I am talking about the standard fats we see in everyday use (triglycerides). It seems that no matter what the composition, saturated or unsaturated, all fats are glossy when liquid. I don't know if this extends to other esters, as I have seen only few of them.

What gives fats this shiny look?

Sorry if the question appears unresearched, but all my attempts to combine "fat" and "glossy" in a search query resulted in nutrition advice.

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    $\begingroup$ Come to think of it, aren't all liquids glossy? $\endgroup$ Dec 15 '15 at 17:47
  • $\begingroup$ If they all are, then I want to know why all liquids are glossy :) But that would be a very hard claim to make. If you are looking at anything liquid as defined in layman terms, then many suspensions and runny emulsions count, and they are not glossy. If you restrict yourself to a single compound in a liquid phase (as long as it has a liquid phase at room temperature - I wouldn't describe molten iron as glossy) then everyday fats are excluded, because they are mixtures. So, the two questions may not have that much of an overlap. $\endgroup$
    – rumtscho
    Dec 15 '15 at 18:10
  • $\begingroup$ OK, let's refine my claim. Aren't all transparent liquids at room temperature glossy? (That would exclude emulsions, suspensions, and even some individual compounds, but leave many mixtures which are true solutions.) $\endgroup$ Dec 15 '15 at 20:18
  • $\begingroup$ It will depend on your angle of incidence. From the Fresnel equations the reflectivity depends greatly on angle of incidence so that even an ordinary flat piece of paper appears reflective if the angle of incidence is almost 90 degrees, i.e. at glancing angle whereas it appears matt at zero degrees. $\endgroup$
    – porphyrin
    Apr 12 '19 at 14:57

Reflected light (gloss) is a maximum when the angle of observation is equal to the angle of incidence: https://en.wikipedia.org/wiki/Gloss_(optics). The image shows reflection from a plane surface with some roughness that gives diffuse reflection.

enter image description here

At Brewster's angle, the reflected ray is strongly polarized. The image is intended to indicate a depth of penetration in transparent materials; the polarization is not relevant here.

enter image description here

In conductors, like metals, and strong absorbers, like iodine, there is essentially no transmission, and almost all the light is reflected from a plane surface, because the penetration depth is exceedingly small.

In a dielectric (i.e., a nonconductor like glass or fats), some light will be transmitted, and reflection occurs after a penetration of some depth - on the order of a wavelength of the light. Glass is atomically uniform on this scale, and a fat molecule (e.g., glyceryl trioleate) would be a glob about 2 or 3 millimicrons in diameter, therefore essentially homogeneous on this scale. Any liquid volume larger than that would have no discernible inhomogeneity and reflection would be a uniform wavefront, specular (mirror-like) rather than diffuse.

But suspensions and runny emulsions and milk are not homogeneous on that scale. They have particles on the order of a wavelength of light (~500 millimicrons) in the liquid, so reflection occurs from sites of different composition within this penetration depth. These materials reflect similar to reflection from a roughened metal surface and give a diffuse reflection rather than a gloss.


The equation for reflection ($\mathrm R_0$) of interfaces of clear mediums is given by:

$$\mathrm R_0 = \left[ \frac{n_1 - n_2}{n_1 + n_2} \right] ^2$$

where $n_i$ is the refractive index of its respective medium. If air is $n_1 = 1.00$ and olive oil is $n_2 = 1.47$ (air-oil interface) then

$$\mathrm R_0 = \left[ \frac{1.00 - 1.47}{1.00 + 1.47} \right]^2 = \left[ \frac{-0.47}{2.47} \right]^2 = 0.1903^2 = 3.62\%$$ Therefore $3.62\%$ of light will be reflected off of the smooth surface of the oil giving it the glossy appearance.


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