Definition: Photon
Photon is a QUANTUM of EM ENERGY
Definition: Photoelectron
Photoelectron is a electron emitted from the SURFACE of a material due to INCIDENT EM RADIATION
Definition: Photoelectric effect
Photoelectric effect is a process in which electron are EMITTED from METAL SURFACE when EM RADIATION of SUFFICIENTLY HIGH FREQUENCY is INCIDENT on the surface
Formula: Intensity
I=P/A = E/tA = nhf/tA
Where P= E/t and E= hf
Photoelectric equation
photon E = work function + KE max of electron
hf eVs
where Vs = stopping potential
Definition: Threshold frequency
Threshold frequency is the Minimum frequency for photoemission
Definition: work function
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Formula
Work function energy of a metal is the minimum amount of E necessary for electron to escape from surface of the material
Work function energy = h x threshold frequency
Condition for photoemission
- Photon E >/= work function energy
- Frequency of incident EM radiation >/= threshold frequency
- Metal surface is clean
- Setup inside vacuum
Definition: Stopping potential (Vs)
Stopping potential is the MAGNITUDE of the -VE POTENTIAL of COLLECTOR wrt EMITTER which prevents the MOST ENERGETIC photoelectron from reaching the collector, hence resulting in 0 PHOTOCURRENT
- MAGNITUDE OF -VE POTENTIAL
- COLLECTOR wrt EMITTER
- PREVENTS MOST ENERGETIC PHOTOELECTRON FROM REACHING COLLECTOR
- 0 PHOTOCURRENT
3 contradictions with classical wave theory
- EXISTENCE of threshold frequency
(wave theory predicts - if light intensity sufficiently high, then electron emitted regardless of frequency)
Particulate nature - light consists of photon with E=hf, hence f must be sufficiently high for photon E to be >/= work function energy
- Max KE of photoelectron independent of light intensity
(wave theory predicts - higher light intensity, higher rate of E transmitted per unit area onto surface, higher max KE)
Particulate nature - light consists of photons with E=hf, hence if f unchanged, by COE, max KE of photoelectron unchanged
- Time delay insignificant - photoelectron emitted almost instantaneously, no time lag between incidence of photon and photoelectric emission
(wave theory predicts - using light of low intensity, electron should take some time to accumulate sufficient E before being emitted)
Particulate nature - Interaction between photon and electrons is like “collision” where momentum and KE is transferred instantaneously
Energy level diagram
Photon Absorption VS particle collision
Photon Absorption
- photon with energy equal to difference between 2 energy levels will be absorbed, causing electron to move from lower to higher energy levels, causing excitation
(note: photons not absorbed will rebound elastically)
Particle Collision
- Incident particle (electron/ atom) collides with (electron/ atom) and transfers part/ all its KE to the electron, causing excitation
Definition: Ionisation energy
Ionisation Energy is the Minimum energy required to remove an electron completely from the atom
Definition: Emission Line spectrum
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Description on formation
Emission line spectrum consists of discrete bright lines of definite wavelength (diff colours) on a dark background
- series of lines observed (definite pattern)
- different gases result in different patterns
- High PD applied Across gas
- gas atoms bombarded by electrons + heated => electrons in gas atoms excite to higher energy levels - electrons de-excite and return to lower E levels
-lose energy by emission of photons with E=hf corresponding to difference between 2 energy levels - As Energy levels are discrete, Energy difference between energy levels also discrete
- only photons of specific E / light of definite wavelength are emitted
Definition: Absorption Spectrum
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Description of formation
Absorption Spectrum is a continuous spectrum crossed by a series of discrete dark lines on a bright background
- White light passes through cool gas
- Cool gas atoms absorb photons of certain frequencies to transit from lower to higher energy levels (E of photon absorbed = difference between 2 discrete energy levels)
- Emergent light, passes through grating, has missing frequencies of light
- Lines are dark - not totally black - After absorption, excited atom de-excites to lower energy state by emitting photon of the same E, however, these emissions occur uniformly in all directions hence light intensity at these spots lower, but not 0
Definition + Equation: Wave particle duality
Theory where matter and waves have particle-like and wave-like characteristics
De broglie wavelength - wavelength associated with wave like properties of a particle
λ=h/p
Experiment where,
- Light behaves like wave
- Light behaves like particle
- Electron behaves like wave
- Electron behaves like particle
- Light behaves like wave - Diffraction of light
- Light behaves like particle - Photoelectric effect
- Electron behaves like wave - Diffraction of electrons
- Electron behaves like particle - collisions, mass & charge
Definition + Equation: Heisenberg uncertainty principle
If measurement of position of particle is made with uncertainty Δx and simultaneous measurement of its x-component of momentum is made with uncertainty ** Δp**, the product of both uncertainties can never be less than or equal to ℏ/2
Δx Δp >= h (approximation)
Hydrogen spectrum (3 ranges + corresponding de-excitation levels)
Visible spectrum corresponding to what de-excitation level?
Lyman (Ultraviolet) - de-excite to n=1
Balmer (VISIBLE) - de-excite to n=2
Paschen (Infrared) - de-excite to n=3
X-ray spectrum
- How is it produced?
- Features/ components?
Produced when high speed electron suddenly slowed down when metal target is struck by electrons that have been accelerated through a pd of several thousand volts
- Continuous spectrum
- Characteristic X-rays
Kα characteristic x-ray
Kβ characteristic x-ray
X-Ray spectrum
Describe and explain formation of continuous spectrum (bremsstrahlung)
Due to EM radiation emitted by high speed electron when they are slowed down in the metal target due to interaction with the nuclei of the target atoms.
When electrons decelerate,loss in KE emerges as energy of the X-Ray photons
Not all electrons stopped in a single collision as different electrons slowed down to different extent
Hence, E of the X-ray photons covers wide range of λ => continuous spectrum
Some electrons loses all of its KE in a single collision => energy of electron completely transferred into E of the X-ray photon => produces most energetic X-ray photon => by E=hc/ λ => λmin (shortest λ of radiation that can be produced)
X-Ray spectrum
Explain and describe the formation of characteristic X-Rays
Accelerated electron collides into the electron of the target atom orbiting in the k-shell
If sufficient E transferred by the accelerated electron to the orbiting electron, orbiting electron ejected form the target atom.
Vacancy in k-shell (n=1) filled by electron from L-shell (n=2)
=> Kα characteristic X-Ray emitted
Vacancy in k-shell (n=1) filled by electron from M-shell (n=3)
=> Kβ characteristic X-Ray emitted
K represent innermost shell of the atom that an electron is able to be liberated from, given the energy supplied
X-Ray spectrum
Why can’t Kα exist without Kβ?
absence of Kβ characteristic X-Ray implies that K-shell is not vacant in the first place
=> no electron from L-shell is able to fill vacancy (there isn’t one)
=> Kα Characteristic X-Ray does not exist
