Ian Ford, of Oamaru, asks :-

How can a brown cow, who eats only green grass, produce only white milk?

John Campbell, a physicist at the University of Canterbury, responded.

How can a black cow, who eats only yellow grain and hay, produce only white milk? You are confusing appearance with output.

We see objects because light (e.g. from the sun) is reflected from the object into our eyes. Sunlight appears white (containing red through purple light) as it contains all the colours (frequencies) our eyes respond to. When reflected off grass we see it as green because different wavelengths are absorbed (or reflected) differently. In grasses case, its function is to capture light (blue and red, reflecting only green) to allow photosynthesis, which is the conversion of light energy into molecules that store energy which an animal can obtain by eating the grass. It is a storage device. In doing so, it takes out of the atmosphere carbon dioxide and gives out a "waste" product called oxygen. So the green of grass is due to the photosynthesis process and plant life is vital to our life on earth.

The cow appears brown because pigments in its hide absorb the other colours from white light.

Milk is composed of tiny globules of fat in water. When white light is scattered by "small" objects (think white clouds of water droplets) the light is scattered in all directions and appears white. When the fat globules are joined together, as in butter, it is yellow.

So the cow eats grass (energy), and its gut produces a watery output (milk) which is dispersed tiny yellow droplets of fat in water which appears white due to the way light is scattered from small droplets.

Sometimes the milk is other colours due to dyes in the eaten food. I heard of one breastfeeding woman who had to freeze her milk when she had too much. One bag was yellow, due to the turmeric in the curry that night.

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