What is sound

Sound is vibration in the air. Sometimes you can see the vibrations; sometimes you can not. If you pluck a taut rubber band, you can see the band twang. If you clap your hands, you see nothing. Vibration is there all the same. But it is not only the band or your hand move, you hear sound because air moves too. As the source of sound moves, it sets air molecules moving too & so vibration is transmitted through air to ears.
Can sound travel in vaccum?
No. Sound can travel through solid, liquid & air. In fact, it can travel faster in solid & liquid than air, because their molecules are closely packed. But there is complete silence in vaccum as there is nothing to vibrate.
What are sound waves?
Sound waves are not like waves in the sea, which go up & down. Sound waves moves by alternately streching & sqeezing. When a sound is made, air molecule near source are sqeezed together. They in turns jostle up against the molecules next to them & then are pulled back into place by molecules behind.
How do we hear sound?
You hear because your ear pick up tiny vibrations in the air made by the sound. Inside ear is a taut, thin window of skin called airdrum, which vibrates with the air & rattles three tiny bones called ossicles. The last of them called stirrup, rattles faster than first to amplify sound. Stirrup shakes another thin skin, sending waves through a fluid filled tube called cochlea, deep inside your head. Ripples in cochlea trigger sensetive nerve receptors to send signals to your brain.

Light concept

What is light?

It is radiations sent out by atoms in light sources such as sun, stars, electric lights. It is visible electromagnetic radiations about 400 to 700 nm. This is straight forward scientefic definition of light. But in ancient theories about light people from different culture think of light differently. Some were defining light as carpuscles, small tiny element of matter.

In ancient India, the philosophical schools of Samkhya and Vaisheshika, from around the 6th–5th century BC, developed theories on light. According to the Samkhya school, light is one of the five fundamental "subtle" elements (tanmatra) out of which emerge the gross elements. The atomicity of these elements is not specifically mentioned and it appears that they were actually taken to be continuous.
On the other hand, the Vaisheshika school gives an atomic theory of the physical world on the non-atomic ground of ether, space and time. (See Indian atomism.) The basic atoms are those of earth (prthivı), water (apas), fire (tejas), and air (vayu), that should not be confused with the ordinary meaning of these terms. These atoms are taken to form binary molecules that combine further to form larger molecules. Motion is defined in terms of the movement of the physical atoms and it appears that it is taken to be non-instantaneous. Light rays are taken to be a stream of high velocity of tejas (fire) atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the tejas atoms. Around the first century BC, the Vishnu Purana correctly refers to sunlight as the "the seven rays of the sun".
Later in 499, Aryabhata, who proposed a heliocentric solar system of gravitation in his Aryabhatiya, wrote that the planets and the Moon do not have their own light but reflect the light of the Sun.
The Indian Buddhists, such as Dignāga in the 5th century and Dharmakirti in the 7th century, developed a type of atomism that is a philosophy about reality being composed of atomic entities that are momentary flashes of light or energy. They viewed light as being an atomic entity equivalent to energy, similar to the modern concept of photons, though they also viewed all matter as being composed of these light/energy particles.

In seventeenth centuary Chrtistiaan Huygens proposed wave theory of light. He considered a luminopherous ether as medium of prapogation of light.

Michel Faraday in 1845 discovered that light is related to electromagnetics. This experiment developed the modern concept of light.

What is photon?

Photons are infinitessimally small particles of light. They have no mass & there are billions of them in a single beam of light. Light energy travels in form of photons.

Light has various properties like reflection, refraction, polorisation.

Concept of Atom

The concept of matter is that it is formed by some discrete units & can not be divided in to smaller parts has been for milleniums, but it was based on philosophy not on experiments & observations. The concept of atom varied considerably over time between cultures, schools. The basic idea of the atom was adopted by scientists thousands of years later because it elegantly explained new discoveries in the field of chemistry.
The earliest mention of concept of atom was in 6th centuary BCE in ancient INDIA. The Nyaya and Vaisheshika schools developed elaborate theories of how atoms combined into more complex objects (first in pairs, then trios of pairs). The references to atoms in the West emerged a century later from Leucippus whose student, Democritus, systemized his views. In approximately 450 BCE, Democritus coined the term átomos (Greek ἄτομος), which means "uncuttable" or "the smallest indivisible particle of matter", i.e., something that cannot be divided. Although the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.
What's inside an atom?
Right in centre of every atom there is a dence core or "nucleus" containing two kinds of tiny particles protons & neutrons.Around nucleus are even tinier particles called electrons, whizzing around at the spped of light.
Atoms are about ten millionth of a millimeter across & wiegh 100 trillionth of a trillionth of a gram. The hydrogen atoms are smallest amogst all.
Protons have positive electrical charge while electrons have negative. They attract each other. Electron-electron or proton-proton repel each other as they have same charges.
Ions are those atoms which gained or lost one or more electrons. When atom looses electrons then it becomes positively charged called cations & when it gains electrons it becomes negatively charge called anions.

'Matter' what is this?

We know every subtance is made up of matter, we heard this. but do we know what is this matter? what form matter have?
Do we wonder with what the whole universe, different things on earth are made up of?
It is 'matter' from which the whole universe, every subtance is made up of. It is not english matter, The scientific meaning of matter is that which has mass & occupies space. Matter is every subtance in universe everything that is not simply empty space.
There are three different states of matter. Solid, liquid & gas.
Solid has strenth & definite shape. Each subtance is made from minute bits called molecules. In solid the molecules are bounded together firmly in regular shape.
A liquid is a subtance in state like water, which flows & takes the shape of any container it is poured into. It does thi because although bounds hold its molecules together, the bounds are loose enough for molecules to roll over each other.
A gas is when a subtance is in a state like air. It does not have any shape, strength or volume. The molecules move so fast that they break any bound that might hold them together.
When a matter in a state get heated or cooled then the distance between molecules increases, results in change of state of the the matter. When solid is heated then it transforms to liquid & when liquid is heated it changes to gas. Exact reverse process is followed when we cool subtances.

Concept of heat & energy

Do we think what is heat, why we feel worm when we touch a hotter subtance?
Heat is a form of energy- energy of molecules moving. The faster they move, hotter it is. When we heat somthing then then the movement (vibration) of molecules in subtance increases. This vibration gives amount of heat.
When we put our hand over a heater, warmth we feel is actually an assualt by billions of fast moving air molecules, spurred along by even faster moving molecules in heater. We measure how hot is by its temperature. Temperature is degree of heat but not same as heat. Heat is combined enegy of all molecules where as temperature is measure of how fast molecules are moving.
Heat moves in three ways conduction, convection & radiation. Conduction involves heat spreading from hot areas to cold areas by direct contact, as moving particles collide onto each other. Convection is as when warm air or water rises as its expands & gets lighter. Radiation is heat rays, invisible rays, of light.
Temperature is measured with thermometer. It is measured in centigrade & farenheit units.

Energy, in general, is capacity to make things happen. We have energy to do something means we have capacity to do that work. Scientifically, it is capacity to do work. It takes various forms. Energy can neigher be created nor be distroyed, means it can be only converted into other form. The total energy of universe is constant.