3 Quark Phenomena Flashcards
what is the photoelectric effect?
emission of electrons from a metal surface when the surface is illuminated by light of frequency greater than a minimum value known as the threshold frequency
what is threshold frequency?
the minimum frequency of light that can cause the photoelectric effect
Explain Einstein’s theory for the photoelectric effect:
- when light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident ray and gains energy (equal to hf)
- the electron can then leave the metal surface if the energy received from the photon exceeds the work function
what is the work function of a metal?
the minimum energy needed by an electron to escape from the metal surface
what happens to excess energy of a photoelectron when it leaves the surface?
it becomes kinetic energy
what is the equation of the maximum kinetic energy of an emitted electron?
Eₖ = hf - 𝜙
what is the stopping potential of a material?
the minimum potential needed to stop photoelectric emission
why is the kinetic energy of an emitted electron reduced to zero when the metal is at stopping potential?
each emitted electron must do extra work to leave the metal surface
why does increased intensity increase photoelectric emission?
intensity - energy per second which is proportional to the number of photons per second
each photoelectron absorbs one photon so the number of photoelectrons emitted is proportional to the intensity of the light
what is ionisation?
any process of creating ions
what is an ion?
a charged atom
what does not affect the maximum kinetic energy of a photoelectron?
the intensity of the incident light
what are some ways ions are created?
- radiation
- electrons passing through fluorescent tubes
what is the electron volt?
unit of energy
equal to work done when an electron is moved through a pd of 1 V.
equal to 1.60 x 10⁻¹⁹ J
what is excitation?
process in which an atom absorbs energy without becoming ionised as a result of an electron inside the atom moving from an inner shell to an outer shell
what are excitation energies?
energy values at which an atom absorbs energy
what happens in excitation by collision?
atoms absorb energy from colliding electrons
the colliding electron makes an electron inside the atom move from an inner shell to an outer shell
what is the ground state of an atom?
the lowest energy state of an atom
what is de-excitation?
process in which an atom loses energy by photon emission, as a result of an electron inside an atom moving from an outer shell to an inner shell or in which an excited nucleus emits a gamma photon
what does fluorescence mean?
glow of light from a substance exposed to UV radiation: the atoms de-excite in stages and emit visible photons in the process
How do fluorescent tubes work?
- ionisation and excitation of metal vapour atoms occur as they collide with each other and electrons in tube
- metal vapour atoms emit UV photons as well as visible photons when the atoms de-excite
- UV photons are absorbed by atoms of the fluorescent coating causing excitation of these atoms
- coating atoms de-excite in steps and emit visible photons
how are line spectra formed?
- each line is due to light of a certain colour (so certain wavelength)
- the photons that produce each line all have the same energy which is different from the energy of the photons which produce other lines
- each photon is emitted when an atom de-excites due to one of its electrons moving to an inner shell
why can line spectra be used to identify elements?
the wavelengths of the lines of a line spectrum of an element are characteristic of the atoms on that element
what are some properties of light which show it’s wave-like nature?
- diffraction
- interference (shown with interference patterns)
