DEFINITIONS : PHYSICS

• Reflection(of light): The bouncing back of a ray of light after striking the surface of an object is called reflection of light.


• Concave Mirror: A spherical mirror, whose reflecting surface is curved inwards, i.e., it faces towards the centre of the sphere, is called a concave mirror.


• Convex Mirror: A spherical mirror, whose reflecting surface is curved outwards, i.e., the centre of the sphere is behind the reflecting surface, is called a convex mirror.


• Pole(of a mirror): The centre of the reflecting surface of a spherical mirror is called the pole of the mirror.


• Centre of Curvature: The centre of the sphere of which the mirror(or lens) is a part, is called the centre of curvature of that mirror(or lens).


• Radius of Curvature: The radius of the sphere of which the mirror(or lens) is a part, is called the radius of curvature of that mirror(or lens).


• Principal Axis of a Mirror: An imaginary straight line passing through the pole and the centre of curvature of a spherical mirror is called the principal axis of that mirror.


• Principal Focus of a Mirror: The point, on the principal axis of a spherical mirror, at which the rays parallel to the principal axis meet or appear to meet after reflection is called the principal focus of that mirror.


• Focal Length of a Mirror: The distance between the pole and the principal focus of a spherical mirror is called focal length of that mirror.


• Aperture of a Mirror: The diameter of the reflecting surface of a spherical mirror is called the aperture of the mirror.


• Magnification: The ratio of the height of the image to the height of the object is called magnification.


• Refraction: The phenomenon in which the direction of a light ray changes when it enters from one medium to another medium, is called refraction.


• Absolute Refractive Index: The ratio of the speed of light in vacuum to that in a medium is called the absolute refractive index of that medium.[Generally absolute refractive index is referred to as Refractive index].


• Relative Refractive Index: The ratio of the speed of light in one medium to that in another medium is called (Relative) refractive index of second medium with respect to the first medium.


• Convex Lens: A lens having two spherical surfaces bulging outwards is called a convex lens.[we use convex in place of biconvex for the sake of convenience].


• Concave Lens: A lens having two spherical surfaces curved inwards is called a concave lens.[we use concave in place of biconcave for the sake of convenience].


• Optical Centre: The central point of a lens is called optical centre of the lens.


• Principal Axis of a Lens: An imaginary straight line passing through the two centres of curvature of a lens is called the principalaxis of the lens.


• Aperture of a Lens: The effective diameter of the circular outline of a spherical lens is called the aperture of the lens.


• Principal Focus of a Lens: The point on the principal axis of a lens at which the rays parallel to the principal axis meet or appear to meet after refraction, is called the principal focus of the lens.


• Focal Length of a Lens: The distance between optical centre and the principal focus of a lens is called the focal length of the lens.


• Power of a Lens: The reciprocal of the focal length of a lens is called power of the lens.


• Power of Accommodation: The ability of the eye lens to adjust its focal length in such a way that the image of the object is formed exactly on retina, is called the power of accommodation (of eyes).


• Least Distance of Distinct vision(Near Point of the Eye): The minimum distance, at which objects can be seen most distinctly without strain, is called least distance of distinct vision.


• Far Point of the Eye: The farthest point up to which the eye can see objects clearly, is called Far point of the eye.


• Angle of Prism: The angle between two lateral faces of a prism is called the prism angle.


• Angle of Deviation: The angle between the incident ray and the emergent ray is called the angle of deviation.


• Dispersion of Light: The splitting of light into its constituent colours is called dispersion of light.


• Spectrum: The group of bands of different component colours of a light beam is called a spectrum.


• Electric Current: The continuous flow of electric charge(electrons) through a conductor(wire) is called electric current.


• Electric Circuit: A continuous and closed path through which electric current can pass is called electric circuit.


• 1-Ampere Current: When one coulomb of charge passes through a point of a conductor per second, the current is said to be 1-ampere current.


• Electric Potential difference: The work done to move a unit charge from one point to the other in an electric field is called the electric potential difference between those two points.


• 1-Volt Potential Difference: When one joule of work is done to move a charge of 1 coulomb from one point to the other in an electric field, the potential difference between those two points is called 1 volt.


• Resistance: It is the property of a conductor by which it(the conductor) opposes the movement of electric charge through it.


• 1-Ohm Resistance: When the potential difference between the terminals of a conductor carrying one ampere current is one volt, its resistance is 1 ohm.


• Resistor: A conductor having some appreciable resistance is called resistor.


• Insulator: A component which offers very high resistance so that practically no current can pass through it is called insulator.


• Resistivity: It is a characteristic property of the material of a conductor on which the resistance offered by the conductor depends.


• Electric power: The rate at which electric energy is dissipated or consumed by an equipment is called its power.


• Direct Current: The current which flows in only one direction and does not change its direction with time is called direct current.


• Alternating Current: The current which changes its direction between two terminals periodically is called alternating current.


• Overloading: A situation in which the current flowing in a circuit increases beyond the current it can hold is called overloading.


• Short-Circuiting A situation created due to the direct contact of the live wire and the neutral wire in which the current increases abruptly resulting in overloading is called short-circuiting.

DEFINITIONS : CHEMISTRY

• Chemical Reaction: When a substance changes chemically during a reaction, the reaction is called a chemical reaction.


• Reactant: The substance that undergoes chemical change is called a reactant.


• Product: The new substance that is formed during a reaction is called a product.


• Skeletal Chemical Equation: A chemical equation which shows both reactants and products but which is not balanced as regards the mass of the substances involved is called a skeletal chemical equation.


• Combination Reaction: A reaction in which a single product is formed from two or more reactants is called a Combination reaction.


• Exothermic Reaction: A reaction in which heat is evolved along with the formation of the products is called an Exothermic reaction.


• Endothermic Reaction: A reaction in which heat is absorbed during the formation of products is called an Endothermic reaction.


• Decomposition Reaction: A reaction in which a single reactant breaks down to give more than one simpler product is called a Decomposition reaction.


• Thermal Decomposition: A reaction in which decomposition of reactant is carried out by heating it is called thermal decomposition.


• Displacement Reaction: A reaction in which a more reactive element displaces a less reactive element from its solution is called a displacement reaction.


• Double Displacement Reaction: A reaction which involves an exchange of ions between the reactants is called a double displacement reaction.


• Precipitation Reaction: A reaction in which an insoluble product(precipitate) is formed is called a precipitation reaction.


• Oxidation: If a substance gains oxygen or loses hydrogen it is said to undergo oxidation.


• Reduction: If a substance gains hydrogen or loses oxygen it is said to undergo reduction.


• Redox Reaction: A reaction in which one substance is oxidised and the other is reduced is called Redox reaction.[NOTE: A reaction cannot be either an oxidation reaction or a reduction reaction independently.]


• Corrosion: When a metal is attacked by substances like moisture, acids, etc. around it and converted into its salt it is said to corrode and the process is called corrosion.


• Rancidity: The change in the smell and taste of fats and oils due to their oxidation is called rancidity.


• Olfactory Indicators: Substances whose odour changes in acidic or basic mediums are called olfactory indicators when they are used to identify whether a substance is an acid or a base.


• Neutralisation Reaction: The reaction between an acid and a base giving a salt and water is called neutralisation reaction.


• Alkali: Bases which are soluble in water are called alkalies.


• Antacid: A base which is used to neutralise excess hydrochloric acid produced in stomach to reduce pain and irritation is called an antacid.


• Water of Crystallisation: The fixed number of water molecules present in one formula unit of a salt(compound) is called water of crystallisation.


• Ductility: The ability of metals to be drawn into thin wires is called ductility.


• Malleability: The ability of metals to be beaten(hammered) inti thin sheets is called malleability.


• Allotrope: Each of structurally different forms of an element is called an allotrope of that element.


• Ionic Compounds: Compounds formed by the transfer of electrons from a metal to a nonmetal are called ionic compounds.


• Covalent Compounds: Compounds formed by sharing of electron pairs between atoms are called covalent compounds.


• Minerals: The elements or compounds which occur naturally in the earth's crust are called minerals.[NOTE: Minerals in chemistry are compounds in general but Minerals in biology are elements. For example Na and Fe are minerals for a biologist]


• Ore: A mineral from which a particular metal can be extracted profitably is called an ore.


• Gangue: Impurities like sand present in an ore mined from the earth are collectively called gangue.


• Calcination: The process of heating carbonate ores strongly in limited supply of air to change them into oxides is called calcination.


• Roasting: The process of heating sulphide ores strongly in presence of excessive air to change them into oxides is called roasting.


• Galvanisation: The process of deposition(coating) of a thin layer of zinc on steel and iron to prevent their rusting is called galvanisation.


• Alloy: A homogeneous mixture of two or more metals or a metal and a nonmetal is called an alloy.


• Amalgam: An alloy containing mercury as one of the constituents is called an amalgam.


• Catenation: The property of carbon atoms by which one carbon atom can form covalent bonds with other carbon atoms giving rise to large molecules is called catenation.


• Isomers: Compounds with identical molecular formula but different structures(atomic arrangements) are called isomers.


• Functional Group: An atom or a group of atoms which confers specific properties to an organic compound regardless of the length and nature of the carbon chain is called a functional group.


• Homologous Series: A series of organic compounds having same functional group and in which there is a difference of -CH₂ between two successive compounds is called homologous series.


• Addition Reaction: A reaction in which an atom or group of atoms enters a molecule without replacing another atom from it is called an addition reaction.


• Substitution Reaction: A reaction in which an atom or group of atoms replaces another atom(or group of atoms) to enter the molecule is called a substitution reaction.


• Catalyst: A catalyst is a substance that causes a reaction to occur or proceed at a different rate without the reaction itself being affected.


• Isotope: Isotopes are atoms of an element with similar chemical properties but different atomic masses due to different number of neutrons in their nuclei.


• Atomic Radius: The distance between the centre of the nucleus and the outermost shell of an isolated atom is called its atomic radius.


• Oxidising Agent: A substance which supplies oxygen to or removes hydrogen from other substance is called an oxidising agent.


• Reducing Agent: A substance which supplies hydrogen to or removes oxygen from other substance is called an reducing agent.