If  the monochromatic source is replaced by a source of white light, then there will be colourful interference fringes,consisting of seven colours.

The same thing we find in case of thin film of soap solution.

Below is the example where,White light incident on an oil film. A colorful interference pattern is observed when light from the sun is reflected from the top and bottom boundaries of a thin oil film.

Interference of Light

When two light waves of similar frequency having a zero or constant phase difference propagate in a medium simultaneously in the same direction, then due to their superposition maximum intensity is obtained at few points and minimum intensity at other few points.

This phenomena of redistribution of energy due to superposition of waves is called interference of light waves.

The interference taking place at points of maximum intensity is called constructive interference.

The interference taking place at points of minimum intensity is destructive interference.

Fringe Width

The distance between the centres of two consecutive bright or dark fringes is called the fringe width.

The angular fringe width is given by θ = λ / d.

where λ is the wavelength of light d is the distance between two coherent sources.

Young’s double slit experiment (YDSE)

The Young’s experiment shows that matter and energy can display both wave and particle characteristics.

Purpose of double slit experiment is as follows:-

1. In order to prove the wave nature of light.
2. To explain the phenomenon of interference.

Two coherent sources of light were taken in order to maintain the 0 or constant phase difference between the sources of light.

• Experimental set-up1:-
  • Young took an ordinary source of light(S) such as light bulb.
  • The light was made to pass through a very small slit S (which was comparable with the wavelength of light).
  • The light coming from Source S was made to pass through two small slits S₁ and S₂ which were separated by a very small distance d.
  • One screen was kept in front of these 2 sources.

Observation1:-

• He observed alternate dark and light bands were formed on the screen.

Setup 2:-

• Now he took 2 light bulbs i.e. 2 non coherent sources of light and placed a screen in front of them.

Observation 2:-

• He observed there were no alternate bands of light formed on the screen.
• Conclusion:-
• When coherent sources of light were taken then the phenomenon of interference is taking place.
• When non-coherent sources were taken phenomenon of interference was not taking place.
• The source S illuminated the sources S₁ and S₂ as a result the light from S₁ and S₂ become coherent.
• S was the source of bothS₁ and S_2, therefore if there is any change in the phase of the source there will be change in the both sources also.
• Therefore both S₁ and S₂ will be always in phase with each other.

Huygens Principle

• Huygens principle states that each point of a wavefront is the source of secondary wavelets (small waves) which spread in all directions with the speed of the wave.
• The new wavefront is formed by drawing a line tangent to all the wavelets.

For example:-

• If a stone is thrown in the river,waves will be formed surrounding that point.
• These waves look like concentric circles and they are known aswavefronts.
• The wavefronts gradually spread in all the directions.
• When the locus of all the waves is joined which are in the same phase, it will be the same as a sphere, and are known as Primary wavefront.
• Secondary wavefront are formed fromeach point on the Primary wavefront.
• The common tangential line that envelopes these secondary wavefronts will further give rise to other secondarywavefronts.
• All wavefronts will gradually fade after some time.

• Conclusion: - According to Huygens principle, every point on a wavefront give rise to secondary wavelets which spread out in all the directions with the speed of a wave.

By the 1970s all the major definitions of tribe were shown to be faulty. It was pointed out that the tribe-peasantry distinction did not hold in terms of any of the commonly advanced criteria: size, isolation, religion, and means of livelihood. Some Indian “tribes” like Santhal, Gonds, and Bhils are very large and spread over extensive territory. Certain tribes like Munda, Hos and others have long since turned to settled agriculture, and even hunting gathering tribes, like the Birhors of Bihar employ specialised households to make baskets, press oil etc. It has also been pointed out in a number of cases, that in the absence of other alternatives, “castes” (or non-tribals) have turned to hunting and gathering.

(i)    Mughal government was a monarchy. The emperor was all-in-all having all executive, legislature and judiciary powers in himself. He was chief commander of the forces. The position of all ministers were just the government officials of the present day government.

(ii)    Though empire was divided in certain provinces, parganas, districts and villages, yet the all governors were appointed by emperor himself.

(iii) The change of the government was not done by the people as it is done by voters today.

Generally succession was either decided by the emperor or the major courtiers or on the basis of sword of might. The Mughal rulers saw themselves as appointed by Divine will. Now-a-days after every five year or even earlier the head of the state i.e. the president or the head of real administration or government is change by the public through ballot. During the Mughal period sometime throne was smashed by force. The example is of Shershah, who has defeated Humayun and force him leave Hindustan. After the gap of fifteen years he defeated the successor of Shershah and again got his throne. The authority of Akbar, Jahangir and even Shah Jahan was challenged by their own sons or blood relations.

On July 7, HRD Minister Ramesh Pokhriyal announced a major CBSE syllabus reduction with 30% of the syllabus slashed for the year 2020-21 for classes 9 to 12 because of the reduction in classroom teaching time due to the Covid-19 pandemic and lockdown.

CBSE has rationalized the syllabus with the help of suggestions from NCERT and the same has been notified by a new CBSE notification as well.

The CBSE syllabus has been reduced but care has been taken so that the learning outcomes remain the same and the students do not miss out on learning core concepts in various subjects.

Deleted syllabus of CBSE Class 12 Mathematics

Q : A converging lens of refractive index 1.5 is kept in a liquid medium having same refractive index. What would be the focal length of the lens in this medium?

A n s w e r :

The focal length of lens in a liquid-medium is given by

i.e., focal length of converging lens is infinity i.e., glass lens behaves as a glass plate.

Colloid: A colloid is a heterogeneous mixture whose particles are not as small as solution but they are so small that cannot be seen by naked eye. When a beam of light is passed through a colloid then the path of the light becomes visible. For example milk, smoke etc.
 Properties of a colloid
• A colloid is a heterogeneous mixture.
• The size of particles of a colloid is too small to be individually seen by naked eyes.
• Colloids are big enough to scatter a beam of light passing through it and make its path visible.
• They do not settle down when left undisturbed, that is, a colloid is quite stable.
• They cannot be separated from the mixture by the process of filtration. But, a special technique of separation known as centrifugation can be used to separate the colloidal particles.

The face-centered cubic (fcc) has a coordination number of 12 and contains 4 atoms per unit cell. The body-centered cubic (bcc) has a coordination number of 8 and contains 2 atoms per unit cell. The simple cubic has a coordination number of 6 and contains 1 atom per unit cell.

The term 'O-Level' in the play refers to the basic test in German taken by students in England, Wales and Northern Ireland. This examination is at the centre of Evans’ successful escape. It is in the guise of taking the O-level exam in German that Evans plans and executes his escape successfully. The holding of the O-level examination by the prison authorities gives Evans the chance to make a successful exit from the prison and it is also the question paper of this examination that helped the Governor track Evans at the Golden Lion from where Evans makes his final successful escape by fooling the Governor.
There is also a play on the term ‘O-Level’, which is short for ordinary level. It is a hint at the preparations that Evans had taken to make his escape successful. Though those were ordinary on the face of it, they helped Evans make a successful escape after four failed attempts earlier.

Concept of circulation of labour:

1. Commercialisation of agriculture has been responsible for the growth of migrant agricultural labour. Seasonal demand for these labourers increased in Green Revolution regions like: PUNJAB.
2. Labourers migrate also due to the increasing inequalities in rural areas.Men migrate out periodically in search of work and better wages, while women and children are often left behind in their villages.
3. Migrant workers come mainly from drought-prone and less productive regions, and  they go to work for part of the year on farms in the Punjab and Haryana, or on brick kilns in U.P., or construction sites in cities such as New Delhi or Bangalore.
4. These migrant workers have been termed ‘footloose labour’ by Jan Breman. However, these landless workers do not have many rights and are also denied minimum wages.
5. The large-scale circulation of labour has had several significant effects on rural society, in both the receiving and the supplying regions.
6. Women are also emerging as the main source of agricultural labour, leading to the ‘feminisation of agricultural labour force. The insecurity of women is greater because they earn lower wages than men for similar work.