## Understanding colour and reflection

Discussions on classical and modern physics, quantum mechanics, particle physics, thermodynamics, general and special relativity, etc.

### Understanding colour and reflection

Hi, I recently posted on another well known science forum and learned something I didn't know as a result, but unfortunately I didn't get any responses to tell me if my interpretation was correct. So I thought I'd ask here. I guess I started out confused and made it worse in trying to explain my understanding so here I will just present how I have come to understand this particular concept.

When I read about why objects appear to have colour, most of the time I read something along the lines that certain wavelengths of EMR are absorbed and some are reflected. Just to clarify, I am not necessarily asking about the perception of colour, I am more trying to grasp the underlying physical phenomenon of how light is affected by different surfaces. In raw physical terms, I thought that the colour of objects depended upon which wavelengths are reflected and which are absorbed, however, it seems there is more to it than that, a fact I hadn't appreciated previously. I will explain.

Note that in my discussion below, I am restricting myself to talking about a simplified representative case, for example a smooth blue cube of a material that can be polished.

Imagine the cube is standing on a table in a room lit by the sun alone shining in through windows. No direct sunlight is falling on the cube, in fact let's say we have thin gauze curtains diffusing the light entering the room and the ambient light is relatively uniform.

If the blue cube's surface is somewhat rough it will reflect light diffusely and will appear blue. That is, light rays striking the surface are reflected at various angles such that at any particular point in space in line of sight to the cube, a measuring device pointed at the cube will measure a greater intensity of blue wavelengths, even though the light from the cube will be composed of all wavelengths in the visible spectrum.

The smoother the cube's surface, the less this is so in the case that other reflecting objects are also present in the room. That is, as we make the cube's surface smoother, the light reflected from other objects that then falls upon the cube will be reflected back at angles approaching the angle of incidence. Here, light is reflected specularly. A measuring device pointed at the smoother cube will tend to record wavelengths that peak in wavelengths corresponding to the "colour" of those surrounding objects, depending on where on the cube the device is pointed.

This means that the colour of an object, as described in terms that relate specifically to the actual physical phenomenon rather than our perception, depends upon both its composition AND its smoothness. The smoother an object, the more its surface will reflect light in specular fashion and hence its colour will be more dependent upon the colour of surrounding objects. That is, when it has a rough surface the cube will appear blue, when it is smoother it will tend to be multi-coloured.

Put into more general physical terms, the field of light visible to a recording device pointed at the rough cube will be dominated by blue wavelengths, whereas the smoother the cube the more the field will be dominated by wavelengths corresponding to wavelengths of light from surrounding objects. For a perfectly smooth surfaced cube, recorded wavelengths will be so dominated by the effects of surrounding light that the wavelength distribution at each point on the cube's surface will more closely match that of the surrounding objects themselves. In perceptual terms, the cube's surface will be dominated by reflected images of surrounding objects.

Is that a good enough approximation of the physical event?
Graeme M
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### Re: Understanding colour and reflection

Graeme M wrote:
Is that a good enough approximation of the physical event?

Your explanation sounds good to me. Some light is selectively absorbed and some light is reflected. Light from a rough surface is reflected in all directions and light from a polished surface is reflected at the same angle as the angle of incidence. A blue object appears blue because it isn't able to absorb certain wavelengths of light from the blue part of the spectrum but it can reflect light of any color because not all of the non-blue light is necessarily absorbed.

A blue object with a polished surface can direct a narrow beam of non-blue light to a detector but not so for one with a rough surface because the light is scattered in all directions.
bangstrom
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### Re: Understanding colour and reflection

bangstrom » March 17th, 2019, 1:24 am wrote:A blue object with a polished surface can direct a narrow beam of non-blue light to a detector but not so for one with a rough surface because the light is scattered in all directions.

By "blue" do you mean light of a particular wavelength? Or the subjective perception of the color blue? Surely the subjective perception does not inhere in the object itself. See https://en.wikipedia.org/wiki/Inverted_spectrum for example.
someguy1
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### Re: Understanding colour and reflection

someguy1 » March 17th, 2019, 5:05 pm wrote:

By "blue" do you mean light of a particular wavelength? Or the subjective perception of the color blue? Surely the subjective perception does not inhere in the object itself. See https://en.wikipedia.org/wiki/Inverted_spectrum for example.

The question was about an object that reflects light mainly in the blue spectrum ignoring our perception of blue. The color of the object is not important. A black object can absorb light of all colors but not all incoming light is absorbed so any color not absorbed is reflected without change so a blue object (or any other color) can reflect any color of light. A smooth, polished surface reflects light in specific directions like a mirror but a rough surface scatters light in all directions.
bangstrom
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### Re: Understanding colour and reflection

Thanks for the reply bangstrom, I am glad I am on the right track. On a bit of a tangent, I gather that as a surface becomes smoother specular reflection comes to dominate. This means light rays are increasingly reflected at close to the same angle from the normal as the original ray. I understand that a "ray" is a kind of idealised model of light propagation, would I be right to believe that any point on a surface that reflects incident light generates an infinite number of rays in all possible directions? And by point, do I mean atom?
Graeme M
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### Re: Understanding colour and reflection

At that scale, quantum reality takes over, and we are dealing with quanta of light, AKA photons. So rays would not be infinite in number.

TheVat

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### Re: Understanding colour and reflection

Graeme M » March 18th, 2019, 5:50 am wrote: I gather that as a surface becomes smoother specular reflection comes to dominate. This means light rays are increasingly reflected at close to the same angle from the normal as the original ray. I understand that a "ray" is a kind of idealised model of light propagation, would I be right to believe that any point on a surface that reflects incident light generates an infinite number of rays in all possible directions? And by point, do I mean atom?

By light “rays” I mean light in parallel directions rather than randomly scattered. Richard Feynman in QED explains how a reflection comes to dominate. We can take a lump of glass and make the surface flat by making scratches in the glass with an abrasive. Large scratches make the surface dull but the surface becomes reflective when we use a finer abrasive and the scratches are closer together. As the scratches come closer together, the surface becomes more reflective until the scratches are less than a wavelength apart. This is the point where the reflection becomes specular. No more finer polishing can improve the reflection beyond this point because it is as good as it can get even though a wavelength of light is much larger than an atom.

Your mention of an infinite number of rays recalls the Wheeler-Feynman Absorber Theory for light. A single photon striking a single atom can be reflected in many different directions, but if the atom is part of a flat surface, all photons reflect parallel in the same direction. So how does the single photon “know” it is on a flat surface well beyond the curved surface of a single atom?

The W-F absorber theory explains that the photon reflects in every possible direction at every possible speed including all speeds in reverse and then all paths taken by the photon that are not consistent with the surface structure and special relativity cancel and adsorb each other leaving only the one path that we observe. The W-F absorber theory explains the properties of light perfectly well but it never gained much traction because of some creditability issues.
bangstrom
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### Re: Understanding colour and reflection

Thanks bangstrom. You raise some interesting points and yes, I expected QM to come into play. I have to admit I didn't quite follow some of what you said. I'm not that knowledgeable about physics.

You said that you mean light rays to be "light in parallel directions rather than randomly scattered". Can you explain for me what you mean by that? I thought that for a given flat surface, light shining on it will be reflected in all directions (ie randomly scattered). That is, all light can be described as being composed of "rays".

Also, you say that all photons reflect in the same direction from a flat surface. Do you mean that across a particular flat surface, all photons from incident light are reflected in parallel? Or do you mean that for the angle of incidence of a reflected ray chosen at random, there is always a subset of all reflected rays in which the rays are parallel to the selected ray? I am not sure I am being clear here, but it's not immediately obvious to me how the set of all reflected rays can be parallel, if light is coming from all directions onto the flat surface.

I guess the problem here is trying to take a classical view of a quantum phenomenon, but clearly light is reflected and we observe certain characteristics so I am really more interested in the macro, classical explanation.
Graeme M
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### Re: Understanding colour and reflection

Photons reflect from a flat surface at the same angle as the angle of incidence if the surface is smooth and polished but they scatter if the surface is rough. Light from a single point arrives and reflects in straight, parallel lines without scatter if the surface is polished until it shines.

The part about reflected light being a subset of all reflected rays comes from the old Wheeler-Feynman “sum over histories” idea which is best forgotten because it is probably wrong and it only adds to the confusion even though the theory works.
bangstrom
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### Re: Understanding colour and reflection

Hi bangstrom, I'm not quite sure I follow your first para in the context of the other comments above but I think you are saying that light from a single point is always reflected at the same angle for a very smooth surface, but more randomly the less smooth the surface. Though, would it also make sense to say that a single ray of light always reflects at the same angle from any surface (ie the same angle from the normal as its angle of incidence), but that rougher surfaces present more and different surface planes than a smooth surface? So light is always reflected at the same angle relative to its reflecting surface, but rougher surfaces have many differently angled reflecting surfaces.
Graeme M
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### Re: Understanding colour and reflection

Hi Graeme, I had to look this one up a bit but found some good videos on YouTube. Specular reflection, sometimes called regular reflection, is due to the light reflecting off a surface such that the angle of incidence is equal to the angle of reflection. Diffuse reflection on the other hand, results as you say from rough surfaces such that the light is reflected in every direction.

Here's a very short vid that explains it:

Here's a much longer one, more detail:

I think this other video gets to the point regarding light and color:

This is really interesting I thought as well.

Best,
Dave.

Dave_C
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### Re: Understanding colour and reflection

Graeme M » March 23rd, 2019, 6:18 pm wrote:Hi bangstrom, I'm not quite sure I follow your first para in the context of the other comments above but I think you are saying that light from a single point is always reflected at the same angle for a very smooth surface, but more randomly the less smooth the surface. Though, would it also make sense to say that a single ray of light always reflects at the same angle from any surface (ie the same angle from the normal as its angle of incidence), but that rougher surfaces present more and different surface planes than a smooth surface? So light is always reflected at the same angle relative to its reflecting surface, but rougher surfaces have many differently angled reflecting surfaces.

Your explanation is classical and correct but up to a limit since no surface is ever truly flat. For light, a surface becomes “flat” at the point where the surface irregularities that cause light to reflect in many directions are closer together than the wavelength of the reflected light. This is the point where the direction of reflection becomes the collective surface of a large number of atoms and the roughness of the surface no longer scatters light in different directions otherwise there would never be such a thing as specular reflection.

Light does not reflect from a “single point” rather it reflects from a collective surface having the radius of a wavelength of light. A single “point” on a material surface would be found on the surface of a single atom, or more correctly, on the surface of a single electron and these are not flat surfaces.
bangstrom
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### Re: Understanding colour and reflection

I think you are agreeing with my suggestion? If a surface has irregularities greater than the wavelength of light then it would be presenting genuine flat planes at varying angles to the incident light, wouldn't it? At significant scales these flat surfaces would dominate, as you say. But if the irregularities are less than the wavelength of light then in effect a surface is close to being a single plane, particularly from the point of view of atoms which are significantly smaller than the scale we are talking about. So it seems correct to say that light always reflects at the same angle as the angle of incidence, what changes things at the macro scale is the degree of surface irregularities and hence possible planes for reflection.

Is that a reasonable layperson's summary of what you've said?
Graeme M
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### Re: Understanding colour and reflection

Your summary is correct. A surface is optically flat and light reflects at the same angle as the angle of incidence when any irregularities from the plane are less than a wavelength apart.
bangstrom
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