Modern Physics Flashcards
What is a photon?
A discrete packet of energy of electromagnetic radiation. Many of them create a** stream of particles** in a beam of electromagnetic radiation.
What is understood by energy level?
It is the total energy of an electron (KE PE) while it is in orbital about the nucleus of an atom.
State the photoelectric effect.
It is the emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequency is shone on it.
(energy of incident photon is sum of max KE of emitted electrons and work function of the metal; hf = ø + KEmax)
What is the work function of a metal?
It is minimum energy needed to remove an electron from the metal surface.
What is threshold frequency?
It is minimum frequency of electromagnetic radiation below which no electrons are emitted from metal surface regardless of intensity of radiation.
(VS threshold wavelength)
What do you understand by wave-particle duality?
It refers to ability of matter to behave like waves in some situations and particles in others.
What is emission line spectra?
They are bright coloured lines against dark background due to energy release in form of photons when electrons in excited state jump to lower energy levels.
(when excited electron return to ground state possessing less energy, due to conservation of energy, the electron emits a photon whose wavelength i.e. colour, is same as photon it initially absorbed in the cool gas)
What do you know by absorption line spectra?
They are dark lines against bright coloured background due to energy of photon being absorbed when a beam of white light is passed through a cool gas.
(gas cloud removes/absorbs certain colours from continuous pattern. Since anything that absorbs also emits, addition of emission and absorption line spectrum gives a continuous spectrum)
Describe briefly how continuous x-ray spectrum is achieved.
When high energy electrons are fired into metal, electrons are slowed down or stopped, emitting photons whose energies are equal to loss of kinetic energies of the electrons.
Account for the characteristic x-ray spectra** **(intense and sharply defined wavelengths of x-ray).
During collision between energetic electron (accelerated by high voltage) and atoms of metal target, one of the inner shell electrons is knocked out. When electron in upper state transits down to fill lower vacated state, a photon is emitted, contributing to the spike.
(VS photoelectric effect which is the ‘opposite’)
What is spontaneous emission?
It is where an electron in excited state (higher energy levels) makes a random transition to a lower energy level, emitting a incoherent photon.
What is stimulated emission?
It is where an incident photon causes electron in excited state to return to ground state with emission of additional coherent photon.
What do you know by population inversion?
It is the condition where there are more atoms in higher excited state than in the lower states.
What is an insulator?
An insulator cannot conduct current because it has large energy gap of about 10 eV between its empty conduction band and completely filled valence band, where electrons cannot gain enough energy to cross to conduction band.
Differentiate between intrinsic semiconductors and metals.
Intrinsic semiconductor has small energy gap of 1.5 eV between conduction and valence band, and its conductivity increase with temperature.
Metals have either their conduction band overlapped with valence band or partially filled with electrons. Those electrons move under presence of electric field to conduct electricity.
