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The absolute refractive index is expressed with the help of the symbol n.
The refractive index of a medium depends upon the refractive index of the surrounding. By surrounding we mean the surrounding in which the light ray is passing from into the medium. The Refractive index also depends upon optical density, the wavelength of the light, and temperature. 

Note:

> The value of the refractive index of light determines the extent to which the light will be refractive when entering or leaving the substance. According to the laws of nature, no substance can travel faster than the speed of light in a vacuum. Those materials with large indices of refraction are known as optically dense media.

> Refractive index is directly proportional to the optical density. It is considered inversely proportional to the wavelength of light.

After firing the bullet, the bullet moves forward with a large momentum and the gun moves backward with an equal momentum. 

Let m and M be the masses of bullet and gun,respectively. If v and V are the velocities of the bullet and gun, respectively after the firing then:- 

0= mv+ MV 

Therefore V=-mv/M 

The negative sign indicates that the gun moves in a direction to that of the bullet.

When we measure speed of light in two different media, relative refractive index of medium 2 with respect to medium 1 is defined as the ratio of speed of light in medium 1 to speed of light in medium 2.

n₂₁= speed of light in 1st medium / speed of light in second medium

Calcium sulphate with half a molecule of water per molecule of the salt (hemi-hydrate) is called plaster of paris (plaster of paris).

It is prepared by heating gypsum (CaSO₄.2H₂O) at 120°C in rotary kilns, where it gets partially dehydrated.

Plaster of paris is a white powder, which when mixed with water sets into a hard mass as crystals of gypsum are produced with evolution of heat. Hence it is used in setting of plaster for fixing broken bones. It is also used in making several decorative items, toys and chemical plugging 

Settling down of heavier insoluble particles/ solids from a mixture is called sedimentation. Like mud settles from muddy water.

Decantation is pouring out of upper clear layer of liquid into another container to separate two immiscible liquids. Like draining oil from a mixture of oil and water after allowing the two to separate and form distinct layers

Filtration is the process of separating insoluble impurities from a solution. When a solution is passed through a filter, particles with pore size bigger than that of the filter are retained while others pass through. Like filtering mud from muddy water

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● Chlorine is used in bleaching powder and liquid bleach, which are used for cleaning purposes.

● Chlorine is also very efficient disinfectant, hence added in public water to kill disease causing pathogens.

● Noble gases such as neon, argon, krypton are used in different types of lights.

● Diamond is used in industries for cutting of glass, etc.

● Coal , another form of carbon, is used as a fuel.

● Nitrogen is used in manufacturing of ammonia.

● Oxygen cylinders are used for breathing in emergency cases.

● iodine is used as tincture iodine