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Why is milk white ?

  1. Nov 17, 2012 #1
    Hello

    Just wondering why milk is white... I thought it was similar to the reason that clouds are white - ie. clouds are white because the water droplets scatter all wavelengths of light.
    So I thought then that milk was white because the droplets of fat suspended in the milk also scatter all wavelengths of light... I had cause to doubt this because I did a quick google search and all answers I found suggested milk was white because it contained lots of substances that are themselves white... .
     
  2. jcsd
  3. Nov 17, 2012 #2

    Drakkith

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    From this link: http://en.wikipedia.org/wiki/Milk#Appearance

    Both the fat globules and the smaller casein micelles, which are just large enough to deflect light, contribute to the opaque white color of milk. The fat globules contain some yellow-orange carotene, enough in some breeds (such as Guernsey and Jersey cattle) to impart a golden or "creamy" hue to a glass of milk. The riboflavin in the whey portion of milk has a greenish color, which sometimes can be discerned in skimmed milk or whey products.[8] Fat-free skimmed milk has only the casein micelles to scatter light, and they tend to scatter shorter-wavelength blue light more than they do red, giving skimmed milk a bluish tint.
     
  4. Nov 17, 2012 #3
    A question on the first line:

    "Both the fat globules and the smaller casein micelles, which are just large enough to deflect light, contribute to the opaque white color of milk."

    The word "deflect" is used - is deflect a synonym for "scatter" in this context?
     
  5. Nov 17, 2012 #4

    Drakkith

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    I assume so.
     
  6. Nov 17, 2012 #5

    sophiecentaur

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    The particles of fat are very large compared with the wavelength of light so the light is all scattered an equal amount. When the particles are really small, comparable with the wavelength, the shorter wavelengths are scattered more than the long. Hence, the sky (with air molecules) looks blue(ish). Although looking 'very blue', it isn't pure spectral (it's very de-saturated) blue but a mixture of all visible wavelengths with slightly more blue than the others. If it were spectral blue, things viewed in the shade (no direct Sunlight) would all look various levels of blue.
     
  7. Nov 17, 2012 #6
    Thanks SophieCentaur!

    Follow up question(s):

    1. When clouds appear gray - is that because of a polarization phenomenon?
    That is, I know scattered light becomes significantly polarized, so would I be correct in saying that grey clouds act as a polarizing filter?

    2. What is the difference in circumstance that leaves us sometimes with white clouds and sometimes with dark or gray clouds? - My guess would be that only certain cloud-types can appear gray - but if you could provide any further info, would be great!
     
  8. Nov 17, 2012 #7
    One additional thing that comes to mind re. fat in milk:

    If you have a block of fat or lard it appears white - but when considering a block of fat or lard it cannot be the same scattering mechanism that makes milk white. Because a block of lard doesn't have little globules of fat that scatter light but is simply one big block of fat.
    Why in the second case does the block of lard appear white?

    Can you be absolutely sure that the reason the single block of lard appears white (it is probably correct to say that the lard has high opacity) is not the reason that the glass of milk does? - that is maybe all of the globules suspended throughout the milk add up so that they too are significantly large in extent to become opaque. (or to put it another way, if you isolated all of the globules of fat and created a block of pure fat out of them, that block of fat would be large enough to be opaque....).
     
  9. Nov 17, 2012 #8

    sophiecentaur

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    Clouds appear grey because they block out light of all wavelengths more or less equally - thicker = darker.

    Lard has many small imperfections in its structure which cause reflections. With many fats and waxes, when they melt they go clearer because the substance becomes more regular. Cloudy again once they solidify. You must have seen candle wax do this.
     
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