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### 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.