FSc ICS Notes Physics XII Short Questions & Definitions Chapter 19 Dawn of Modern Physics

FSc ICS Notes Physics XII Short Questions & Definitions Chapter 19 Dawn of Modern Physics 2nd Year Physics Notes Online Taleem Ilmi Hub

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Modern Physics: New Physics which is more general and more basic than the Newtonian Physics.

Classical Physics (or Newtonian Physics): The part of Physics that was developed before, and therefore does not include, either quantum theory or the theory of relativity.

Black body (or Full radiator): A body that absorbs all the radiation falling on it.

Black body radiation: The thermal radiation from a black body at a given temperature, having a spectral distribution of energy with some specific characteristics.

Photoelectric effect: The emission of electrons by a substance when illuminated by electromagnetic radiation.

Quantum theory: According to it the radiant energy of a system or the exchange of radiant energy between different systems occurs not in a continuous fashion permitting all possible values, as demanded by the wave theory but in a discrete quantified form, as integral multiples of an elementary quantum of energy.

Theory of relativity: A theory formulated by Einstein, that recognizes the impossibility of determining absolute motion and leads to the concept of a four-dimensional space-time continuum.

Special theory of relativity: The part of relativistic mechanics as developed by Einstein which pertains to uniform motion.

General theory of relativity: The part of relativistic mechanics as developed by Einstein, which pertains to accelerated motion.

Electromagnetic radiation (or Electromagnetic waves): Transverse waves in space having an electric component and a magnetic component, each being perpendicular to each other and both  perpendicular to the direction of propagation. These do not require any medium for its motion.

Globe: A spherical representation of our planet the earth.

Coordinate: One of the quantities used to define the position of a point relative to a frame of reference. There are three main coordinate systems.

Cartesian coordinate system: Three mutually perpendicular lines/axis are drawn through a point O known as the origin.

Cylindrical polar coordinates: The position of a point P is specified by three coordinates: radial distance r, azimuthal angle θ , and axial distance z, these being related to the Cartesian system by

x = rcosθ, y = rsinθ, z ≡ z

Spherical polar coordinates: The coordinates of P are the radius r, the azimuthal angle θ, and the angle of longitude φ. These are related to the Cartesian system by x = r sinθ cosφ, y = r sinθ sinφ, z = r cosθ

Frame of reference: A coordinate system in which measurements are made.

Inertial frame of reference: A frame of reference in which law of inertia holds.

Accelerated frame of reference: A frame of reference in which law of inertia does not hold.

Law of inertia (or 1st law of motion): A body at rest remains at rest unless an unbalanced force produces acceleration in it.

Ether: A hypothetical fluid, formerly thought to permeate all space and to be the medium through which electromagnetic waves were propagated.

Absolute motion (or velocity): Velocity (or motion) measured relative to hypothetical medium ether.

Absolute rest: The state of a body in which its position with respect to hypothetical medium ether does not change.

Relative velocity: The vector difference of the two velocities.

Relativistic velocity: Any velocity that is sufficiently high to make the mass of an object significantly greater than its rest mass.

Uniform motion: Motion of a body with equal distances in equal intervals of time.

Time dilation: Time is not absolute, but depends upon the motion of the frame of reference according to the equation, t = to/(√ 1 – v2 / c2)

Length contraction: In relative motion, length of two points fixed distance apart, appears shorter length than if we are at rest relative to them, it is according to the equation, l = lo /(√ 1 – v2 / c2)

Mass variation: .The mass of an object is a varying quantity and depends upon the speed of the object. They are related by, m = mo / (√ 1 – v2 / c2)

Energy-mass relation: Mass and energy are different entities but are interconvertible, according to the equation, E = m c(square)

NAVSTAR GPS:. Formally known as the Navstar Global Positioning System, was initiated in 1973. it is joint service effort directed by the United States Department of Defense. Navstar GPS is a space-based radio-positioning system consisting of 24 orbiting satellites that provide navigation and timing

information to military and thousands of civilian users worldwide. It is based on Einstein’s theory of relativity.

Navigation: The art or science of directing a ship, boat, or aircraft, spacecraft, etc.; any of several methods of determining or planning a craft's position and course by geometry, astronomy, etc.

Radiation: Emission of energy from the surface of a body in the form of electromagnetic waves.

Black body radiation: The thermal radiation from a black body at a given temperature, having a spectral distribution of energy with some specific characteristics.

Cavity: A hollow place; a hollow; an empty space in a solid body.

Black body (or Cavity radiator): A substance that absorbs completely radiations of all frequencies, and can also emit radiations of all frequencies.

Cavity absorber: A device that may take the form either of a narrow tube or cavity through which sound is passed, or a hollow resonator placed in the sound field.

Radiator: A thing that radiates something, especially, light, heat, or sound waves.

Stefan’s law: Former name of Stefan-Boltzmann law.

Stefan-Boltzmann law: The total amount of heat radiated by a perfectly black body per second per unit area is directly proportional to the fourth power of its absolute temperature; E = σ T4

Stefan’s constant: Former name for Stefan-Boltzmann constant.

Stefan-Boltzmann constant: The constant σ in Stefan-Boltzmann law, equal to σ = 2π5 k4 / 15 h3 c2

Nobel prize/laureate: A fund of $ 9 million was set up. The interest of which is annually distributed among six important discoveries or inventions in Physics, Chemistry, Medicine, Literature, Economics, and for peace among nations. It is according to will of Alfred Bernhard Nobel (1833 -1896).

Quantum (plural Quanta): The smallest amount of energy that a system can gain or lose.

Photon: The basic energy packet of electromagnetic radiation.

Granular: Consisting of grains or granules. Having a granulated surface or structure.

Electromagnetic spectrum: Complete array of known electromagnetic radiations.

Radio waves (or Radio frequency): The frequency of electromagnetic radiation within the range used in radio, i.e. from 3x10(7) to 3x10(9) Hz.

Electromagnetic radiation (or Electromagnetic waves): Transverse waves in space having an electric component and a magnetic component, each being perpendicular to each other and both perpendicular to the direction of propagation. These do not require any medium for its motion.

Photoelectric effect: The emission of electrons by a substance when illuminated by electromagnetic radiation.

Monochromatic light (or Monochromatic): Light consisting of only one colour.

White light: Light such as day light, containing all wavelengths of the visible spectrum at normal intensities so that no colouration is apparent.

Photoelectrons: Electrons emitted from a light sensitive material when it is properly illuminated.

Photoelectric current: The current flowing in a material due to photoelectrons.

Threshold frequency (νo ): Minimum frequency required for a photon of radiation to remove an electron from the surface of a material.

Stopping potential: Voltage corresponding to the maximum energy of the photoelectrons.

Classical theory: The part of physics that was developed before, and therefore does not include, either quantum theory or the theory of relativity.

Threshold frequency: Minimum frequency required for a photon of radiation to remove an electron from the surface of a material.

Quantization of energy: According to it, light energy is emitted or absorbed in small packets of energy , called quantum of energy. These quanta are known as photons.

Work function: The minimum energy required to remove an electron from the surface of a material and send it into field-free space.

Einstein’s photoelectric equation: An equation which relates maximum energy of the photoelectrons to the frequency of incident light; mathematically, ½ m vo2 = hν − φo

Newton’s corpuscular theory: According to it light consists of streams of minute particles in motion.

Huygen’s wave theory: According to it light travels from one place to another in the form of waves.

Electromagnetic wave theory: According to it light waves are electromagnetic in nature and they consists of an oscillating electric field and an oscillating magnetic field, both are perpendicular to each other and have the same frequency and phase.

Quantum theory of light: According to it light is carried from one place to another in wave packets called ‘quanta’ or ‘photons’, each having a definite energy and momentum.

Photocell: Any device for producing an electric signal from electromagnetic radiation.

Cutoff wavelength: In an electrical network, possessing no internal source of energy, the wavelength at which the attenuation quickly changes from a small value to a much higher value.

Compton effect: The phenomenon in which a photon is scattered by an electron and the scattered photon has a frequency less than its original frequency.

Photon:

• The basic energy packet of electromagnetic radiation.
• A quantum of electromagnetic radiation that has zero rest mass, and energy equal to the product of the frequency of the radiation and Planck’s constant.

Scattering: The ‘spreading out’ of a beam of radiation as it passes through matter, reducing the energy moving in the original direction.

Compton shift: The change in wavelength of the scattered photon. This Compton shift depends only upon the scattering angle and is independent of the frequency of the incident photon.

Pair production (or Materialization of energy): When high energy radiations, like cosmic rays, γ-rays, are absorbed by matter, under certain conditions, there is production of material particles, the electron-positron pair.

X-rays (or Inverse photoelectric effect):

• The emission of photons of radiations due to the bombardment on a material with high speed electrons.
• Electromagnetic radiation of extremely short wavelength, having great penetrating power.

Annihilation of matter: The process in which positrons and electrons unite to be annihilated as material particles and produce radiant energy.

Antiparticle: A counterpart of a subatomic particle having the same properties but opposite charge and spin direction.

De Broglie wave: A wave associated with a particle, such as an electron or proton.

De Broglie relation (or equation): A particle of mass m moving with a velocity v will under suitable experimental conditions exhibit the characteristics of a wave of wavelength λ given by the equation λ = h / mv, where h is the Planck constant.

Diffraction: The bending ore spreading of waves around the edge of an opening or obstacle.

Interference: The phenomenon in which the two waves support each other at some points and cancel at others.

Wave particle duality: The phenomenon whereby electromagnetic radiation and particles can exhibit either wave-like or particle-like behaviour, but not both.

Principle of complementarity: That both wave and particle aspects are required for the complete description of both radiation and matter.

Electron microscope: An apparatus for producing a magnified image by using a beam of electrons focused by electron lenses.

Optical microscope: Ordinary microscope; a device for producing large images of close small objects with a set of lenses. Light is directed onto the object through the lenses.

Resolution: Ability to distinguish two close objects separately, rather than as one single object.

Heisenberg uncertainty principle: It is impossible simultaneously to determine exactly both the position of an object and its momentum.

Planck’s constant: A universal proportionality constant relating photon energy to the frequency of radiation; 6.6256 x 10(-34) Jsec.

Subatomic: A constituent part of an atom, a subatomic particle, e.g., electrons, protons and neutrons. Occurring in the atom; smaller than an atom. Also, dealing with or involving particles, forces, or phenomena on this scale.

Written By: Asad Hussain