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.

How does an image change in a 3D hologram depending on angle of viewing?

 A Hologram is made by taking a single coherent beam, usually from a LASER, and splitting it into two beams.

One of the beams called a reference beam, directly hits a photographic plate where as other is reflected off an object (whose image needs to be stored in the Hologram). The interference pattern of these two beams is stored as Hologram.

When light is focussed on this Holographic plate, it reflects off the plate, but after mixing with the stored pattern.

 So if the original light beam that was used is directed at the corrected angle, it cancels out the component corresponding to reference beam and we see the object.

However, in a typical room, Light hits hologram from all angles and is also reflected back in all angles. Hence, there will be a beam that will fall on plate at same angle as that of reference and reflect the image of the object. If our eye happens to be in the path of the reflected beam then we can see the stored object.

If two interference patterns were stored simultaneously with different reference beam, we can see two different images depending on the angle of viewing. Because the light reaches each eye is not exactly the same, the 3D effect or perception of depth is produced.