Optical Activity | Physical Properties And Molecular Structure | Explained | Learn About Chemistry |

Hello Everyone, Am Manjeet Singh and Today we will Discuss about Optical Activity And Its Physical Properties And Molecular Structure... So Let's Start...

PHYSICAL PROPERTIES AND MOLECULAR STRUCTURE
 
The physical properties like density, optical activity, dipole moment, refractive index, magnetic susceptibility etc. can be determined quantitatively and play an important role in providing information about the molecular structure. 
The basic fact that these physical properties are additive, constitutive or both in nature helps us to determine the molecular structure.

(1) Additive property: The physical property the total value of which is equal to the sum of values of corresponding properties of the constituent atoms is called an additive property.
For example molecular mass of a compound.

(2) Constitutive property: The physical property which depends upon the constitution of the molecule i.e. the arrangement of atoms and bond structure in a molecule is called constitutive property.
For example optical activity, surface tension, viscosity.

(3) Additive and Constitutive property: If is defined as an additive property that also depends upon the arrangement and bond structure of atoms in a molecule.
For example surface tension, viscosity, magnetic susceptibility etc.

OPTICAL ACTIVITY

Some common terms: 

(1) Plane polarised light: A beam of ordinary light consists of electromagnetic waves vibrating in all planes perpendicular to its direction of propagation. When ordinary light is passed through a Nicol prism (a special type of prism made up of crystalline calcium carbonate) its vibrations in all directions except the direction of axis of the prism, are cut off. The emergent rays  come out with their vibrations only in one plane. A beam of light having vibrations only in one plane is said to be plane polarised light.

(2) Optical activity: If plane polarised light is passed thorough quartz or solutions of some substances like sugar, camphor, etc. the path of plane polarised light gets rotated. The substances which rotate the plane of polarised light are called optically active substances and the property of a substance to rotate the plane of polarised light is called optical activity. All optically active substances do not rotate there Plane of polarised light in the same direction. Some rotate the plane of polarised light to the right and are called dextro rotatory written as d or ( + ) as prefix to the name of the compound. On the other hand, the substances which rotate the  plane of polarised light towards left after called leavo rotatory written as l or( - ) as prefix to the name of the compound.

# The principle behind rotation of plane polarised light:

 
if two nicol prisms are placed with their polarisation planes parallel to each other, the planes polarised light emerging from first prism passes through the second and bright field is seen through the eye piece but if the second prism is turned through 90° so that the planes of polarisation of two prisms are at right angles, then the light rays coming out of the first prism are stopped by the second and a dark field is seen through the eye piece.

The first prism is called POLARISER and the second prism is called ANALYSER. If an optically active substance is placed between the polariser and analyser kept at dark field position some light starts coming through the analyser and the optical view appears some what bright. In order to get a complete dark view again, the analyser had to be rotated through an angle (alpha). From this observation it is clear that optically active substances has the power of rotating the plane of polarised light through a certain angle and hence it is said to be optically active.

To obtain dark view when the analyser is turned towards right( i.e. clockwise) the optically active substances is said to be dextro rotatory and when analyser is turned to left ( anticlockwise) the substances is called leavo rotatory.

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BLACK BODY AND BLACK BODY RADIATION | Explained | Learn About Chemistry |

BLACK BODY AND BLACK BODY RADIATION

Hello Everyone, Am Manjeet Singh and Today we will Discuss about Black Body And Black Body Radiation... So Let's Start...

To understand the concept of black body we consider

following two situation.

(1) Absorption of radiation by a body.
(2) Emission of radiation by a body.

When a radiation falls on the surface of a body, a part of it is reflected, some port of it is absorbed and the rest is transmitted. Whole of radiant energy is not absorbed because generally the surface of the objects are not perfect Absorbers of radiation. However if the radiation falls on a blackened metallic surface, it is almost completely absorbed. So, a body which completely absorbs all the radiations falling on it is called a black body.
Now we take the case of emission of radiation by the object. Radiation is emitted by any solid body at any temperature as a result of vibrations of its particles. At low temperature the emitted radiations are of low frequency. As the temperature is raised the emitted radiation goes from a lower frequency to the higher frequency. Different solids emit radiation at different rates at same temperature, the emission rate is maximum when the solid is perfect black. In other words, when a body is heated, it radiates maximum amount of radiant energy. So a black body is a perfect emitter of radiant energy. At a given temperature, a black body radiates maximum amount of radiant energy. Thus a black body is defined as an object that  completely absorbs all the radiations falling on it, it is also a perfect emitter of radiation.
The radiation emerging from a black body is called " black body radiation". 

Experimentally, a nearest approach to black body is a hollow sphere blackened from inside and having a small hole for the entry of radiations. The inner surface of the sphere is blackened with lamp black. This blackened surface helps in complete absorption of radiations falling on it. The black body designed by Fery is shown in figure. It consists of a hollow double walled metallic sphere having a hole. The inner surface has a projection to prevent the direct reflection of the radiations entering from the hole. The inside surface is blackened with lamp black. Its exterior is completely insulated from surroundings so as to avoid the loss of radiation in the form of heat. Any radiation that enters black body through a small hole is reflected repeatedly from the walls and reabsorbed inside the sphere untill all of the energy is absorbed.

When a black body is heated, it glows from inside and cannot lose any energy to the outside world. It soon attains a state of thermal equilibrium and no external heat energy is required to maintain a constant temperature. But the inner walls are radiating all types of wavelength. The radiation emitted by a black body at a particular temperature are called Black Body Radiation. The origin of these black body radiations lies in the vibrations of particles of the body ( the oscillators) that emit the radiation energy in the form of electromagnetic waves.

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