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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.
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.
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.
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.
