Why the Sky is blue ?

The daytime sky on Earth appears blue in colour. Light is made up of PHOTONS which are matterless particles. White light is made up of seven colours: red, orange, yellow, green, blue, violet & indigo.

This light travels at different wavelengths. Some colours are better at passing through gases than others. Red light is seen at longer wavelengths and indigo has the shortest wavelength.

When photons from the Sun travel into Earth's atmosphere some of this light hits and bounces the nitrogen and oxygen molecules there. The result is that light is split up and scattered.

Red, orange, yellow and green light carries straight on through this atmosphere but blue, violet and indigo get bounced around from molecule to molecule.

We see a blue sky as a result of all this bouncing around.

Why don't we see an Indigo or Violet sky? Simply because our eyes do not see these colours very well. Blue is a more dominant colour.

Further evidence of this can be seen during sunset and sunrise. The sky appears red then as the light travels through more of Earth's atmosphere. Most of the blue light is scattered away. Also during a lunar eclipse the Moon appears red as light travels through Earth's atmosphere.

What is a light-year and how is it used?

A light-year is a unit of distance. It is the distance that light can travel in one year. Light moves at a velocity of about 300,000 kilometers (km) each second. So in one year, it can travel about 10 trillion km. More p recisely, one light-year is equal to 9,500,000,000,000 kilometers.

Why would you want such a big unit of distance? Well, on Earth, a kilometer may be just fine. It is a few hundred kilometers from New York City to Washington, DC; it is a few thousand kilometers from California to Maine. In the universe, the kilometer is just too small to be useful. For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km. That's 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance.

In our solar system, we tend to describe distances in terms of the Astronomical Unit (AU). The AU is defined as the average distance between the Earth and the Sun. It is approximately 150 million km (93 million miles). Mercury can be said to be about 1/3 of an AU from the Sun and Pluto averages about 40 AU from the Sun. The AU, however, is not big enough of a unit when we start talking about distances to objects outside our solar system.

For distances to other parts of the Milky Way Galaxy (or even further), astronomers use units of the light-year or the parsec . The light-year we have already defined. The parsec is equal to 3.3 light-years. 

Using the light-year, we can say that :

• The Crab supernova remnant is about 4,000 light-years away.
• The Milky Way Galaxy is about 150,000 light-years across.
• The Andromeda Galaxy is 2.3 million light-years away.

What is Fraunhofer Spectram?

Fraunhofer Spectrum also called as Fraunhofer lines are a set of spectral lines named after the German physicist Joseph von Fraunhofer. The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun.

In 1802, a scientist called W.H. Wollaston noticed that the visible spectrum from the Sun had several dark lines in it. Not long afterwards, Joseph von Fraunhofer built the first spectrometer. This focused sunlight from a small telescope onto a narrow slit. The light then passed through a prism, which produced the spectrum. Fraunhofer later invented the diffraction grating, which is used in most spectrometers today. Fraunhofer not only confirmed Wollaston's results, but also found that there were far more dark lines in the spectrum than Wollaston had suspected. Fraunhofer showed that these were a feature of sunlight and not an illusion nor an optical effect, and he labelled them with letters of the alphabet (A,B,C etc.). We now call these dark lines Fraunhofer lines.

what is fermion?

Fermions are particles which have half-integer spin and therefore are constrained by the Pauli exclusion principle. Particles with integer spin are called bosons. Fermions include electrons, protons, neutrons. The wavefunction which describes a collection of fermions must be antisymmetric with respect to the exchange of identical particles, while the wavefunction for a collection of bosons is symmetric.

The fact that electrons are fermions is foundational to the buildup of the periodic table of the elements since there can be only one electron for each state in an atom (only one electron for each possible set of quantum numbers). The fermion nature of electrons also governs the behavior of electrons in a metal where at low temperatures all the low energy states are filled up to a level called the Fermi energy. This filling of states is described by Fermi-Dirac statistics.

What exactly is the Higgs boson?

Particle physics usually has a hard time competing with politics and celebrity gossip for headlines, but the Higgs boson has garnered some serious attention. That's exactly what happened on July 4, 2012, though, when scientists at CERN announced that they'd found a particle that behaved the way they expect the Higgs boson to behave. Maybe the famed boson's grand and controversial nickname, the "God Particle," has kept media outlets buzzing. Then again, the intriguing possibility that the Higgs boson is responsible for all the mass in the universe rather captures the imagination, too. Or perhaps we're simply excited to learn more about our world, and we know that if the Higgs boson does exist, we'll unravel the mystery a little more.

In order to truly understand what the Higgs boson is, however, we need to examine one of the most prominent theories describing the way the cosmos works: the standard model. The model comes to us by way of particle physics, a field filled with physicists dedicated to reducing our complicated universe to its most basic building blocks. It's a challenge we've been tackling for centuries, and we've made a lot of progress. First we discovered atoms, then protons, neutrons and electrons, and finally quarks and leptons (more on those later). But the universe doesn't only contain matter; it also contains forces that act upon that matter. The standard model has given us more insight into the types of matter and forces than perhaps any other theory we have.

Give a brief account of Newton’s law of motion.

Newton’s law of motion is given below-

In 1687 in his book Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) the greatest English physicist Sir Isaac Newton first introduce the three laws of motion.

The laws are-

1. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

2. The vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration vector a of the object. Mathematically we can express the law as F=ma.

3. Every action has an equal and opposite reaction. In other word we can say that when one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

What is the difference between Reversible and Irreversible process?

Ans: The difference between Reversible and Irreversible process is given bilow.

Reversible Process	Irreversible Process
1. It takes place in infinite number of infinitesimally small steps and it would take finite time to occur.	1. It takes place infinite time.
2. It is imaginary as it assumes the presence of frictionless and weight less piston.	2. It is real and can be performed actually.
3. It is in equilibrium state at all stage of the operation.	It is in equilibrium state only at the initial and final stage of the operation.
4. All changes are reversed when the process is carried out in reversible direction.	4. After this type of process has occurred all changes do not return to the initial stage by themselves.
5. It is extremely slow.	5. It proceeds at measureable speed.
6. Work done by a reversible process is greater than the corresponding irreversible process.	6. Work done by a irreversible process is smaller than the corresponding reversible process.