Lecture- 5 : CLASS-X: SCIENCE : Chapter: Reflection of light & concave mirror

CLASS X   |    SCIENCE    |    LIGHT

      Notes prepared by Subhankar Karmakar

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SCIENCE: CLASS X: CBSE (CLASS NOTES)

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 • Rules for obtaining images formed by Concave Mirror:

The image will be formed at the point where atleast two reflected rays intersect or appear to intersect. 

1. A ray of light which is is parallel to the principal axis of a concave mirror, passes through its focus after reflection from the mirror. 

2. A ray of light passing through the centre of curvature of a concave mirror is reflected back along the same path. Arrow pointing from left to right indicates the direction of incident ray and the arrow pointing from right to left indicates the direction of reflected ray.

3. A ray of light passing through the focus of a concave mirror becomes parallel to the principal axis after reflection.

4. A ray of light which is incident at the pole of a concave mirror is reflected back making the same angle with the principal axis. 

• If a ray of light is incident on a concave mirror along its principal axis, then it is reflected back along the same path. 

• FORMATION OF DIFFERENT TYPES OF IMAGES BY A CONCAVE MIRROR

The type of image formed by a concave mirror depends on the position of object in front of the mirror. At different places, an object produces different types of images. 

a. When the object is in between Pole (P) and focus (F):

When an object is placed between the pole (P) and focus (F) of a concave mirror, the image formed is:

i. Behind the mirror

ii. Virtual and erect, and

iii. Larger than the object or magnified.

Uses of concave mirror using this type of images:

1. A concave mirror can be used to magnify objects. Therefore, it will be used as a magnifying glass.

2. A concave mirror can be used as a makeup mirror. It magnifies a part of the face.

3. Dentist's mirror is a small concave mirror fitted in a frame with a long handle. It gives magnified image of tooth.

b. When the object is placed at the focus (F) of a concave mirror:

When an object is placed at the focus of a concave mirror, the image formed is:

i. At infinity, 

ii. Real and inverted, and

iii. Highly magnified.

Uses of concave mirror using this type of images:

1. When a light bulb is placed at the focus of a concave mirror reflector, the diverging light rays off the bulb are collected by the concave reflector and then reflected to produce a strong, parallel beam of light. 

c. When the object is placed between focus (F) and centre of curvature (C):

When an object is placed between the focus (F) and the centre of curvature (C) of a concave mirror, the image formed is:

i. Beyond the centre of curvature

ii. Real and inverted, and

iii. Larger than object or magnified.

d. When the object is placed at the centre of curvature (C) of a concave mirror:

When an object is placed at the centre of curvature (C) of a concave mirror, the image formed is:

i. At the centre of curvature (C),

ii. Real and inverted, and

iii. Same size as the object.

e. When the object is is beyond the centre of curvature (C) of the concave mirror:

When an object is placed beyond the centre of curvature (C) a concave mirror, the image formed is:

i. Between the focus (F) and the centre of curvature (C),

ii. Real and inverted, and

iii. Smaller than the object or diminished.

f. When the object is at infinity:

When an object is at infinity from a concave mirror, the image found is:

i. At the focus (F), 

ii. Real and inverted, and

iii. Much smaller than the object or highly diminished. 

"This means that a concave mirror can concentrate all The parallel rays of light to its focus."

Uses of concave mirror using this type of images:

1. A concave mirror is used as a "head mirror" by the doctors to concentrate light coming from a lamp onto the body part of a patient like ear, nose, throat etc. to be examined. 

2. The concave "metal dishes" are used in dish antenna of televisions to receive TV signals from the very distant communication satellite which are high up in the sky. 

USES OF CONCAVE MIRRORS:

1. A concave mirror can be used to magnify objects. Therefore, it will be used as a magnifying glass.

2. A concave mirror can be used as a makeup mirror. It magnifies a part of the face.

3. Dentist's mirror is a small concave mirror fitted in a frame with a long handle. It gives magnified image of tooth.

4. When a light bulb is placed at the focus of a concave mirror reflector, the diverging light rays off the bulb are collected by the concave reflector and then reflected to produce a strong, parallel beam of light. 

5. A concave mirror is used as a "head mirror" by the doctors to concentrate light coming from a lamp onto the body part of a patient like ear, nose, throat etc. to be examined. 

6. The concave "metal dishes" are used in dish antenna of televisions to receive TV signals from the very distant communication satellite which are high up in the sky. 

LECTURE -2 : CLASS VIII : SCIENCE : CHAPTER 4 : MATERIALS : METALS & NON-METALS

CLASS VIII   |    SCIENCE    |    CHAPTER 4

     Notes prepared by Subhankar Karmakar 

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CHEMICAL PROPERTIES OF METALS & NON METALS:

REACTION OF METALS:

 a. Reactions of metal with Oxygen (O₂):

 Metal reacts with oxygen to form metal oxides. Metal oxides are basic in nature. 

The basic metal oxides turn red litmus to blue. 

*(Metals and R & B)

∴ Metal + Oxygen (from air) = Metal Oxide (basic oxide) 

Magnesium burning in air: 

I. When Magnesium (Mg) burns in air, it combines with the oxygen (O₂) of air to form magnesium oxide. 

• Mg + O₂ = MgO (a basic oxide)

II. Magnesium oxide dissolves partially in water to form magnesium hydroxide Mg(OH)₂  solution:

• MgO + H₂O = Mg(OH)₂ (a base)

Sodium (Na) reacts with Oxygen in air and produces Sodium Oxide (Na₂O)

• Na + O₂ = Na₂O (a basic oxide) 

Water solution of Sodium Oxide forms Sodium Hydroxide (NaOH)

• Na₂O +  H₂O  = NaOH

 Reaction of iron with oxygen of air:

During the rusting of iron, iron (Fe) metal combines slowly with the oxygen (O₂) of air in the presence of water or moisture to form a compound called iron oxide (Fe₂O₃). This iron oxide is called rust. Damp air contains Oxygen (O₂) + water (H₂O). 

• Iron (Fe) + Oxygen (O₂) + water (H₂O) ⟹ Iron Oxide or rust (Fe₂O₃) (basic oxide)

• Reaction of copper metal with moist air:

When a copper object is exposed to moist air for a long time, then copper (Cu) reacts with water (H₂O), carbon dioxide (CO₂) and oxygen (O₂) present in moist air to form a green coating on the copper object. The green coating is a mixture of copper hydroxide [Cu(OH)₂] and copper carbonate (CuCO₃) which is formed by the action of moist air on copper object.

• 2Cu + H₂O + CO₂ + O₂ = Cu(OH)₂ + CuCO₃ 

• Corrosion of copper: The formation of green coating of basic copper carbonate on the surface of copper objects on exposure to moist air is called corrosion of copper. 

 

b. Reactions of metal with water:

 When a metal reacts with water, then a metal hydroxide and hydrogen gas are formed. 

Metal + water = Metal hydroxide + Hydrogen

Not all metals react with water. Some of the metals reacts with cold water, whereas some metals reacts with hot water and steam. It depends upon reactivity of metals.

Sodium and potassium very quickly reacts with cold water. 

·        Magnesium reacts slowly with cold water and quickly with hot water and zinc and iron slowly react with steam. 

·        Sodium (Na) + water (H₂O) → Sodium Hydroxide (NaOH) + Hydrogen (H₂)

·       Sodium (Na) is a very reactive metal. It reacts with moisture, oxygen and other gases present in air. So, if sodium metal is kept exposed to air, it will react with the various components of air and get spoiled. In order to prevent its reaction with the moisture and other gases of air, sodium metal is always told under kerosene. Potassium metal is also very reactive and also kept in kerosene. 

c. Reactions of metals with acids:

Most of the metals react with dilute acids to form salts and hydrogen gas. 

Metal + Acid → Salt + Hydrogen gas.

Only less reactive metals like Copper, silver and gold do not react with dilute acids. 

• Magnesium reacts with dilute hydrochloric acid to form magnesium chloride (salt) and hydrogen gas.

Magnesium + hydrochloric acid → magnesium chloride + hydrogen gas

Mg + HCl → MgCl₂ + H₂

 When foodstuffs containing acids like orange juice, pickles, and curds are kept in iron, aluminium or copper containers, the acids present in them react with the metal of the container slowly to form toxic salts. That's why acidic foodstuffs should not be kept in metal containers.

d. Reactions of metal with bases:

Only some metals react with bases to form salts and hydrogen gas. Like aluminium is a metal and Sodium hydroxide is a base. When aluminium is heated with sodium hydroxide solution, then sodium aluminate which is a salt and hydrogen gas is formed. 

Sodium hydroxide + aluminium → sodium aluminate + hydrogen

NaOH + Al → NaAlO₂ + H₂

Zinc also reacts with bases like sodium hydroxide to produce hydrogen gas. 

REACTION OF NON METALS:

a. Reaction of nonmetals with oxygen:

 Non metals react with oxygen to form non metal oxides. Non metal oxides are acidic in nature. Non metal oxides water solution turn blue litmus into red. 

Non metal + oxygen → non metal oxide

 1. When sulphur burns in air, it combines with the oxygen of air to form sulphur dioxide. Sulphur dioxide is a acidic oxide. 

Sulphur + oxygen → sulphur dioxide

S + O₂ → SO₂

Sulphur dioxide dissolves in water to form sulphurous acid solution

SO₂ + H₂O → H₂SO₃

b. Reactions of nonmetals with water:

 Non metals do not react with water. Therefore, highly reactive nonmetals like phosphorus cannot be kept open in the air as it reacts with oxygen of air and catches fire. So, in order to protect phosphorus from atmospheric air, it is stored in a bottle containing water.

 c. Reactions of nonmetals with acids:

 Non metals do not react with dilute acids. 

 d. Reactions of nonmetals with bases:

 Some of the nonmetals react with bases but no hydrogen gas is produced.

 Difference between metal oxides and non metal oxides:

 Metal oxides are basic in nature and turn red litmus to blue. 

Non metal oxides are acidic in nature and turn blue litmus to red. 

 REACTIVITY SERIES OF METALS:

The arrangement of metals in a vertical column in the order of decreasing reactivities is called the reactivity series of metals.  

In reactivity series, the most reactive metal is placed at the top whereas the least reactive metal is placed at the bottom.

Potassium is the most reactive metal, so it has been placed at the top of the reactivity series. Gold is the least reactive metal so it has been placed at the bottom of the reactivity series.

 

Potassium (K) (most reactive)

Sodium (Na)

Calcium (Ca)

Magnesium (Mg)

Aluminium (Al)

Zinc (Zn)

Iron (Fe)

Lead (Pb)

Copper (Cu)

Silver (Ag)

Gold (Au) (least reactive)

Reactivity of the metals decreases as we go down in the above series. 

 

LECTURE -1 : CLASS VIII : SCIENCE : CHAPTER 4 : MATERIALS : METALS & NON-METALS

CLASS VIII   |    SCIENCE    |    CHAPTER 4

      notes prepared by subhankar Karmakar

                                                                         

• Element:

A substance which cannot be broken down into two or more simpler substances by chemical reactions is is called an element. 

Some of the common elements are:

Hydrogen, helium, carbon, nitrogen, oxygen, sulphur, phosphorus, silicon, chlorine, bromine, iodine, sodium, potassium, magnesium, calcium, zinc, iron, copper, silver, gold and mercury. 

Every element is represented by a symbol. No two elements can have the same symbol.

Symbols of common elements:

1. Hydrogen - H

2. Helium - He

3. Carbon - C

4. Nitrogen - N

5. Oxygen - O

6. Sulphur - S

7. Phosphorus- P

8. Silicon - Si

9. Chlorine - Cl

10. Bromine - Br

11. Iodine - I

12. Sodium - Na

13. Potassium - K

14. Magnesium - Mg

15. Calcium - Ca

16. Zinc - Zn

17. Iron - Fe

18. Copper - Cu

19. Silver - Ag

20. Gold - Au

21. Mercury - Hg

• Atom:

The smallest particle of an element is called atom. An element is a substance which is made up of only one kind of atoms. 

There are as many type of atoms as are elements. So different elements are made up of different kinds of atoms. For example, sulphur element is made up of only sulphur atoms. This means an amount of oxygen is totally made of atoms of oxygen only. 

There are only 92 naturally occurring elements known to us at present. Other elements in the periodic table are synthesized elements. 

Properties of elements:

Different elements have different properties. Some of the most important properties of elements are malleability, ductility, brittleness, lustre, Sonorousness, conductivity, strength, hardness, toughness etc. 

On the basis of their properties, all the elements can be divided into two main groups: 

Metals and nonmetals.

Characteristics of metals:

Metals are malleable and ductile elements. They are good conductors of heat and electricity. Metal are lustrous or shiny. Metals are usually hard and strong. All the metals are solids except Mercury which is a liquid metal. Metals have high densities which means they are heavy. Metals have high melting points and boiling points. Metals are sonorous which means that metals make a ringing sound when we strike them with a hard object. 

Some of the examples of metals are: iron, copper, aluminium, zinc, silver, gold, Platinum, chromium, sodium, potassium, calcium, magnesium, nickel, Cobalt, tin, Mercury, tungsten, manganese, uranium etc. Out of 92 naturally occurring elements, 70 elements are metal.

Characteristics of nonmetals:

Non metals are the elements which are neither malleable nor ductile, they are brittle. Non metals do not conduct heat and electricity. Non metals are not lustrous or shiny. Non metals can be solid, liquid or gases at the room temperature. Non metals have usually low melting points and boiling points. Non metals have low densities which means they are light. Non metals are not sonorous, which means non metals do not make ringing sound when we strike them with a hard object. 

Some of the examples of non metals are: Carbon, sulphur, phosphorus, hydrogen, oxygen, nitrogen, chlorine, fluorine, bromine, iodine, helium, neon, argon, Krypton and xenon. Out of of 92 naturally occurring elements, 22 elements are non metals. Out of these, 10 non metals are solids, 1 non metal is a liquid (bromine), and 11 non metals are gases. 

Metalloids:

There are some elements which show some properties of metals and the other properties of nonmetals. The elements whose properties are intermediate between those of metals and nonmetals are known as metalloids. 

The example of metalloids are: silicon, germanium, arsenic and tellurium.

Physical properties of metals and nonmetals:

a. Malleability:

The property which allows the metals to be hammered into thin sheets is called malleability. Most of the metals are malleable. Gold and silver are the best malleable metals and can be hammered into very fine sheets or foils. Aluminium and copper are also highly malleable. It is due to the property of malleability that metals can be bent to form objects of different shapes by beating with a hammer. 

Non metals are not malleable.

b. Brittleness:

The property due to which non-metals  break on hammering is called brittleness. This means non metals can not be hammered into a thin sheets, it break into small pieces when hammered. All the non metals are brittle.

c. Ductility:

The property which allows the metals to be drawn into wires is called ductility. Ductility is another characteristics property of metals. Generally all the metals are malleable and ductile. 

Gold and silver are among the best ductile metals. Copper and aluminium metals are also very ductile and can be drawn into to tin Copper and aluminium wires.

Non metals are not ductile. 

d. Conductivity:

Heat and electricity can easily flow through metals. Therefore metals are good conductors of heat and electricity. ( as they allow heat and electricity to pass through them easily).

Silver metal is a best conductor of heat. Copper, gold, aluminium and iron metals are good conductors of heat.

Except graphite and diamond all the non metals are bad conductors of heat and electricity. Therefore, they are called insulators. Diamond is a good conductor of heat whereas graphite is a good conductor of electricity. 

e. Lustre:

All the metals have a shiny appearance. This property of metal is known as Lustre of metal. 

All the non metals are not lustrous. Only iodine has a lustre. 

f. Strength:

Metals are usually strong, they have high tensile strength. Metals can hold large weights without snapping. Iron is one of the most strongest material, hence iron is used in construction purposes. 

Non metals are not strong. They have low tensile strength.

g. Sonorousness:

All the metals make a ringing sound when we strike them. This property of metal is known as Sonorousness. Sonorous means capable of producing a ringing sound. 

Non metals are not sonorous. They do not produce ringing sounds when we strike them. 

h. Hardness:

Metals cannot be cut very easily. This property is known as hardness of metal. Only sodium and potassium metals are soft and can be easily cut with a knife. 

Most of the solid non metals are quite soft.

Differences in physical properties of metals and nonmetals

1. Metals: metals are malleable and ductile. 

    Non metals: non metals are neither malleable nor ductile. They are brittle.

2. Metals: metals are good conductors of heat and electricity.

Non metals: nonmetals are poor conductors of heat and electricity except graphite is a good conductor of electricity and diamond is a good conductor of heat.

3. Metals: metals are lustrous. 

Non metals: non metals are not lustrous. They are dull. Only iodine has shiny appearance.

4. Metals: Metals are strong and have a high tension strength. Only sodium and potassium are not strong and have low tensile strength.

Non metals: non metals are not strong. They have a low tensile strength.

5. Metals: metals of sonorous. They make a ringing sound when struck.

Non metals: non metals are not Sonorous. They do not make a ringing sound when struck.

6. Metals: Metals are generally hard. Only sodium and potassium are soft metals.

Non metals: solid nonmetals are quite soft. Only diamond are very hard, in fact it is the hardest material. 

Lecture-1, 2, 3 and 4 : CLASS-X: SCIENCE : Chapter: Reflection of light & concave mirror

CLASS X   |    SCIENCE    |    LIGHT

      Notes prepared by Subhankar Karmakar

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SCIENCE: CLASS X: CBSE (CLASS NOTES)

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CLASS- X; PHYSICS; CHAPTER-1

Topic: REFLECTION OF LIGHT

LECTURE-1

Characteristics of Light:

1. Light is a form of energy.

2. Light always travelled in a straight line which is called Rectilinear Propagation of Light.

3. Light can travel through a medium as well as through vacuum also.

4. Light carries Energy from one point to another point.

5. Physics of light is called Optics.

6. Light exhibits two phenomena called Reflection and Refraction of Light.

REFLECTION OF LIGHT

• When a ray of light travelling in one medium falls on the surface of the second medium and turned back into the first medium, then it is called reflection of light. 

Important characteristics of reflection of light:

• The objects having polished, shining surfaces reflect more light than objects having unpolished, dull surfaces.

• Silver metal is one of the best reflectors of light.

• Ordinary mirrors are made by depositing a thin layer of silver metal on the back side of a plane glass sheet. The Silver layer is then protected by a coat of red paint. The reflection of light in a plane mirror takes place at the Silver surface in it. 

• A ray of light it is the straight line along which light travels.

• A bundle of light rays is called a beam of light.

INCIDENT RAY: The ray of light travelling in one medium when falls on the surface of the second medium is called incident ray.

REFLECTED RAY: The ray of light which after striking the surface of the second medium and turned back into the first medium is called reflected Ray.

POINT OF INCIDENCE: The point where incident ray strikes the surface of the second medium is called point of Incidence.

NORMAL: Perpendicular to the surface of the second medium drawn at the point of Incidence is called Normal.

ANGLE OF INCIDENCE: The angle between incident ray and normal is called angle of incidence. It is denoted by ∠i.

ANGLE OF REFLECTION: The angle between reflected ray and normal is called angle of reflection. It is denoted by ∠r.

LAWS OF REFLECTION:

Reflection of light takes place according to the following two laws:

First law of reflection:

1. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

Second law of reflection:

2. The angle of incidence is always equal to the angle of reflection, i.e., ∠i = ∠r.

• A ray of light which is incident normally or perpendicularly on a mirror, is reflected back along the same path ( because the angle of incidence as equal as the angle of reflection for such a Ray of light are zero).

• The laws of reflection of light applied to all kinds of mirrors, plane mirrors as well as spherical mirrors.

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LECTURE-2

Regular reflection:

• In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction. Regular reflection of light occurs from smooth surfaces like that of a plane mirror or highly polished metal surfaces. A plane mirror always produces regular reflection of light.

Diffuse reflection of light:

• In diffuse reflection, a parallel beam of incident light is reflected in different directions. The diffuse reflection of light takes place from rough surfaces like paper, cardboard, chalk, table, chair and unpolished metal surfaces.

 

Objects: Anything which gives out light rays either its  own or reflected by it, is called an object. For example, a bulb, a candle, a pinhead, an arrow etc. 

Images: Image is an optical appearance produced when light rays coming from an object are reflected from a mirror for refracted through a lens. For example, when we look into a mirror, we see the image of our face, while watching a movie in the cinema hall, we see the images of actors and actresses on the cinema screen. 

Types of images:

There are two types of images

a. Real image 

b. Virtual image

Real image: The image which can be obtained on a screen is called a real image. The image formed on a cinema screen is an example of real image. 

Virtual image: The image which cannot be obtained on a screen is called a virtual image. A virtual image can be seen only by looking into a mirror or a lens. The image of our face in a plane mirror is an example of virtual image. A virtual image is just an illusion. 

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LECTURE-3

Formation of image in a plane mirror: 

Consider a small object O placed in front of a plane mirror MM'. The mirror will form an image I of the object O. The object O gives out light rays OA and OB. OA coming from the object O is incident on the plane mirror at point A and it gets reflected in the direction AX according to the laws of reflection of light, making the angle of reflection r₁ equal to the angle of incidence i₁. Another ray of light OB coming from the object O strikes the mirror at point B and gets reflected in the direction BY, again making the angle of reflection r₂ equal to the angle of incidence i₂. 

The two reflected rays AX and BY are diverging from each other so they cannot meet under left side of the mirror. Let us produce the reflected rays AX and BY backwards. They meet at point I behind the mirror. Now, when the reflected rays AX and BY enter the eye of a person at position E, the eye sees the rays of light in the straight line direction in which the reflected rays enter it. So, the person looking into the mirror from position E sees the reflected rays as if they are coming from the point I behind the mirror. Thus, point I is the image of the object O formed by the plane mirror.

• The nature of image formed by a plane mirror is virtual and erect. The size of the image formed by a plane mirror is equal to that of the object. 

• The image formed in a plane mirror is at the same distance behind the mirror as the object is in front of the mirror.

Lateral inversion: 

• When an object is placed in front of a plane mirror, then the right side of object appears to become the left side of image, and the left side of object appears to become the right side of image. This change of sides of an object and its mirror image is called lateral inversion. 

• The phenomenon lateral inversion is due to the reflection of light. 

Characteristics of an image formed by a plane mirror:

a. The image formed in a plane mirror is virtual. It cannot be received on a screen.

b. The image formed in a plane mirror is erect. It is the same side up as the object.

c. The image in a plane mirror is of the same size as the object.

d. The image formed by a plane mirror is at the same distance behind the mirror as the object is in front of the mirror. 

e. the image formed in a plane mirror is laterally inverted.

Uses of plane mirrors:

a. Plane mirrors are used to see ourselves. The mirrors on our dressing table and in bathroom are plane mirrors.

b. Plane mirrors are fixed on the inside walls of certain shop to make them look bigger.

c. Plane mirrors are fitted at blind turns of some busy roads so that drivers can see the vehicle coming from the other side and prevent accidents.

d. Plane mirrors are used in making periscopes. 

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LECTURE-4

SPHERICAL MIRRORS:

A spherical mirror is a reflecting surface which forms part of a hollow sphere. 

(a) A hollow sphere cut by a plane                          (b) concave mirror  (c) convex mirror

Spherical mirrors are of two types: 

(i) Concave Mirror: A spherical mirror in which the outer bulged surface is silvered polished and the reflection of light takes place from the inner hollow surface is called a concave mirror.

(ii) Convex Mirror: A spherical mirror in which the inner hollow surface is silvered polished and the reflection of light takes place from the outer bulged surface is called convex mirror.

Terms related with Spherical Mirrors

1. POLE: It is the middle point P of the spherical mirror.

2. CENTRE OF CURVATURE: It is the centre C of the sphere of which the mirror forms a part.

3. RADIUS OF CURVATURE: It is the radius ( R= AC or BC) of the sphere of which the mirror forms a part.

4. PRINCIPAL AXIS: The line PC passing through the pole and the centre of curvature of the mirror is called its principal axis.

5. APERTURE: It is the diameter AB of the circular boundary of the spherical mirror.

6. PRINCIPAL FOCUS: A narrow beam of light parallel to the principal axis either actually converges to or appears to diverge from a point F on the principal axis after reflection from the spherical mirror. This point is called the principal focus of the mirror.

CONCAVE MIRROR:

7. Principal focus of a concave mirror:

The principal focus of a concave mirror is a point on its principal axis to which all other light rays which are parallel and close to the axis,  converge after reflection from the concave mirror

8. Focus of a concave mirror:

A concave mirror has a real focus. The focus of a concave mirror is in front of the mirror. 

9. Focal length of a concave mirror: 

The focal length of a concave mirror is the distance between its pole and principal focus. It is denoted by f. 

10. Relation between radius of curvature (R) and focal length (f) of a spherical mirror:

The focal length of a spherical mirror is equal to half of its radius of curvature. It is true for both concave and convex mirror. 

∴ f = R/2

Principal focus and focal length of a convex mirror: 

• The principal focus of a convex mirror is a point on its principal axis from which a beam of light rays, initially parallel to the axis, appears to diverge after being reflected from the convex mirror. 

• A convex mirror has virtual focus, focus of a convex mirror is situated behind the mirror. 

Lecture-3 : CLASS-X: SCIENCE : Chapter: Electricity

CLASS X: SCIENCE: ELECTRICITY

notes prepared by Subhankar Karmakar

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ELECTRIC CHARGE: (Q)

Electric charge is the property that causes electrically charged particles to attract or repel each other.

There are two types of electric charge, positive and negative. Protons are positively charged particles, while electrons are negatively charged particles. Neutrons have no electric charge.

Opposite charges attract each other, while like charges repel each other. The strength of the electric force between two charged particles depends on the magnitude of the charges and the distance between them. The greater the charge and the closer the distance, the stronger the electric force between them.

Electric charge is measured in coulombs (C). The unit of electric charge is the amount of charge that flows through a wire when a current of one ampere flows for one second.

Electric charge plays a fundamental role in many areas of science and technology, including electricity, electronics, magnetism, and chemistry.

ELECTRIC CURRENT: (I)

Electric charge flows through a wire if potential difference is applied between the end of the wire. 

When electric charge flows through a wire , it is called electric current. Therefore, the electric current is a flow of electric charges in a conductor such as a metal wire. 

The magnitude of electric current in a conductor is the amount of electric charge passing through a given point of the conductor in one second. 

If a charge of Q coulombs flows through a conductor in time t seconds, then the magnitude I of the electric current flowing through it is given by:

FORMULA OF ELECTRIC CURRENT:

Current, I = Q/t

UNIT OF ELECTRIC CURRENT:

The SI unit of electric current is ampere. It is denoted by the symbol A. 

DEFINITION OF 1 AMPERE CURRENT:

1 A current : When 1 coulomb of charge flows through any cross section of a conductor  in 1 second, the electric current flowing through it is said to be 1 ampere. 

1 ampere = 1 coulomb / 1 second

Or, 1 A = 1 C/1 s

There are two smaller unit of current called

1. milliampere (mA) and 2. microampere (μA)

1 milliampere (1 mA)= 1/1000 A = 10⁻³ A 

1 microampere (1 μA) = 1/1000000 A = 10⁻⁶ A

AMMETER:

Electric current is measured by an instrument called ammeter. An ammeter is always connected in series with the circuit in which the current is to be measured. Since the entire current passes through the ammeter, therefore, an ammeter should have very low resistance, so that it may not change the value of the current flowing in the circuit. 

CONTINUOUS FLOW OF ELECTRIC CURRENT:

If we maintain a steady potential difference between the two ends of a conductor so as to get a continuous flow of current is to connect the conductor between the terminals of a cell or a battery.

DIRECTION OF ELECTRIC CURRENT:

The conventional direction of electric current is from positive terminal of a cell or a battery to the negative terminal through the outer circuit. The actual flow of electrons which is really responsible for the electric current is however from negative terminal to positive terminal of a cell which is opposite to the direction of conventional current.

Electric current flows through a conductor due to the presence of free electron in the conductor. The electron which can move easily through a conductor is known as a free electron. 

ELECTRIC CIRCUITS:

A continuous conducting path consisting of wires and other resistances like electric bulb and a switch, between the two terminals of a cell or a battery along which an electric current flows is called an electric circuit.

Symbols for Electrical Components:

a. Cell : It supplies a continuous supply of potential difference.

b. Battery or Combination of cells:

c. Connecting Wire: It is made of conductors with a covering of insulator through which electric current can flow

d. A Wire Joint: A joint where two connecting wire is joined together. 

e. Wires crossing without connection:

When two wires cross each other without touching each other

f. Fixed Resistance or Resistor:

When the value of the resistor does not change.

g. Variable Resistance or Rheostat:

When the value of the resistor can change with time.

h. Ammeter: instrument which measures current.

i. Voltmeter: instrument which measures potential difference.

j. Galvanometer: instrument which detects the flow of current through a wire.

k. An open switch: which prevents the flow of current through a circuit.

l. A closed switch: which enables electric current to flow through a circuit.

m. Electric bulb: which emits light when electric current flow through it.

Circuit Diagrams:

A diagram which indicates how different components in a circuit have been connected by using the electrical symbols for the components, is called a circuit diagram. 

A simple electric circuit. In this circuit, a resistor R has been connected to the two terminals of a cell through a switch (which is closed). An ammeter A has been put in series with the resistor R. This is to measure current in the circuit. A voltmeter V has been connected across the ends of the resistor R, that is, voltmeter is connected in parallel with the resistor. This voltmeter is used to measure potential difference or voltage across the ends of the resistor R.

LECTURE -4 : CLASS VIII : SCIENCE : CHAPTER 3 : SYNTHETIC FIBRES & PLASTICS

CLASS VIII   |    SCIENCE    |    CHAPTER 3

      notes prepared by subhankar Karmakar

                                                                         

PLASTICS:

A Plastic is a synthetic material which can be moulded into desired shape when soft and then hardened to produce a durable article. Plastics are also polymers. Plastics are made from petroleum products called petrochemicals. 

Some of the examples of plastics are: Polythene, polyvinyl chloride (PVC), bakelite, melamine and Teflon. Nylon is also a plastic.

Polythene: 

It is a plastic obtained by the polymerization of a chemical compound known as ethene. Polythene is tough and durable. Polythene is used in making Polythene bags, waterproof plastic sheets, bottles, buckets and dustbins. Polythene is also used for packaging.

Polyvinyl chloride (PVC): 

It is a strong and hard plastic. It is not as flexible as polythene. PVC is used for making insulation of electric wires, pipes, garden hoses, rain coats, seat covers etc.

Bakelite: 

It is a very hard and tough plastic. Bakelite is a poor conductor of heat and electricity. Bakelite is used for making the handles of various cooking utensils. 

Bakelite is used for making handles cooking utensils because

1. It is a poor conductor of heat and

2. It does not become soft on getting heated as it is a thermosetting plastic.

Bakelite is also used for making electrical fittings such as electric switches, plugs and sockets etc. because

1. It does not conduct electricity and

2. It does not become soft on getting heated.

Melamine: 

It is a plastic which can tolerate heat better than other plastics and resists fire. 

Melamine is used for making floor tiles, unbreakable kitchenwares, ashtrays and fire resistant fabrics. 

Melamine is a fire resistant plastic, hence, the uniforms of fire man heavy coating of melamine plastic to make them fire resistant. 

Special plastic cookwares made of melamine is used in microwave oven for cooking food. In microwave oven the heat cooks the food but does not affect the plastic vessel.

Teflon:

Teflon is a special plastic on which oil and water do not stick. Oil and water do not stick on Teflon plastic because it has a slippery surface. Teflon also withstands high temperature. Teflon is used for giving non stick coating on cookwares like non stick frying pans. Teflon is also used for making soles of electric irons.

TYPES OF PLASTICS:

There are two types of plastics

1. Thermoplastics

2. Thermosetting plastics

a. Thermoplastics:

A Plastic which can be softened repeatedly by heating and can be moulded in two different shapes again and again, is called a thermoplastic. 

Properties of thermoplastics:

Thermoplastics are flexible so they can be bent easily without breaking. Thermoplastics are also known as thermosoftening plastics. Some of the examples of thermoplastics are: polythene and polyvinyl chloride (PVC).

Uses of thermoplastics:

Thermoplastics are used for making those articles which do not get too hot and are flexible. Thermoplastics are used for making insulation of electric wires and cables, various types of plastic containers like plastic bottles, plastic jars, combs, toys, plastic bags, raincoats, seat covers, bristles of brushes, packaging materials and chairs. 

Thermoplastics are used for making the insulation of electric wires because

1. They do not conduct electricity, and

2. They are flexible. 

b. Thermosetting plastics:

A plastic which once set, does not become soft on heating and cannot be moulded a second time, is called a thermosetting plastic. Once Seth in a given shape and solidified, a thermosetting plastic cannot be re-softened or remoulded. Thus, and article or object made of thermosetting plastic will retain its original shape permanently, even on heating. Thermosetting plastics are also known as thermosets.

Some of the examples of thermosetting plastics are: bakelite and melamine. 

Properties of thermosetting plastics:

Most of the thermosetting plastics hard and rigid. Thermosetting plastics are not flexible. Therefore, thermosetting plastics cannot bend. When an article made of thermosetting plastic is forced to bend, it breaks. Thermosetting plastics do not become soft on heating. Thermosetting plastics are used for making those articles which may get too hot during use and are hard and rigid so that they do not bend at all. 

Uses of thermosetting plastics:

Thermosetting plastics are used for making handles of cooking utensils, plates, cups, floor tiles, electrical fittings like electrical switches, plugs and sockets, ballpoint pens and telephone instruments. 

Reasons of using thermosetting plastics for making the handles of cooking utensils:

Thermosetting plastics are used for making the handles of cooking utensils because

a. They do not softened on getting heated, and

b. They are poor conductors of heat.

Reasons of using thermosetting plastics for making electrical fittings:

Thermosetting plastics are used for making electrical fittings such as electric switches, plugs and sockets because

a. They do not become soft on getting heated, and

b. They do not conduct electricity.

Plastics use for making toothbrush:

The handle and bristles of a toothbrush cannot be made of the same plastics, because the handle of a toothbrush has to be hard and rigid whereas the bristles of a toothbrush have to be soft and flexible. This means that the handle of a toothbrush should be made of thermosetting plastic whereas its bristles should be made of thermoplastic.

Structures of thermoplastics and thermosetting plastics:

In thermoplastics long polymer chains are not cross linked with one another but in thermosetting plastics the long polymer chains are cross linked with one another.

As in thermoplastic long polymer chains are not cross-linked, hence, the individual polymer chains can slide over one another and thermoplastic material become soft and ultimately melts.

As in thermosetting plastics, long polymer chains are cross linked, these cross-links prevent the displacement of individual polymer chains on being heated. Due to this, thermosetting plastics do not become soft on heating once they have been set into to a particular shape.

Useful properties of plastics:

Some of the most useful properties of plastics are as follows. 

a. Plastics are chemically unreactive. 

Plastics do not react with air and water. Therefore, plastics are resistant to corrosion. Due to this property, plastic containers are used to store various kinds of materials, including many chemicals.

b. Plastics are bad conductors of heat and electricity:

Plastics neither conducts heat nor electricity. They are used as insulators. This properties make it the material for making of handles of the cooking utensils and electrictical fittings like switches, plugs and sockets as well as it is used as the covering of the electrical wires. 

c. Plastics can be moulded into different shapes.

Since plastics can be very easily moulded, they are used to make a large variety of articles like buckets, mugs, furniture, bags, sheets, slippers, electrical fittings, toys, combs, toothbrushes etc.

d. Plastics are quite cheap and easily made:

Plastics a generally cheaper than metals and wood. Plastics production is also easy comparing to the extraction and purification of metals. These property made plastics as the first choice for making many of the household as well as industrial articles. It replaces metals in most of the cases. 

e. Plastics are light, strong and durable:

As the plastics has low density, they are lighter than metals. Plastics has good strength and they are durable too. Due to their low cost, high strength, easy availability, lightweight, long durability and corrosion resistant properties, plastics are now-a-days widely used for the making of most of the articles we use. 

BIODEGRADABLE & NON-BIODEGRADABLE MATERIALS:

A material which gets decomposed through natural processes such as action of bacteria and other other natural factors are called biodegradable material. They are environment friendly. Plant and animal wastes, paper, cotton, cloth, wool, jute, wood are some examples of biodegradable materials. 

A material which is not easily decomposed by natural processes are called non-biodegradable material. They are not environment friendly. They directly or indirectly pollute nature. Plastics, glass, metal foils, aluminium cans are some examples of non-biodegradable materials. 

PLASTICS POLLUTE ENVIRONMENT:

Plastic materials have a bad effect on the environment. The excessive use of plastic materials affect our environment in the follwing ways. 

a. Plastics are non-biodegradable, so, the waste plastic articles keep on accumulating in the surroundings and pollute the environment badly.

b. The waste plastic articles thrown in the drain choked them and causes them to overflow and create an unhygienic condition. 

c. Plastics may be eaten by the animals, which causes permanent damage to the animals. 

d. If plastic materials are burnt they produce toxic gases which pollutes air and ultimately become the causes of many diseases.

The ways to prevent environment pollution from excessive plastic wastes:

Plastics are very useful materials but use of plastic articles is not good for the environment. Therefore, some steps should be taken to save the environment from plastic waste. They are as follows:

1. We should try to reduce or minimise the use of plastic by using other alternative materials. Like instead of using Polythene bags we can use jute or paper bags.

2. We should not throw Polythene bags, wrappers of chips, biscuits or other eatables in water bodies, on the roads, in parks or picnic places. We should use dustbin for solid waste.

3. We should reuse the plastic containers which come with jams, pickles, oils and other packed food. 

4. Plastic wastes should be recycled. All the plastic waste in in the different places should be collected and sent for recycling to plastic making factories. In plastic factories, the waste plastic articles are melted and used to make new plastic articles. All the recycled plastic product are given certain colours, so that buyers can understand they are recycled plastic product and they don't use them for stories of food.

5. We should follow 3R principle. It means Reduce, Reuse and Recycle for plastic products.