Periodic Classification of Elements

 

Periodic Classification of the Elements

 

Classification:

Grouping of elements into different classes is called periodic classification of elements. Elements are arranged into groups (columns) and periods (Rows )based on their properties.  This method is requires arranging the elements that are alike and separating the elements that are unlike.

Need of Classification :

 It helps us understand how different elements form different compounds. The number of elements discovered has increased, making it difficult to remember the behavior and properties of these elements. Hence it is important to classify elements according to their properties.

 

Dobereiner’s  law Triads:
Johann Wolfgang Dobereiner, a German chemist, classified the known elements in groups of three elements on the basis of similarities in their properties. These groups were called triads.

According to this law, when elements are arranged in order of increasing atomic masses, groups of three elements, having similar properties are obtained. The atomic mass of middle element of the triad is nearly  equal to the average of the atomic masses of the other two elements.
For Example Li (6.9),         Na (23),    K (39).

 

Characteristics of Triads:

1.   Properties of elements in each triad were similar.

2.   Atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.

Limitations:

 Dobereiner could identify only three triads. He was not able to prepare triads of all the known elements.

 

 

Newlands’ Law of Octaves:

John Newlands’, an English scientist, arranged the known elements in the order of increasing atomic masses. He found that the physical and chemical properties of every first element is very similar to that every eighth element.  He compared this to the octaves found in music. Therefore, he called it is the ‘ Law of Octaves’. It is known as ‘ Newlands' law of octaves ’ .

Characteristics of Newlands’ Law of Octaves:

·       It contained the elements from hydrogen to thorium.

·       Properties of every eighth element were similar to that of the first element.

Limitations of Newlands’ law of Octaves:

1.   The law was applicable to elements up to calcium (Ca).

2.   In order to fit elements into the table, Newlands’ adjusted two elements like cobalt and nickel in the same slot.

3.   At the time of Newlands' only 56 elements were known in nature and Newlands' assume that no more elements will be discovered the future But, later on , several new elements were discovered, whose properties did not fit into the law of octaves.

4.   Newlands' law of Octaves worked well with lighter elements only.

 

 Mendeleev’s Periodic Table:

 

 D. I. Mendeleev, a Russian chemist, was the most important contributor to the early development of a periodic table of elements wherein the elements were arranged on the basis of their atomic mass and chemical properties.

Mendeleev use the following criteria to formulate his periodic law.

1.   All the elements were arranged on the basis of their atomic masses.

2.   He reacted all then known elements with hydrogen (to form hydrides) and oxygen (to form oxides)

 

On the bases of the above two criteria he formulated the law called as the Mendeleev’s law of the periodic table:

“The properties of the elements are periodic functions of their atomic masses.”

 

Characteristics of Mendeleev’s Periodic Table:

1.   Mendeleev arranged all the 63 known elements in increasing order of their atomic masses.

2.   The table contained vertical columns called ‘groups’ and horizontal rows called ‘periods’.

3.   The elements with similar physical and chemical properties came under the same groups.

4.   Mendeleev’s stated that the properties of elements are the periodic function of their atomic masses.

 

 

 

Achievements of Mendeleev’s Periodic Table:

 

1.   Through this table, it was very easy to study the physical and chemical properties of various elements.

2.   Mendeleev left some gaps in his periodic table.
He predicted the existence of some elements that had not been discovered at that time. Later on elements like scandium, gallium and germanium were discovered to fill these gaps.

3.   The Nobel gases like helium, neon and argon, which were discovered later, were placed in a new group without disturbing the existing order.

Limitations of Mendeleev’s Periodic Table : 

1.   No fixed positions were given to hydrogen in the Mendeleev’s periodic table.

2.   Positions of Isotopes of all elements was not certain according to Mendeleev’s periodic table.

3.   Atomic masses did not increase in a regular manner in going from one element to the next.

4.   Anomalous pairs of elements: Cobalt (Co) has higher atomic weights but was placed before Nickel (Ni) in the periodic table.

5.   He could not explain the cause of periodicity among the elements

 

Modem Periodic Table:

 

Henry Moseley, gave a new  property of elements, ‘atomic number’ and this was I adopted as the basis of Modem Periodic Table.

Modern Periodic table.

1.   Elements are arranged in increasing order in their atomic numbers.

2.   These are 7 period and 18 groups.

3.   Insert gases are placed in a separate group i.e. 18 group.

4.   All the transitional elements are placed in 3 to 12 groups.

5.   Metal and non metals are widely separated from one another.

6.   Hydrogen is given a special position in modern Periodic table

 

Modem Periodic Law: 

“Properties of elements are a periodic function of  their atomic numbers”.

Characteristics of the modern periodic table

or

Long form of periodic table

1.   The modem periodic table consists of 18 groups and 7 periods.

2.   Elements present in any one group have the same number of valence electrons.

3.   The number of shells increases as we go down the group.

4.   Elements present in any one period, contain the same number of shells.

5.   Each period marks a new electronic shell getting filled.

 

Trends in the Modern Periodic Table:

 

Valency:  

Valency of an element is determined by the number of valence electrons present in the outermost shell of its atom. In  simple terms the combining capacity of the atom is called as valancy.

Group: Valency of elements in a particular group is same.

Period: Valency of elements in a period first increases from one to four and then decreases to zero.

Atomic Size: 

 

Atomic size refers to the radius of an atom. It is the distance from the center of the nucleus to the outermost electron present in the atom.

Group: In a group, atomic size and radii increases from top to bottom.

Period: In a period, atomic size and radii decreases from left to right.

 

Metallic and Non-metallic Properties:

 

The tendency to lose electrons from the outermost shell of an atom, is called metallic character of an element.

The tendency to gain electrons from the outermost shell of an atom, is called non-metallic character of an element.

 

Note:

Groups in Modern Periodic Table

The modern periodic table contains 18 vertical columns known as groups.

Group 1 elements are known as alkali metals.
Group 2 elements are known as alkaline earth metals.
Group 15 elements are known as pnicogens.
Group 16 elements are known as chalcogens.
Group 17 elements are known as halogens.
Group 18  elements are known as noble gases.

 

 

 

 

Non- metallic character:- The tendency to gain electrons from the outermost shell of an atom is called non-metallic character of an element.

Ionization energy:-

It is defined as the energy required to remove an electron completely from an isolated gaseous atom of an element. The energy required to remove the first electron is called first ionization energy.

Variation in a group:- Ionisation energy goes on decreasing down a group.

Variation in a period:- It goes on increasing generally along a period from left to right.

 

 

Page no. 112

Q.1. Yes, Dobereiner’s  triads also exist in the column of Newland’s octaves.

E.G.; Li(Lithium), Na(Sodium), K(Potassium) is a triad of Dobereiner also exist in the coloumn of Newland’s octaves.

Q.2. See back.

Page no. 116

Q.1.   Elements                                    Oxides

        1. K (Potassium)                            K20

        2. C (Carbon)                                  C02

             3. Al (Aluminium)                          Al203

        4. Si (Silicon)                                   Si02

        5. Ba (Barium)                                Ba0

Q.2. (i) Scandium (EKKA- boron): Sc

         (ii) Germanium (EKKA- silicon): Ge

Q.3. Mendeleev use the following criteria for his periodic table:

 (i) Mendeleev arranged the elements on the basis of their fundamental property i.e., the atomic mass.

(ii)  Mendeleev concentrated on the compound formed by elements with oxygen(oxides) and hydrogen (hydrides). He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements.

Q.4. Noble gases are very less reactive element. These gases were discovered very late, because they are very inert and placing them in a separate group, does not disturb the existing order put forward by Mendeleev.

Page no. 120

Q.1. Modern periodic table removes various anomalies of Mendeleev’s periodic table.

         Achievements of modern periodic table:- See back

Q.2. The two elements  that show the chemical reactions similar to magnesium are:-

(i) Beryllium

(ii) Calcium

Q.3. (i)

(i) Sodium

(ii) Hydrogen

(iii)Lithium

(ii)

Beryllium

Magnesium

Calcium

Helium

Neon

Argon

 

Q.4. (i) Lithium(Li), Sodium(Na), Potassium(K) atoms belong to same group and have same numbers of electrons in their outermost shells.

(ii) Helium(He) and Neon (Ne) both belong to last group and have their outermost shell filled.

Q.5. Among the first ten elements only Lithium and Beryllium are metals.

Q.6. Be and  Ga are excepted to be most metallic out of be and Ga . Ga is bigger in size and hence has greater tendency to lose electrons than Be. Therefore, Ga is more metallic than Be.

Excerise 121

Q.2. ‘X’ is a metal element and is in the same group as (B) Mg(Magnesium) metal.

Q.3.( a) Neon (2,8)

(b) Magnesium(2+8+2=12)

(c) Silicon (2,8,4)

(d) Boron(2,3)

(e) Carbon(2,4)

Q.4. (a) All elements of this column have 3 electrons in their valence shell like Boron.

(b) All elements of this column have 7 electrons in their valence shell like fluorine.

Q.5. (A) its atomic number is 17(equal) to total number of electrons i.e. (2+8+7=17)

(B) Electron configuration of these elements are:-

N=2,5        F= 2,7          P= 2,8,5             AR= 2,8,8

These properties of this element are similar to F (Fluorine) element as both have same number of valence electrons (electrons is the outermost shell)

Q.6. ‘A’ is an non metal because group 17 contain non metal.

(b) ‘c’ is less reactive than ‘A’ as non metallic properties decreases on moving down in a group.

(c) ‘c’ will smaller in size than ‘B’ as atomic size decreases as we move from left to right in a period.

(d) Anion will be formed by A because non metal form anions only.

Q.7. The electronic configuration of nitrogen and phosphorus

Elements

Nitrogen

Phosphorus

Atomic No.

7

15

Electronic configuration

2,5

2,8,5

  Nitrogen will be more electro-negative because due to increases in size the electro negativity of non metals goes on decreasing on moving down the group

Q.8. We can determine the position of any element in the modern periodic table be its electronic configuration on number of electrons in the outermost shell shows the group number and total number of shell of an atom shows the period to which it belongs.

E.g. Atomic number of solution=11

Electronic Configuration=2,8,1

It contains 1 electron in outermost shell. So it belongs to group 1 and total number of shell are 3 So, it belong to 3rd period.

Q.9 The electronic configuration of all the elements are given below.

Atomicno.

       1  20

            21

                38

 

Electronic confuration

 2,8,2

 

 

2,8,8,1

 

 

2,8,8,1

 

 

2,8,8,3

 

 

2,8,8,8,2

 

So the elements having atomic number 12 and 38 resemble with the properties of calcium as both the elements have some number of valence electrons as calcium.

 

Q10.

Mendeleev periodic table

1.   Elements are arranged in increasing order of their atomic masses.

2.   These are 6 period and 8 groups.

3.   Inert gases were not known at the time of Mendeleev.

4.   All the transitional elements are placed in 8 groups.

5.   Many metals and non metals are group together.

6.   No fixed position was given to hydrogen in Mendeleev periodic table.

An Excellent Father

 

An Excellent Father

Q.No1. What does Mrs Bennett tell her husband?

Ans. Netherfield Park is a house which is vacant for a long time. It was to let. A rich young gentleman, Mr Bingley, rents the house. When Mrs Bennet comes to know about it, she is extremely delighted because she hopes that Mr Bingley might marry one of her daughters. She tells all this to her husband. She wants him to form an acquaintance with Mr Bingley for the sake of their daughters.

Q.No.2. Who takes over Netherfield park? Why is Mrs Bennet happy about it?

Ans. Mr Bingley takes over the Netherfield Park. He is a young man of large fortune from the north of England. He earns four or five thousand a year and is unmarried. Mrs Bennet has got three daughters of marriageable age and hopes that Mr Bingley might marry one of her daughters. Therefore she is happy for having a young and rich man in her neighbourhood.

Q.NO.3. Why does Mrs Bennet insist on her husband to meet Mr Bingley?

Ans. Mrs Bennet insists her husband to make an acquaintance with Mr Bingley, who is young and unmarried man of large fortune, because she does not want to miss the golden opportunity of marrying one of her daughters with Mr. Bingley.  .

 

Q.NO.4. ’You take delight in vexing me. You have no compassion on my poor nerves’. Explain.
Ans. Mrs Bennet out of disappointment says these words to her husband. She wants her husband to form an acquaintance with Mr Bingley. However, Mr.  Bennet pretends to have no interest at all and teases his wife that the young man would be more delighted to see her and she should rather go and visit him. On hearing this from her husband Mrs Bennet gets vexed and says the given words or lines.

Q.NO.5. Why does Mrs Bennet think that Mrs Long is selfish?
Ans. Mrs Long had promised that she would introduce Mrs Bennet and her daughters to Mr Bingley at the assemblies (gatherings). But Mrs Bennet doesn’t think that Mrs Long would do any such thing because Mrs Long, has got two nieces of her own and she would not lose a chance of marrying one of them with Mr. Bingley. So, she says that Mrs Long is a selfish lady and she would like Mr Bingley to marry one of her nieces.

Q.NO.6. How does Mr Bennet react to the plan designed by his wife?
Ans. Mrs Bennet comes to know that a young, unmarried and wealthy man has come to live in their neighbourhood. She sees in Mr Bingley a perfect match for one of her daughters. She insists her husband to make an acquaintance with Mr Bingley for the sake of their daughters.

 

Q.NO.7. Why does Mrs Bennet says her daughters that they have an excellent father?
Ans. Mrs Bennet has got three daughters of marriageable age and hopes that Mr Bingley might marry one of her daughters. So, she asks rather insists her husband to pay the young man a visit as to form an acquaintance with him. But Mr Bennet pretends to have no interest in Mr Bingley. He rather teases his wife that Mr Bingley would be more pleased to see her. He even teases his daughters with witty remarks. However, at last seeing his wife annoyed, Mr bennet reveal that he has already paid a visit to Mr Bingley. It comes as a great surprise to the ladies. Mrs Bennet exclaims delightfully, “WHAT AN EXCELLENT FATHER YOU’VE GOT”.

Q.8. Write briefly about the humorous situation in “An Excellent Father”.

The story ‘An Excellent father’ is full of humour but the situation I liked most is that Mr. Bennet knows everything but pretends to have no interest in Mr Bingley. He rather teases his wife that Mr Bingley would be more pleased to see her. He even teases his daughters with witty remarks. However, at last seeing his wife annoyed, Mr bennet reveal that he has already paid a visit to Mr Bingley. It comes as a great surprise to the ladies

  

Q.9. Give the Character sketch of Mrs Bennet

The character of Mrs Bennet is very humorous. When she comes to know that a rich young gentleman, Mr Bingley, has taken the Nether field Park, she gets extremely delighted because she hopes that Mr Bingley might marry one of her daughters. She compels her husband Mr Bennet not to miss such an acquaintance for the sake of their daughters. She gets vexed when there is no response from her husband. She is a mother but lacks the foresight. She gets irritated quickly and thereby proves to be very short-tempered. She is very firm that her design will bear fruit.

 

MAGNETIC EFFECT OF ELECTRIC CURRENT

 

 MAGNETIC EFFECT:-  The term magnetic effect of electric current means that or electric current flowing in a wire produces in a magnetic field around it. In other words, electric current can produce magnetism.

MAGNET:-  A magnet is an object which attract pieces of iron, steel, nickel and cobalt. Magnets come in various shapes and sizes depending on their in tended use.

BASIC  LAWS OF MAGNETISM:

1] There are two poles namely north pole and south pole.

2] Like poles repel each other, while unlike poles attract each other.

LODE STONE:- The black ore of iron (fe3o) called magnetic capable of attracting  similar pieces of iron is called lode stone.

MAGNETIC SUBSTANCE:-

Those substances which are attracted by magnet are called magnetic substances.

NON MAGNETIC SUBSTANCES:-

Those substances which are not attracted by a magnet are called non magnetic substances.

MAGNETIC FIELD:-

 The space surrounding a magnet in which magnetic force is exerted is called a magnetic field. The magnetic field has both, magnitude as well as direction.] A compass needle placed near a magnet gets deflected due to the magnetic force exerted by a magnet. The north end of the needle of a compass indicates the direction of magnetic field at a point where it is placed. [The S I unit of magnetic field is telsa.

 

MAGNETIC FIELD LINES:-

 A magnetic field is described by drawing the magnetic field lines. (The magnetic field lines are the lines drawn in a magnetic field along which a north magnetic pole would move.) The magnetic field lines are also known as magnetic lines of force. (The magnetic field lines always begin from N-pole of a magnet and end on the S-pole) of a magnet.

Properties of the magnetic field lines:-

(i)           The magnetic field lines originate from the north pole of a magnet and end at its South pole.

(ii)       The magnetic field lines come close to one another near the poles of a magnet but they are widely separated at other places.

(iii)    They exert lateral pressure on each other.

(iv)    They magnetic field lines do not intersect one another. It is because if they do so, it will give two different direction at the same point which is not possible.

 

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Human Eye

Human Eye    

Human eye is one of the most valuable and sensitive organs. It resembles a camera. The human eye works on the refraction of the light through a natural convex lens. Main parts of human eye :

 

 

Structure of Human Eye

 

     Cornea

·       Iris

·       Pupil

·       Cillary muscles

·       Eye lens

·       Retina

·       Optic nerve

      1)   Cornea: The front part of the eye is called as cornea. It is made of a transparent substance .It is a thin membrane of eye, through which light enters the eye first and gets refracted.

     2 )   Iris: Iris is suited just behind the cornea. Iris is a flat, coloured, ring shaped membrane. It control the size of pupil.

     3)   Pupil: There is a hole in the middle of the iris which is called the pupil of the eye. The pupil appears black because no light is reflected from it. Pupil regulates and controls the amount of light entering the eye.

     4)   Cillary muscles : It holds the eye lens and lens by focusing. The focal length of the convex lens present inside the eye can be changed by the action of the Cillary muscles.

     5)   Eye lens: Eye lens is a double convex lens made of a transparent , soft and flexible material like a jelly made of proteins . Being flexible the eye lens can change its shape (it can become thin or thick) to focus light on the retina.

     6)   Retina : The screen on which the image is formed in the eye is called retina. The retina is behind the eye lens. It is a delicate membrane having a large number of light, sensitive cells called ‘rods and cones’ which response to the intensity of light and colour of object respectively by generating electrical signals. 

   7)   Optic nerve: It is a nerve connecting eye and brain. It sends the electrical signals to brain.

              Working of Human Eye

The light rays coming from the object kept in front of us enter through the cornea of the eye, pass through the pupil of the eye and fall on the eye lens so it converges the light rays and produces a real and inverted image of the object on the retina is converged to the brain by the optic nerve and gives rises to the sensations of vision.

 

     

 

 

NOTE : Rod and Cone cells: The retina of our eye has a large number of light sensitive cells. These are two kinds of light sensitive cells on the retina : rods and cones.

Rods are the rod – shaped cells present in the retina of an eye which are sensitive to dim light. Rods are the most important for vision in dim light. Natural animals which sleep during the day and come out at night like the owl have a large number of rod cells in their retina which help them to see properly during the night where there is not much light.

Cones are the cone shaped cells which are sensitive to bright to bright light for normal light. The cone cells of our retina also respond to colour.

Power of Accomodation

An eye can focus the images of the distant object as well as the nearby object on its retina by changing the focal length (or converging power of its lens. The focal length of the eye lens is changed by the action of the Cillary muscles). The Cillary muscles can change the thickness of the soft and flexible eye lens and hence its focal length which in turn, changes the converging power of the eye lens.

               The ability of an eye to focus the distant object as well as the nearby objects in the retina by changing the focal length for converging power of its lens is called accommodation. An normal eye has a power of accommodation which enables objects as far as infinity and as close as 25 cm to be focused on the retina.

Far point : the farthest point from the eye at which an object can be seen clearly is known as the “ far point” of the eye. The far point of a normal human eye is at infinity.

Near point: The minimum distance at which an object must be placed so that a normal eye may see it clearly without any strain, is called the least distance of distinct vision. The least distance of distant vision for a normal eye may see it clearly without any strain, is called the least distance of distant vision for a normal eye is about 25 cm .

Note: colour combination:

Red + green= yellow

Blue + green = Peacock blue

Red + Blue= Magenta

Defect of vision

Sometimes the eye of a person cannot focus on the retina properly.  In this case the person is said to have a defect of vision . There are three common defects of vision.

Myopia:    Myopia ( or short – sightedness ) is that defect of vision due to which a person cannot see the distant objects clearly through he can see the nearby objects clearly. The far point of an eye suffering from myopia is less than infinity.

Causes of myopia

The defect of eye called myopia (for short – sightedness) is caused

1)   Due to high converging power of eye lens.

2)   Due to eye ball being too long.

Correction of myopia

Myopia (short –sightedness or near –sightedness ) is corrected by using spectacles containing concave lenses

    

Diagram

Hypermetropia

Hypermetropia or ( long – sightedness) is that defect of vision due to which a person cannot see the nearby objects clearly though he can see the distant objects clearly. The near point of a hypermetropia is centimetres away from retina.

Causes of  Hypermetropia

 1 Due to low converging power of eye lens.

2  Due to eye – ball being too short.

Correction of hypermetropia

Hypermetropia ( long – sightedness or far – sightedness) is corrected by using spectacles combining convex lenses.

Diagram

Presbyopia

Presbyopia is that defect of vision due to which an old person cannot see nearby objects clearly due to the loss of power of accommodation of the eye.( The near point in this case become less than 25 cm ) .

Cause

1 Due to Cillary muscles becoming weak

2 Due to the eye lens becoming rigid

Correction

Presbyopia is corrected by using spectacles containing cylindrical lens.

There are few modes of defect of vision

Cataract

 It is a defect of vision in which usually comes in oblage is cataract. The medical condition in which the lens of the lens of eye of a person becomes progressively resulting in blurred vision is called cataract.

Cause

Cataract develop when the eye lens of a person becomes cloudy due to the formation of a membrane over it.

Correction

The opaque  lens is removed from the ye of the person by surgical operation and a new artificial lens is inserted in the place.

 Colour blindness

It is a genetic disorder caused due to the absence of cone cells in the retina. In this defect, vision is normal, but eye cannot between colours.

Q Why do we have 2 eyes for vision and not just one?

Ans There are many advantages of having two eyes instead of one. Some of them are given below:

1.  Having two eyes gives a wider field of view

2.  Having two eye enables us to judge distance more accurately.

Q What is night blindness?

Ans. Some persons have the difficulty to see the objects in dim light during night. This defect of eye is called night blindness.

 Cause

1 Lack of vitamin a in the food

2 Improper functioning of rods – shaped cells. The rods shaped cells respond to intensity variation OF LIGHT.

Q What is total internal reflection ?

 Ans  When light passes from an optically denser medium to a rarer medium beyond certain angle of incidence it undergoes reflection instead of refraction. This is called total internal reflection and that certain minimum angle of incidence is called critical angle condition for total internal reflection.

1      Light should pass from denser to rarer medium.

2      Angle of incidence should be greater than critical angle.

Atmospheric  refraction : The refraction of light by the Earth’s atmosphere is called as atmospheric refraction.

Twinkle stars

The twinkling of stars is due to the atmospheric refraction of starlight. The starlight on entering the Earth’s atmosphere undergoes refraction continuously before it reaches the Earth. The continuously changing atmosphere refracts the light from the stars by different amounts from one moment to the next. When the atmosphere refracts more star light toward us, the star appears to be bright and when the atmosphere refracts less star light, then the star appears to be dim. In this way, the star light reaching our eyes increases and decreases continuously due to atmospheric refraction and the star appears to twinkle at night.

·      Advance sunlight and delayed sunset

Sun is visible to us about 2 min after the actual sunset because of atmospheric refraction. By actual sunset because of atmospheric refraction. By actual crossing of the horizon by the sun. The time difference between the actual sunset and the apparent sunset is about 2 min.

Rayleigh law of scattering

According to ray light, the amount of scattering is inversely proportional to the fourth power of the wavelength . Therefore the light of shorter wave length is scattered much more than the light of longer wavelength. There is no change in the wavelength of light rays during scattering.

 Note: thE MOST BEAUTIFUL PHENOMENA OF NATURE SUCH AS BLUE COLOUR OF SKY. White colour of clouds, Red of sunrise and sunset can be explained in terms of scattering of light.

Blue colour of sky

Blue colour have a shorter wavelength than red. So according to Ray light scattering law, Blue colour of sunlight scattered much more strongly by the larger no. Of molecules present in the Earth’s atmosphere. Hence the sky appears blue.

 Mirage

 The apparent inverted images formed due to not surface in deserts is called mirage. This is caused by total reflection of light in atmosphere.

TYNDALL EFFECT

The phenomenon of scattering of light by the colloidal particles known as Tyndall effect. This effect can be observed when;

1      A fine beam of sunlight enters a room containing suspended particles of dust, the path of the beam of light visible .It is due to the scattering of light.

2      Sunlight passes through a canopy of dense forest, mist contains tiny droplets of water which acts as particles of colloid dispersed in air.

Dispersion of light

In the year 1665 Newton found that if a beam of white light is passed through a triangular glass prism the white light splits to form a band of seven colours. The splitting of white light into its constituents colours is called dispersion.

            The band of seven colours formed on a white screen, when a beam of white light is passed through a glass prism, it is called spectrum of white light. The seven colours of the spectrum are red, orange, yellow, green, blue, indigo and violet.

Note: When the light rays change its path due to some particles it is termed as Scattering of light.

Causes of Dispersion : The dispersion of white light occurs because colour of white light travels at different speed through a glass prism. When white light consisting of seven colour falls on a glass prism, each colour in it is refracted by  different angles, with the result that seven colours are spread out to form a spectrum.

Ques. How rainbow is formed ?

Ans. A Rainbow is produced by the dispersion of white (sun) light by raindrops in the atmosphere. Each raindrop acts as a tiny glass prism splitting the sunlight into a spectrum. A Rainbow is always  formed in a direction opposite to that of the Sun.

Ques. Why does not glass prism produce a dispersion of light?

Ans. A glass slab acts as a combination of two identical glass prism is in an inverted position with respect to the first. The first prism splits the white light into the seven colours. The colour components fall on the second prism, where it recombines them to form white light.

Ques. Why does the sun appear reddish early in the morning?

Ans. At sunset or sunrise, the sun rays pass through a maximum length of atmosphere. Much of the blue light and shorter wave length has been taken out by scattering. Only the red colour of light reaches the observer. That is why when sunset and sunrise appear red.

Ques. Explain why the planets do not twinkle?

Ans. The planets are much closer to the Earth and are thus seen as extended sources. If we consider a planet as a collection of a large number of point sized sources of light, the total variation in the amount of light entering our eye from all individual point -sized sources will average out to zero thereby nullifying twinkling effects of planets.

Ques. Why does the sky appear dark instead of blue to an astronaut?

Ans. The sky appears dark instead of blue to an astronaut because at such a huge height there is no atmosphere to scatter the light. Hence the sky appears completely dark.