THE SCIENTIFIC MIND

July 11th,2007

WHY IS THE MILK WHITE?

With due credit to the professor who has answered this question………

The simple answer to this straight question is: “The Tyndall Effect gives milk its white colour”. Now let us see what is this Tyndall Effect and how it is associated with milk and its white ‘colour’.

Milk is an admixture of several nutritious and mineral constituents that are essential to the newborn babies of the respective mammalian mother. Water constitutes more than 80% of milk’s mass and volume. Proteins (mainly casein), carbohydrates (mainly lactose sugar), lipids, phospholipids, etc make the rest. Some inorganic salts, vitamins and cholesterol are also present at low concentrations. Whereas the salts and some low molecular weight organic ingredients are present at molecular dimensions, the macromolecular nutrient constituents, such as proteins, carbohydrates, lipids, etc, are as suspended aggregates, micelles and polymeric globules and in micron scale. Such a system of admixture wherein the micron sized particles are suspended uniformly in a large volume of a hosting medium is called a ‘colloid’. Milk is a colloid.

Depending on the natural state of the suspended particles, called the ‘disperse phase’ and that of the host, called the ‘disperse medium’ (in the case of milk, it is water), we have many names such as gel, emulsion, sol, foam, etc. among others for colloids. On this classification milk is partly an emulsion (liquid guest in liquid host) and partly a sol (solid guest in liquid host) because the fat contents are naturally liquids whereas protein and carbohydrate constituents are solids at ambient conditions.

The important optical property of most of the colloids is that the disperse phase exists at dimensions comparable to the size of the wavelengths (400 to 800 nanometers or in fraction of a micron) of visible region of light. Whenever light falls on particles of a size comparable to or slightly larger than the wavelength of the incident light, the particles scatter the radiation in all directions after initial absorption and re-emission without changes in wavelengths. This is known as Raleigh Scattering (unlike Raman scattering where light is absorbed but wavelengths are altered before scattering). Since every part (all seven colours) of the visible light is scattered in all directions by milk, milk appears white. Such a non-absorptive Raleigh Scattering performed by colloids is called ‘Tyndall Effect’. That we see a piece of chalk, mist, fog, smoke, crushed ice, ground glass, talcum powder, cotton wool, rice dough, projector light path in a theatre, etc, white is just due to this Tyndall Effect. Phenol water, certain tree saps, etc also appear white because of the same effect. Many crystals, glass sheets, water, kerosene, salt, sugar, air, etc do not appear white because their constituents are at atomic and molecular scale (about 1 nanometer) and are not in sizes comparable to the wavelengths of visible light. The visible light which eventually finds its way through the medium, is simply transmitted. So we call these substances clear and transparent.

June 8th,2008

I LIKE THE SMELL OF THE GRASS AFTER IT RAINS

Rain Rain Come Again

Little Johnny wants to play

May be little johnny likes rain and loves to play in it,Nirveda doesn’t.I don’t like rain as such,but there are certain things..the cool breeze,the rainbow,the smell of the grass after it rains and other similar things that i almost love to experience.The smell of the grass after it rains is my favorite.so,i’ve just been wondering the science that  goes behind it..here’s what i found out

It is actually caused by bacteria! Actinomycetes, a type of filamentous bacteria, grow in soil when conditions are damp and warm. When the soil dries out, the bacteria produces spores in the soil. The wetness and force of rainfall kick these tiny spores up into the air where the moisture after a rain acts as an aerosol (just like an aerosol air freshener). The moist air easily carries the spores to us so we breathe them in. These spores have a distinctive, earthy smell we often associate with rainfall. The bacteria is extremely common and can be found in areas all over the world, which accounts for the universality of this sweet “after-the-rain” smell. Since the bacteria thrives in moist soil but releases the spores once the soil dries out, the smell is most acute after a rain that follows a dry spell, although you’ll notice it to some degree after most rainstorms.

There is another explanation to this stuff…..read on

After-the-rain smell comes from volatile oils that plants and trees release. The oil then collects on surfaces such as rocks. The rain reacts with the oil on the rocks and carries it as a gas through the air. This scent is like the bacteria spores in that most people consider it a pleasant, fresh smell. It has even been bottled and sold for its aromatic qualities!

ok,now that’s some nature science for today…whatever may the theory behind the happenings…the most aromatic smell which no man made scent can replace is the after-the -rain smell.No wonder it is so widely referred  to by poets…especially the romantic ones!