Saturday, August 04, 2007

Why the sky looks blue

When I observe the manner in which science is taught to my daughters today, I am disappointed to find that there is hardly any change in the teaching methods in the last several decades. True, the subject is a lot wider now, lessons have changed and many things that I learned in college are included in the syllabus of the eight standard. But, the lessons are still imparted in the same boring manner; the teacher does nothing to fire the student’s imagination; both do not have the luxury of time.

These thoughts came to me when I read this piece written, in 1931, by Sir James Jeans, a reputed astronomer,. This was originally part of his book, “Stars in their courses”, but I found it in a book called “Province of Prose”, where it was showcased as an example of good prose writing and how a difficult subject can, in the hands of an expert, lend itself to clarification by analogy.

I remember the drab explanation that my own teachers had provided to explain why the sky was blue. With a smattering of Rayleigh scattering, followed by some long equations. Look how Sir James Jeans has handled the same question, seventy five years back.

Imagine that we stand on an ordinary seaside pier and watch the waves rolling in and striking against the iron columns of the pier. Large waves pay very little attention to the columns- they divide right and left and reunite after passing each column, much as a regiment of soldiers would if a tree stood in their road; it is almost as though the column had not been there. But the short waves and ripples find the columns of the pier a much formidable obstacle. When the short waves impinge on the columns, they are reflected back and spread as new ripples in all directions. To use the technical term, they are ‘scattered’. The obstacle provided by the iron columns hardly affects the long waves at all, but scatters the short ripples.

We have been watching a sort of working model of the way in which sunlight struggles through the earth’s atmosphere. Between us on Earth and outer space, the atmosphere interposes innumerable obstacles in the form of molecules of air, tiny droplets of water and small particles of dust. These are represented by the columns of the pier.

The waves of the sea represent the sunlight. We know that sunlight is a blend of many colors- as we can prove for ourselves by passing it through a prism, or even a jug of water or as nature demonstrates to us when she passes it through the raindrops of a summer shower and produces a rainbow. We also know that light consists of waves, and that the different colors of light are produced by waves of different lengths, red light by long waves and blue light by short waves. The mixture of waves which constitutes sunlight has to struggle past the columns of the pier. And these obstacles treat the light waves, much as the columns of the pier treat the sea-waves. The long waves which constitute red light are hardly affected but the short waves which constitute blue light are scattered in all directions.

Thus the different constituents of sunlight are treated in different ways as they struggle through the earth’s atmosphere. A wave of blue light may be scattered by a dust particle and turned out of its course, and so on, until finally it enters our eyes by a path as zigzag as that of a flash of lightning. Consequently the blue waves of the sunlight enter our eyes from all directions.

And that is why the sky looks blue.


Philip said...

Beautiful explanation

Shruthi said...

Ah, lovely! Thanks!

Raj said...

philip: true.

Shruthi, thanks

Revathi said...


Raj said...

revathi, thanks

Anonymous said...

Raj The first time that I have partly understood the concept of scattering of light. As I have always mentioned in the recent past - you are a prolific reader, fantastic blogger, a very very humorous and intelligent guy. Am I sounding too gooey. So be it, but just stating facts.

Anonymous said...

Raj, thank you for posting this short essay. I am an English professor and have been looking for it for quite awhile... you made my life easier.

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