Particles 2 Flashcards
Describe the photoelectric effect
the emission of photoelectrons from a metal when light above a threshold frequency is shone on it
What does the photoelectric effect give evidence for?
evidence for the quantum model of light
Why does the photoelectric effect give evidence for the quantum model of light?
- emission is instant (if at all)
- emission occurs above a threshold frequency (at any intensity)
What is meant by intensity here?
the number of photons per second
Explain what happens during the photoelectric effect?
- one electron absorbs one photon (one-to-one interaction)
- 100% of the photon energy is absorbed (or none of it)
- photon energy = hf
What does ϕ equal in the equation: hf = ϕ + KE(max)? What is its definition?
- work function
- the minimum energy required for an electron to escape from the surface
What does the work function depend on?
depends on the metal
Why is is KE(max) in the equation hf = ϕ + KE(max)?
because it is the KE for an electron emitted from the surface
What is the equation for ϕ?
ϕ = h(fo)
What does fo equal?
the threshold frequency
Define the threshold frequency
the frequency at which electron emission just starts
What is the stopping potential?
the reverse p.d. needed to just stop electrons with maximum kinetic energy
What does the work done on electrons equal?
- = KE(max)
- = QV
- = e(stopping potential)
Define an excited atom
an atom is excited if its electrons occupy higher energy levels with vacancies below
How much energy can orbital electrons have?
they can only have certain discrete energies when they orbit a nucleus (the energy levels are unique to each element)
What are the two ways an orbital electron can become excited?
- if it absorbs a photon of exactly the energy difference between two levels
- if it collides with a free electron of sufficient energy
Explain the first step of how fluorescent tubes works
- a high potential difference accelerates free electrons through a low pressure mercury vapour
- free electrons collide and excite mercury atoms
- excited electrons in mercury atoms fall down, de-excite, emitting photon(s)
- when electrons fall directly from higher energy levels down to lower we get UV photons
Why in fluorescent tubes is a low pressure mercury vapour used?
it is low pressure so the electrons can still accelerate fast enough to have enough energy to collide with mercury atoms
Explain the second step of how fluorescent tubes work
- the tube is coated
- the coating absorbs UV photons, exciting its electrons
- when electrons in the coating fall back down, they may emit several visible photons by falling via the intermediate levels
What does the emission spectrum show?
the frequencies emitted by an excited atom, unique to that element, due to unique energy levels
What does the absorption spectrum show?
the frequencies emitted are missing from the continuous spectrum
Give evidence for light as a wave
it diffracts
Give evidence for light as a particle
the photoelectric effect
Give evidence for an electron as a wave
it diffracts
Give evidence for an electron as a particle
- they are charged (e.g. they are deflected by magnetic fields)
- they have a rest energy (non-zero)
What is meant by duality of electrons?
electrons behave like both waves and particles / electrons have characteristic of both waves and particles
