Modern Physics - Graduate level important notes for competitive exams - PART1

  • Aston’s Mass Spectroscope enables identification of isotopes.
  • Packing fraction (P) is mass defect for elementary particle, P = Mass defect/ Mass number.
  • Negative Packing Fraction implies exceptional Nuclear Stability.
  • Binding energy of Neutron is 2.23 MeV, Spin = 1.
  • Binding energy of Lithium is 37.7 MeV. Binding energy per Nucleon is 37.7/7 = 5.4MeV.
  • Diameter of Nucleus is of order of 10⁻¹⁴ m.
  • Intensity of X- rays varies with current.
  • The quality of X- rays is a function of Potential difference between Cathode and Anode.
  •  Moseley’s law is based on Bohr’s theory of atom.
  • Wave length of X – rays ranges from 10⁻¹⁰ to 10⁻⁸ m.
  •  Speed of X- rays = 3*10⁸ m/sec.
  • The short wave length of X- rays emitted from X- ray tube depends on current in tube.
  • The operating Voltage in a typical X- ray tube is of order 10 KV.
  • Stopping Potential is independent of intensity of incident Radiation.
  • Saturation current is directly proportional to Intensity of Incident Radiation.
  • Compton Effect is observed with X-Rays and Gamma Rays but not with Visible and UV Radiation.
  • The constant factor of probability of transition is known as "Spontaneous Emission". It leads to broad Spectrum.
  • The variable factor of probability of transition is known as "Stimulated Emission". 
  • Spontaneous emission was postulated by "Neil Bohr".
  • Stimulated emission was postulated by "Einstein".
  • Probable rate of occurrence of absorption transition from state 1 to state 2 depends on properties of two states and is proportional to energy density.
  • Probability rate of "spontaneous emission" depends on properties of states but does not depend on energy density.
  • In Ruby(Al2O3) LASER, Chromium ions(Cr3+) acts as "active" material.
  • Ruby(Al2O3) LASER is a “pulsed" LASER . The output beams of it have principal wave length in visible spectrum.
  • Gas LASER(He-Ne) is a continuous LASER with high Monochromaticity.
  • In He– Ne LASER, Helium atoms are in majority while ‘Ne’ atoms are in minority.
  • In He–Ne LASER, we observe emission between 5S to 3P.
  • In He- Ne LASER, Helium gas serves as a mediator in producing Population Inversion.
  • Bragg’s law is a result of “ Periodicity of Lattice Points”.
  • Fine structure of H⍺ line includes 7 no. of valid transitions.
  • X- ray emission is referred to as inverse photo electric effect.
  • Ratio of Einstien's Spontaneous & stimulated coefficients = 8Πh𝝂³/C³.
  • Spectrum of non- rigid diatomic molecule is similar to that of Rigid molecule except that each line is displaced slightly to lower frequency.
  • Pure "Vibrational spectra" are observed only in liquids. Because interactions between neighboring molecules prevent this Rotational motion.



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