Glossary
Every technical word in the course and what it means in plain language, with flashcards for practice.
Every technical term in the course, with a plain-language meaning. On every page these words are marked with a dotted underline: hover or tap one to see its meaning. The “Used in” links point to the sections where each term appears.
Ch 1 · Light – Reflection and RefractionCh 2 · The Human Eye and the Colourful WorldCh 3 · ElectricityCh 4 · Magnetic Effects of Electric Current
Ch 1 · Light – Reflection and Refraction
| Term | Meaning | Used in |
|---|---|---|
| angle of emergence | The angle between the emergent ray and the normal at the surface where light leaves a slab or prism. For a slab with parallel faces it equals the angle of incidence. | 1.3 2.2 |
| angle of incidence | The angle between the incident ray and the normal (the line at 90° to the surface) at the point where the ray strikes. | 1.1 1.3 2.2 |
| angle of refraction | The angle between the refracted ray and the normal at the point where the ray crosses the boundary between two media. | 1.3 2.2 |
| aperture of a lens | The effective diameter of the outline of a lens or spherical mirror. The lens and mirror formulas hold for small apertures (rays close to the principal axis). | |
| centre of curvature | The centre $C$ of the sphere of which a spherical mirror (or lens surface) is a part. The line from $C$ to any point of the mirror is the normal there. | 1.1 1.2 1.4 |
| concave lens | A lens thinner at the middle than at the edges. It spreads parallel rays out as if from a virtual principal focus, so it is also called a diverging lens; its focal length and power are negative. | 1.4 2.1 |
| concave mirror | A spherical mirror whose reflecting surface curves inwards, like the inside of a spoon. It makes parallel rays meet at a real focus. | 1.1 1.2 1.4 |
| convex lens | A lens thicker at the middle than at the edges. It bends parallel rays to meet at a real principal focus, so it is also called a converging lens; its focal length and power are positive. | 1.4 2.1 |
| convex mirror | A spherical mirror whose reflecting surface bulges outwards, like the back of a spoon. It spreads rays out and always gives a smaller, erect, virtual image. | 1.1 1.2 1.4 |
| dioptre | The SI unit of the power of a lens, symbol D: $1\ \text{D}=1\ \text{m}^{-1}$, the power of a lens of focal length $1\ \text{m}$. | 1.4 2.1 |
| emergent ray | The ray that comes out of a block, slab, prism or lens after refraction at its second surface. | 1.3 2.2 |
| focal length | The distance $f$ from the pole of a mirror (or optical centre of a lens) to its principal focus. For a spherical mirror $f=R/2$. | 1.1 1.2 1.4 2.1 |
| image distance | The distance $v$ of the image from the pole of a mirror or the optical centre of a lens, with its sign given by the New Cartesian sign convention. | 1.1 1.2 1.4 |
| lateral inversion | The left–right swap seen in a plane-mirror image: your right hand appears as the image's left hand. | |
| lateral shift | The perpendicular distance between the incident ray (produced) and the parallel emergent ray from a slab with parallel faces. It grows with thickness, angle of incidence and refractive index. | 1.3 |
| lens formula | The relation $\frac1v-\frac1u=\frac1f$ between object distance $u$, image distance $v$ and focal length $f$ of a thin lens (New Cartesian sign convention). The magnification is $m=v/u$. | 1.2 1.4 2.1 |
| magnification | The ratio $m=h'/h$ of image height to object height. For a mirror $m=-v/u$; a negative $m$ means a real, inverted image and a positive $m$ a virtual, erect one. | 1.2 1.4 |
| mirror formula | The relation $\frac1v+\frac1u=\frac1f$ between object distance $u$, image distance $v$ and focal length $f$ of a spherical mirror (New Cartesian sign convention, small aperture). | 1.2 1.4 |
| New Cartesian sign convention | The rule for signs in optics: distances are measured from the pole or optical centre, positive in the direction of the incident light and negative against it; heights are positive upwards. | 1.2 1.4 2.1 |
| optical centre | The central point $O$ of a lens on its principal axis. A ray passing through it goes straight on without deviation; lens distances are measured from it. | 1.4 |
| optically denser medium | Of two media, the optically denser one has the higher refractive index (light travels more slowly in it); the other is optically rarer. This is not the same as mass density. | 1.3 2.3 |
| power of a lens | The reciprocal of the focal length in metres, $P=1/f$, measured in dioptres. Positive for a convex lens, negative for a concave lens; for thin lenses in contact the powers add. | 1.4 2.1 |
| principal axis | The straight line through the pole and the centre of curvature of a mirror, or through the optical centre of a lens and its two centres of curvature. | 1.1 1.2 1.4 |
| principal focus | The point $F$ on the principal axis where rays parallel to the axis meet after reflection or refraction (real focus), or from which they appear to spread out (virtual focus). | 1.1 1.4 |
| real image | An image formed where light rays actually meet after reflection or refraction. It can be caught on a screen; for a single mirror or lens it is inverted. | 1.2 1.4 2.1 |
| refraction of light | The change in direction of light when it passes obliquely from one transparent medium into another, caused by the change in the speed of light. | 2.3 |
| refractive index | A number that measures how much a medium slows light: the absolute refractive index is $n=c/v$; the index of medium 2 relative to medium 1 is $n_{21}=v_1/v_2=n_2/n_1$. It has no unit. | 1.3 2.2 2.3 |
| Snell's law | The second law of refraction: for a given colour and pair of media, $\sin i/\sin r$ is constant, equal to $n_{21}$. Equivalently $n_1\sin i=n_2\sin r$. | 1.3 2.2 2.3 |
| virtual image | An image formed where light rays only appear to meet when traced backwards. It cannot be caught on a screen; for a single mirror or lens it is erect. | 1.2 1.4 2.1 |
Ch 2 · The Human Eye and the Colourful World
| Term | Meaning | Used in |
|---|---|---|
| advance sunrise and delayed sunset | Because atmospheric refraction lifts the Sun's image by about half a degree, the Sun is seen about 2 minutes before it truly rises and about 2 minutes after it truly sets, lengthening the day by about 4 minutes. | 2.3 |
| angle of deviation | The angle $\delta$ between the direction of the incident ray (extended forwards) and the emergent ray. A prism deviates light towards its base. | 2.2 |
| angle of the prism | The angle $A$ between the two refracting surfaces of a prism — $60^\circ$ for an equilateral prism. | 2.2 |
| atmospheric refraction | The bending of light by the Earth's atmosphere, whose layers and moving pockets of air have slightly different refractive indices. It makes stars twinkle and lifts the apparent position of stars and the Sun. | 2.3 |
| ciliary muscles | The ring of muscle that holds the eye lens and changes its curvature, and so its focal length: relaxed for distant objects, contracted for near ones. | 2.1 |
| colloid | A mixture whose particles are much larger than molecules but too small to settle or be seen individually, such as milk, smoke, mist or fog. Its particles scatter light. | 2.3 |
| cornea | The clear, curved front surface of the eye. Light enters the eye through it, and it does most of the bending (refraction) of that light. | 2.1 |
| dispersion of light | The splitting of white light into its component colours when it is refracted, because the refractive index of the medium is different for each colour (largest for violet, smallest for red). | 2.2 |
| far point | The farthest point at which the eye can see an object clearly — at infinity for a normal eye, nearer for a myopic eye. | 2.1 |
| hypermetropia | Far-sightedness: nearby objects look blurred because the near point is beyond 25 cm and their image would form behind the retina. Corrected with a convex lens. | 2.1 |
| least distance of distinct vision | The near point of a normal eye, taken as 25 cm. It is the usual reading distance used in spectacle calculations. | 2.1 |
| monochromatic light | Light of a single colour (a single wavelength). A prism deviates it but cannot split it further. | 2.2 |
| myopia | Near-sightedness: distant objects look blurred because their image forms in front of the retina. The far point is nearer than infinity; corrected with a concave lens of focal length $f=-(\text{far point})$. | 2.1 |
| near point | The nearest point at which the eye can see an object clearly without strain — about 25 cm for a normal young adult eye. | 2.1 |
| power of accommodation | The ability of the eye lens to adjust its focal length so that both near and distant objects are focused sharply on the retina. | 2.1 |
| presbyopia | The loss of the power of accommodation with age, as the ciliary muscles weaken and the eye lens stiffens; the near point recedes. Corrected with convex reading lenses, or bifocals if myopia is also present. | 2.1 |
| rainbow | A natural spectrum seen in the sky opposite the Sun after rain, formed by refraction with dispersion, internal reflection and refraction again in raindrops. Red is on the outside (about $42^\circ$ from the antisolar point), violet on the inside (about $41^\circ$). | 2.2 |
| recombination of the spectrum | Putting the colours of a spectrum back together into white light, for example with a second identical prism placed inverted after the first. | 2.2 |
| retina | The light-sensitive layer at the back of the eye, on which the eye forms a real, inverted image. Its rods and cones turn light into electrical signals sent along the optic nerve to the brain. | 2.1 |
| scattering of light | The sending of light in all directions by small particles in its path. Very fine particles scatter blue light much more than red; large particles scatter all colours about equally. | 2.3 |
| spectrum | The band of colours obtained when light is dispersed. The spectrum of white light runs violet, indigo, blue, green, yellow, orange, red (VIBGYOR). | 2.2 |
| triangular prism | A block of transparent material with two triangular ends and three rectangular faces. Light enters one rectangular face and leaves through another, bending towards the base. | 1.3 1.4 2.2 |
| true solution | A solution such as salt or sugar dissolved in water, in which the dissolved particles are single molecules or ions — too small to scatter a beam of light appreciably. | 2.3 |
| twinkling of stars | The flickering of a star's brightness and position, caused by changing atmospheric refraction of light from a point source. Planets, which appear as small discs, do not twinkle. | 2.3 |
| Tyndall effect | The scattering of a light beam by the particles of a colloid (smoke, mist, diluted milk), which makes the path of the beam visible. | 2.3 |
| VIBGYOR | Memory aid for the colours of the spectrum of white light in order: Violet, Indigo, Blue, Green, Yellow, Orange, Red. In a prism violet is deviated most and red least. | 2.2 |
Ch 3 · Electricity
| Term | Meaning | Used in |
|---|---|---|
| ammeter | An instrument that measures electric current. It has a very low resistance and is always connected in series with the component. | 3.1 3.2 3.3 4.2 |
| ampere | The SI unit of electric current, symbol A: a current of $1\ \text{A}$ is a flow of $1$ coulomb of charge per second. | 3.1 4.2 4.3 |
| branch | One of the separate paths between two junctions in a parallel circuit; the branch currents add up to the current in the main wire. | 3.3 |
| circuit diagram | A drawing of an electric circuit using standard symbols for the cells, keys, resistors, meters and other components, joined by straight lines for the wires. | 3.1 3.3 |
| conventional current | The agreed direction of current: from the positive terminal of a cell, round the external circuit, to the negative terminal — opposite to the direction in which electrons drift. | 3.1 |
| coulomb | The SI unit of electric charge, symbol C. One coulomb is the charge of $6.25\times10^{18}$ electrons; the charge of one electron has size $e=1.6\times10^{-19}\ \text{C}$. | 3.1 3.3 |
| electric current | The rate of flow of electric charge through a cross-section of a conductor, $I=\frac Qt$. Measured in amperes with an ammeter. | 3.1 4.2 4.3 |
| electric power | The rate at which electrical energy is converted into other forms, $P=VI=I^2R=\frac{V^2}{R}$. Measured in watts. | 3.4 4.3 |
| energy meter | The meter at the point where the mains enters a building that records the electrical energy used, in kilowatt-hours (units). | 3.4 4.3 |
| equivalent resistance | The resistance of the single resistor that could replace a combination of resistors between two points and draw the same current from the same potential difference, $R_{\text{eq}}=\frac VI$. | 3.3 |
| free electron | An electron in a metal that is only loosely held by its atom and can move through the whole conductor. Free electrons carry the current in metal wires. | 3.1 3.2 3.3 |
| fuse | A safety device: a short wire of low melting point connected in series with a circuit. If the current exceeds its rating, the wire melts and breaks the circuit. | 3.3 3.4 4.3 |
| heating effect of current | The production of heat in a conductor when a current flows through its resistance. Used in heaters, irons, kettles, filament bulbs and fuses. | 3.4 |
| Joule's law of heating | The heat produced in a resistor $R$ carrying a steady current $I$ for a time $t$ is $H=I^2Rt$: proportional to the square of the current, to the resistance and to the time. | 3.4 |
| junction | A point in a circuit where three or more wires meet, shown by a dot in a circuit diagram. The current flowing into a junction equals the current flowing out. | 3.1 3.3 |
| kilowatt-hour | The commercial unit of electrical energy, one "unit" on the electricity bill: the energy used by a $1\ \text{kW}$ appliance in $1$ hour, $1\ \text{kWh}=3.6\times10^6\ \text{J}$. | 3.4 |
| nichrome | An alloy of nickel and chromium with a high resistivity (about $1.0\times10^{-6}\ \Omega\ \text{m}$) that does not oxidise easily when red-hot; used for the heating elements of irons, toasters and heaters. | 3.2 3.4 |
| ohm | The SI unit of resistance, symbol $\Omega$: a conductor has a resistance of $1\ \Omega$ if $1\ \text{V}$ across it drives a current of $1\ \text{A}$, so $1\ \Omega=1\ \text{V/A}$. | 3.2 3.3 3.4 |
| Ohm's law | At constant temperature, the potential difference across a metallic conductor is directly proportional to the current through it: $V=IR$. Its V–I graph is a straight line through the origin. | 3.2 3.3 3.4 |
| overloading | Drawing more current from a circuit than it is designed to carry, usually by switching on too many appliances at once; the fuse should then melt. | 3.4 4.3 |
| parallel combination | Resistors each connected between the same two junctions: each has the same potential difference across it, the branch currents add, and $\frac1{R_p}=\frac1{R_1}+\frac1{R_2}+\frac1{R_3}$. | 3.3 |
| potential difference | The work done in moving a unit positive charge from one point of a circuit to another, $V=\frac WQ$. Measured in volts with a voltmeter; also called voltage. | 3.1 3.2 3.3 3.4 |
| resistance | The opposition a conductor offers to the flow of charge, $R=\frac VI$. Measured in ohms; for a uniform wire $R=\rho\frac lA$. | 3.1 3.2 3.3 3.4 |
| resistivity | A property of a material, $\rho=\frac{RA}{l}$: the resistance of a piece $1\ \text{m}$ long with cross-section $1\ \text{m}^2$. Unit ohm metre ($\Omega\ \text{m}$); it does not depend on the size of the wire. | 3.2 3.4 |
| resistor | A component made to have a particular resistance, used to control the current in a circuit. Drawn as a zig-zag in circuit diagrams. | 3.1 3.2 3.3 3.4 |
| rheostat | A variable resistor: a coil of resistance wire with a sliding contact, used to change the current in a circuit without changing the battery. | 3.1 3.2 3.3 4.3 |
| series combination | Resistors joined end to end so that there is only one path: the same current flows through each, the potential differences add, and $R_s=R_1+R_2+R_3$. | 3.3 |
| volt | The SI unit of potential difference, symbol V: $1\ \text{V}=1\ \text{J/C}$, the potential difference when $1$ joule of work moves $1$ coulomb of charge. | 3.1 |
| voltmeter | An instrument that measures potential difference. It has a very high resistance and is always connected in parallel, across the component. | 3.1 3.2 3.3 |
| V–I graph | A graph of potential difference $V$ against current $I$ for a component. For an ohmic conductor it is a straight line through the origin whose slope is the resistance. | 3.2 |
| watt | The SI unit of power, symbol W: $1\ \text{W}=1\ \text{J/s}=1\ \text{V}\times1\ \text{A}$. $1\ \text{kW}=1000\ \text{W}$. | 3.4 |
Ch 4 · Magnetic Effects of Electric Current
| Term | Meaning | Used in |
|---|---|---|
| alternating current (AC) | Current that reverses its direction again and again. The Indian mains is AC of 50 Hz: it changes direction 100 times a second. Its power can be transmitted over long distances without much loss of energy. | 4.3 |
| clock rule | Looking at a face of a current-carrying coil or the end of a solenoid: if the current flows anticlockwise that face is a north pole; if clockwise, a south pole. | 4.2 |
| compass needle | A small, light magnet pivoted so that it can turn freely. Its north pole points in the direction of the magnetic field where it is placed — northwards when only the Earth's field acts. | 4.1 4.2 4.3 |
| direct current (DC) | Current that always flows in the same direction round a circuit, such as the current from a cell or battery. | 4.3 |
| earth wire | A safety wire joined to a metal plate buried in the ground and to the metal bodies of appliances. It carries current only during a fault. | 4.3 |
| earthing | Connecting the metal body of an appliance to the ground through the earth wire, so that a leakage current flows away safely and the body never stays at a high voltage. | 4.3 |
| electromagnet | A solenoid wound on a soft-iron core. It is a strong magnet only while a current flows; its strength can be changed and its poles reversed with the current. | 4.2 4.3 |
| Fleming's left-hand rule | Stretch the thumb, forefinger and middle finger of the left hand at right angles: forefinger = magnetic field, middle finger = current, thumb = force (motion) on the conductor. | 4.3 |
| iron filings | Tiny pieces of iron. Sprinkled on a card over a magnet and tapped, each becomes a small magnet and lines up with the field, showing the pattern (but not the direction) of the field lines. | 4.1 4.2 |
| live wire | The wire that brings the current into a house, at 220 V above the Earth. Switches and fuses are placed in it. | 3.4 4.3 |
| magnetic field | The region around a magnet (or a current) in which its magnetic force can be detected. It has a strength and a direction: the direction in which the north pole of a compass needle points. | 4.1 4.2 4.3 |
| magnetic field line | A curve drawn so that its tangent at every point gives the direction of the magnetic field there. Outside a magnet it runs from N to S, inside from S to N; lines never cross and are crowded where the field is strong. | 4.1 4.2 |
| magnetic force on a conductor | The force a magnetic field exerts on a conductor that carries a current. It is greatest when the conductor is at right angles to the field and zero when it is parallel to it. | 4.3 |
| magnetic material | A material that is attracted by a magnet and can itself be magnetised, such as iron, steel, nickel and cobalt. Copper, aluminium, wood and plastic are not magnetic materials. | 4.1 4.3 |
| magnetic pole | One of the two regions near the ends of a magnet where its attraction is strongest: the north-seeking (N) and south-seeking (S) pole. Like poles repel, unlike poles attract; poles always come in pairs. | 4.1 4.3 |
| Maxwell's corkscrew rule | Another form of the right-hand thumb rule: a right-handed screw driven forward in the direction of the current turns in the direction of the magnetic field. | 4.2 |
| miniature circuit breaker (MCB) | An automatic switch in the live wire that opens (trips) when the current exceeds its rating; it is switched back on after the fault is cleared. | 3.4 4.3 |
| neutral point | A point where the magnetic fields acting there cancel, so the total field is zero and a compass needle has no preferred direction — e.g. midway between two equal like poles facing each other. | 4.1 4.2 |
| neutral wire | The wire that takes the current back to the supply, at about the same potential as the Earth (0 V). | 4.3 |
| permanent magnet | A magnet, usually of steel or a special alloy, that keeps its magnetism without any current, e.g. a bar magnet or a compass needle. | 4.2 4.3 |
| right-hand thumb rule | Hold a current-carrying straight wire in the right hand with the thumb along the current: the curled fingers give the direction of the magnetic field lines round it. | 4.2 4.3 |
| short-circuiting | The live and neutral wires touching directly, so that the resistance of the path is very small and the current becomes very large. | 3.1 4.3 |
| soft iron | Nearly pure iron that is magnetised strongly by a current-carrying coil but loses almost all its magnetism when the current stops — the core material for electromagnets. | 4.1 4.2 4.3 |
| solenoid | A coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder. With a current it behaves like a bar magnet, with a uniform field inside. | 4.1 4.2 |
| uniform magnetic field | A field with the same strength and direction at every point of a region, drawn as straight, parallel, equally spaced lines — e.g. between the pole faces of a horseshoe magnet or inside a long solenoid. | 4.1 4.2 4.3 |