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Flashcards in quantum physics Deck (16):

two theories of nature of light in 17th century

corpuscular theory (supported by newton) as it being a tiny stream of particles travelling at high speeds in straight lines.
wave theory


differences between corpuscular theory and wave theory

both explained refraction and reflection.
waves needed a transmitting medium that was undetected.
interference and diffraction began to explain wave.
fluctuating electric and magnetic fields suggested, which didn't need a transmitter


photoelectric effect

emission of electrons from a metal surface when light shines on it. electrons are able to escape because repelled by other negative charge once liberated


the photoelectric effect's impact on wave theory

would expect uniform distribution of the whole area the light hit, but:
only observed when at a certain light frequency threshold that is dependant on the metal regardless of intensity.
emission instantaneous even with faint light.
maximum speed of electrons only dependant on frequency.
only explains number of electrons emitted per time is proportional to intensity.


Einstein's photoelectric equation

extension of quanta theory (E=hf [h plank's constant]).
assumes electromagnetic radiation emitted and absorbed as quanta called PHOTONS. where they can give all energy to an electron and cease to exist, but only at a certain energy level, giving the THRESHOLD. extra energy becomes kinetic
hf=Φ (work function hf0) + Emax


energy threshold radiation (hf0)

energy needed to release lest tightly bound electrons


energy of radiation (hf)

energy to release electron (variant on electron shell) +kinetic energy
special case it = energy release least tightly bound electron (hf0) + Emax


within Einstein's theory, greater intensity

allows more photons to arrive per second and area, but not change their energies


experiment to test Einstein's photoelectric model

Milikan's experiment.
monochromatic light from a spectrometers enters vacuum chamber and falls on electrode of the test metal, photoelectrons emitted collected by a second electrode with a sensitive ammeter
the stopping potential V was found for different frequencies


stopping potential V in Milikan's experiement

has to be applied to prevent most energetic photoelectrons causing current flow


effect of frequency on IV graph

max current is decreased and stopping potential V stretched left/right


effect of intensity on IV graph

maximum current increased


why hf=hf0+eV

because the most energetic electrons will just transfer the kinetic energy to Ep as the fail to reach second electrode





workfunction of a metal=



decay constant

the prob that a nucleus will decay in a unit time