Hello! This Wednesday, we discussed a very simple looking question...Why the sky is blue? The answer to this question comes in one word as "Scattering of light"! But what exactly this scattering is and what is a thing scattering the light? Well, when we talk about sky color we are essentially talking about Rayleigh scattering!

Rayleigh scattering is scattering of light due to particles much smaller than wavelength of light. It occurs when light travels in transparent solids and liquids, but is most prominently seen in gases. The intensity of scattered light varies as inverse fourth power of wavelength of light. So for shorter wavelengths , scattering is more! In the visible spectrum, red light has a wavelength of 720 nm whereas blue has 380 nm. So the if we calculate, the blue light is scattered almost 10 times more than red light and this is a reason why our sky is blue.

Well, our eye structure also plays an important role in the sky looking blue. There are 3 types of receptors in our eyes - Red(R),Green(G) and Blue(B). Out of these 3, B receptors respond more strongly as compared to R and G.

Now, one common misconception is this scattering is due to dust particles! But that's not the case...had it been due to dust particles or droplets of water, the sky color would have changed every now and then as the humidity or haze conditions changed! It was later proved by Einstein that the oxygen and nitrogen molecules produce this Rayleigh scattering.

And then, if smaller the wavelength of light, more the scattering then sky should look violet but this isn't the case! The reason is the violet light scattered in a high amount, is absorbed in high atmosphere only so what reaches earth is the blue light only!

Now the obvious question comes...why the sky is red at sunrise and sunset? The reason again lies in scattering of light! At Sunrise or Sunset, the Sun is at horizon and hence, the light rays travel greater distance. In between blue color is lost and what we see is a red light and similar shades!

In the end, we come down to a very basic question -
What is exactly happening in "scattering" or why molecules "scatter" the light? Well, the molecules scatter light because the electromagnetic field of light waves induces electric dipole moments in the molecules.

Going along the same line, we can find out why clouds are white and why fire looks red? Any guesses?

Stellar Population

Last Wednesday (4th July) at Sion we discussed about the Population of stars. There were around 10-15 people present for the discussion. Sukhada started the topic and gave some information and then we discussed about few things.

Stars are divided into three populations according to metallicity. Metallicity is the proportion of the elements present in the star other than hydrogen and helium to the mass of the star. The population 3 (or generation 1) stars were formed just after the big bang and were very massive i.e. about 100 Solar masses. They lived for very short period of few million years and died off. They did not have any metal content (here metal means elements other than hydrogen and helium) and there is no star found to be population 3 in today’s universe. They formed little amount of elements upto iron in their core which increased the metallicity of further stars.

Population 2 (and generation 2) stars were formed after the death of the population 3 stars and hence they contain very little amount of metallicity (about 0.1%). Population 2 stars are mainly seen near the centre of Galaxies and hence most of the globular clusters are formed by these stars. Population 1 (or generation 3) stars are metal rich stars which show metallicity of about 2-3 %. The metallicity is also responsible to form the planetary systems to form around these stars. Our Sun is population 1 star and it’s metallicity is around 2 %.

Unsolved questions:
1. Metallicity of stars is calculated with respect to Sun’s metallicity, then how Sun’s metallicity is calculated?
2. How to calculate the mass of particular element in a star or say Sun by analyzing the spectrum?

Topics for next week:
1. Initial Mass Function.
2. Scattering of light.
3. How the number of electrons for the particular element is determined or basically in History how they determined the number of electrons present in the atom of a particular element.