PHYSICS DICTIONARY

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Moon

The moon is Earth's only natural satellite. The moon is a cold, dry sphere whose surface is studded with craters and strewn with rocks and dust (called regolith). The moon has no atmosphere. The moon is about 238,900 miles (384,000 km) from Earth on average. The moon's diameter is 2,159 miles (3,474 km), 27% of the diameter of the Earth. The moon's mass is (7.35 x 10 22 kg), about 1/81 of the Earth's mass. The moon's gravitational force is only 17% of the Earth's gravity. The moon's density is 3340 kg/m 3. This is about 3/5 the density of the Earth. 


Moseley’s Law

The frequency of a spectral line in characteristic X-ray spectrum varies directly as square of atomic number ‘Z’ of the element emitting it.


Moseley’s Law for X-rays

The frequency of a spectral line in characteristic X-ray spectrum varies directly as square of atomic number Z of element emitting it.


Mosley’s Experiment

Mosley in 1913 carried out a systematic study of Kα X-rays produced by elements from aluminum to gold using Bragg technique of X-rays scattering from a crystalline lattice of a potassium ferro-cyanide crystal. The characteristic Kα X-rays were produced by bombardment of targets with energetic electrons. The results of Mosley’s experiments confirmed the Bohr’s atomic theory. 


Mossbauer Effect

Rudolf Mossbauer, in 1958, discovered the recoil-free emission and absorption of gamma rays by nuclei. The Mossbauer effect states that when some atoms are held tightly in crystalline atomic structures, the gamma radiation emitted by their nuclei are very close to being recoil-free.  This implies that the emitted photon has the exact frequency that corresponds to the transition energy between the nuclear ground state and the excited state.  When this photon carrying the full nuclear transition energy strikes another similar nucleus also embedded in a tight crystalline structure, absorption may occur.  Because the nuclear energy levels are very sharply defined, only such a photon - one that carries the exact transition energy - may excite another nucleus. Because of the very narrowly defined nuclear energy states, the smallest change in the energy of the photon will destroy the resonance.  Thanks to this extreme sensitivity, Mossbauer spectroscopy has proven itself an excellent method to measure otherwise impossible to detect nuclear energy differences.  Minuscule energy variations, magnetic fields at the nucleus, and even distortions or asymmetries in the shape of the lattice that encloses the atom are enough to produce shifts and split tings in the absorption line.  All these alterations of the nuclear environment that produce an observable effect are called the nuclear hyperfine interactions.  


Most Probable Speed

It is the speed possessed by maximum number of molecules in a gas sample.


Motion

Change in position of an object with respect to time seen by an observer from a frame of reference.


Multiwire Drift Chamber

A multiwire drift chamber is a tracking device that was built to track particles in high energy physics collisions. It uses the timing information to determine the position of an ionizing particle. In a drift chamber the cathode is made up of either closely spaced wires or strips that are kept at distributed potentials. The advantage of this design is that the electrons produced by the incident radiation are directed towards the nearest anode wire with a constant drift velocity. The incident radiation produces electron ion pairs inside the active volume that drift towards the nearest anode and cathode. After passing through the chamber, the radiation is detected by a fast detector, such as scintillation counter, which produces a pulse. This pulse initiates a logic-enable pulse that starts a timing counter. The counter keeps on counting until it gets a stop signal that is generated when the electrons eventually reach the anode, produce avalanche, and produce an anode pulse. The number of ticks of the counter determines the time it has taken the electrons to reach the anode wire. Now, if the drift velocity of the electrons in the chamber gas is known, one can determine the distance traveled by the electron and hence position at which the electron was produced by the radiation.


Muon

It is an elementary particle, a lepton having spin ½ and about 200 times the mass of electron. It is usually formed from decay of Pion.  


Muonic Atom

It is an atom in which an electron is replaced by a negative muon orbiting close to or within the nucleus. Muonic atom consisting of a proton and negative with a reduced mass of 186 Me.   


Muonium (Mu)

It is a light, Hydrogen like neutral atom consisting of a positive muon μ+ and an orbital electron. Chemists consider Muonium to be a light unstable isotope of Hydrogen. It is formed when an energetic positive Muon slows down in an absorber and attracts an electron which then revolves about Muon. The reduced mass of Muon is within 0.5% of electron mass.


Musical Interval

The ratio of frequencies of two tones is called musical interval.


Musical Scale

If several tones in increasing order of their frequencies are arranged in a series such that they have a definite musical interval and their resultant effect is pleasant to the ear, it is called the musical scale.


Mutual Induction

When two coils are placed near each other such that the current flowing in one changes, emf is induced in the second coil. Such phenomenon is known as mutual induction. 


Myopia

Term used for short sight. Light from a distant object forms an image before it reaches retina. A myopic person has clear vision when looking objects close to them but distant objects will appear blurred. 

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