions of all values between limits can have any value. It is important to recognise that the H Atom model is not enough to deal with solid state situations. The reflection will be coloured slightly because of the details of the metal structure where certain frequencies may lose a different amount of energy in the reflection process.

Interesting. Thanks for that correction.

 

[jerromyjon]

A basic example is green plants, the chlorophyll in the leaves absorbs the red and blue for energy and the green is reflected.

I googled and found this: https://www.itp.uni-hannover.de/~zawischa/ITP/origins.html

Haven't gone though that page yet, but I'm pretty sure it has to do with the surface texture for the simple case of reflection, various atoms also absorb distinct narrow bands as the energy per photon has to be exactly right to bump an electron n shells further from the nucleus (like phinds said), and just the first few seconds of scanning that webpage shows it can get a lot more complicated than just simple "this makes it red, this makes it green, this makes it blue"

 

[sophiecentaur]

A basic example is green plants, the chlorophyll in the leaves absorbs the red and blue for energy and the green is reflected.

I googled and found this: https://www.itp.uni-hannover.de/~zawischa/ITP/origins.html

Haven't gone though that page yet, but I'm pretty sure it has to do with the surface texture for the simple case of reflection, various atoms also absorb distinct narrow bands as the energy per photon has to be exactly right to bump an electron n shells further from the nucleus (like phinds said), and just the first few seconds of scanning that webpage shows it can get a lot more complicated than just simple "this makes it red, this makes it green, this makes it blue"

This only applies in the case of gases. In dense media, there is a continuum of energy states within 'bands', rather than isolated levels. Often, within a wide band of energies, the light (other EM wavelengths also ) can be absorbed and give the surface 'colour'. You never get monochromatic light with pigments and dyes.

I agree with you that statements like "this makes it red, this makes it green, this makes it blue" are wild over-simplifications and are only fit for Primary School classrooms.

 

[oreo]

In a dense medium like a metal, the electrons do not "jump up an energy state". There is a continuum of energy levels (an energy band) and transitions of all values between limits can have any value. It is important to recognise that the H Atom model is not enough to deal with solid state situations. The reflection will be coloured slightly because of the details of the metal structure where certain frequencies may lose a different amount of energy in the reflection process.

Thank you. You have given a very good and logical explanation. Thanks again for the answer.

 

[jerromyjon]

This only applies in the case of gases

I was debating on the gas chromatograph approach with colored flames, etc. or the organochemical basis which really seems simple from what I'm reading... I'm into the energy diffusion widening the spectral bands (like you were heading it seems) related to the freedom of electrons transiting the double bonds in molecules.

 

[oreo]

In a dense medium like a metal, the electrons do not "jump up an energy state". There is a continuum of energy levels (an energy band) and transitions of all values between limits can have any value. It is important to recognise that the H Atom model is not enough to deal with solid state situations. The reflection will be coloured slightly because of the details of the metal structure where certain frequencies may lose a different amount of energy in the reflection process.

It means that it depends on the type of atoms but what about that lizard which changes its colour.

 

[sophiecentaur]

I was debating on the gas chromatograph approach with colored flames, etc. or the organochemical basis which really seems simple from what I'm reading... I'm into the energy diffusion widening the spectral bands (like you were heading it seems) related to the freedom of electrons transiting the double bonds in molecules.

I put it down to the Paui Exclusion Principle, basically. Nearby atoms cannot have the same states so they mutually widen the lines into bands.

 

[sophiecentaur]

It means that it depends on the type of atoms but what about that lizard which changes its colour.

Lizards (chamaeleons?) and several fish / octopi, have tiny sacs of different pigment that they can squeeze and control which pigment is closest to the surface. It works like Pointilistic Painting and an inkjet printer. No actual 'chemistry' involved to vary the colour.

 

[my2cts]

For the general answer study solid state physics. This is a summary.
The valence electrons in constituents of material absorb certain wavelengths. The remaining wavelengths are reflected or transmitted so that the material displays a colour. Insulators, such as diamond, kitchen salt, glass and pure water do not absorb optical wavelengths and are transparent. The can acquire colour if impurities are present. An example is ruby, which is transparent aluminium oxide with dissolved cromium ions. Metals do not transmit but reflect all optical wavelengths because their valence electrons can move freely. Some color is exhibited if the reflection is not equally intense across the optical spectrum.