Theme E — Nuclear and Quantum Physics
E.1.4 Photon Energy — IB Physics HL Revision Notes & Topical Questions
5 min read 11 parts Lesson 6 of 45 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
E.1.4 Photon Energy — IB PHYSICS SL notes: Use E = hf = hc/λ to find photon energy, wavelength and photon rates, and use the electronvolt.Photon — a massless packet (quantum) of electromagnetic energyRadiation transfers energy in discrete packets, each delivering its energy in on…
- Charge and mass
- What the symbols mean
- Useful constants
- Comparison
- Electronvolt (eV)
- Conversion
Section 1 of 11The photon model
Photon
- Photon a massless packet (quantum) of electromagnetic energy
- Radiation transfers energy in discrete packets, each delivering its energy in one go.
- Waves, in contrast, transfer energy continuously.
Charge and mass
- Charge and mass photons are neutral with no rest mass, yet carry energy and momentum, p = E/c = h/λ
Section 2 of 11Photon energy
Equation
- E = hf = hc/λWhat the symbols mean: E = energy of one photon (J) · h = Planck constant (6.63 × 10−34 J s) · f = frequency (Hz)
- E is the photon energy in J or eV.
- h is the Planck constant, 6.63 × 10−34 J s.
- f is the frequency in Hz, c is 3.00 × 108 m s−1 and λ is the wavelength in m.
Frequency
- Frequency higher frequency means a more energetic photon
Wavelength
- Wavelength energy is inversely proportional to wavelength, so shorter wavelength gives higher energy
Useful constants
- Useful constants hc = 1.99 × 10−25 J m, or 1240 eV nm
Comparison
- Comparison a gamma-ray photon carries far more energy than a radio-wave photon
Section 3 of 11The electronvolt
Electronvolt (eV)
- Electronvolt (eV) the kinetic energy gained by an electron accelerated through a potential difference of 1 V
- 1 eV = 1.60 × 10−19 J.
Conversion
- Conversion divide joules by 1.60 × 10−19 to get eV and multiply eV by it to get joules
Prefixes
- 1 keV = 103 eV, 1 MeV = 106 eV and 1 GeV = 109 eV
Typical photons
- Typical photons visible light is about 1.8 eV (red, 700 nm) to 3.1 eV (violet, 400 nm)
- X-ray photons are of order keV and gamma photons of order MeV.
Section 4 of 11Atomic energy levels
Lowest state
- Lowest state electrons occupy the lowest possible level, the most stable configuration
Excitation
- Excitation an electron absorbs a photon and moves to a higher level
De-excitation
- De-excitation an electron emits a photon and moves to a lower level
Ionisation
- Ionisation an electron is removed from the atom
- The ionisation energy is the minimum energy to remove an electron from the ground state.
Energy level diagram
- Energy level diagram horizontal lines with the ground state at the bottom (most negative)
- The top line, 0 eV or infinity, represents ionisation.
Section 5 of 11Photon energy in transitions
Energy of the photon
- E = hf = E2 − E1
Wavelength
- λ = hc/(E2 − E1)
- The larger the energy gap, the shorter the wavelength.
Greatest and smallest
- Greatest and smallest the largest gap gives the shortest wavelength and the smallest gap gives the longest
Lines
- Lines each emission line corresponds to one transition
- Four levels allow six possible wavelengths.
Temperature
- Temperature at low temperature most electrons are in the ground state, and at high temperature electrons may reach the most excited state
Evidence for quantisation
- Evidence for quantisation atomic spectra consist of discrete wavelengths, so photon energies are discrete and atoms must have discrete energy levels
Section 6 of 11Photon rate and intensity
Number per second
- N = P/E = Pλ/(hc) for a source of power P
- A 1 W green source emits about 2.8 × 1018 photons per second, so light seems continuous.
Photon flux
- I/(hf) photons per unit area per unit time for intensity I
Intensity change
- Intensity change doubling the intensity at fixed wavelength doubles the number of photons per second but not the energy of each photon
Section 7 of 11Energy thresholds
Energy thresholds
Ionising hydrogen
- Ionising hydrogen at least 13.6 eV
Ejecting electrons from a metal
- Ejecting electrons from a metal a few eV
Pair production
- Pair production a photon needs at least 2mc2, which is 1.02 MeV for an electron–positron pair
- The reverse process is annihilation.
Chemical bonds
- Chemical bonds they have energies of a few eV, so ultraviolet can damage molecules and cause sunburn while visible light cannot
Radio waves
- Radio waves a 100 MHz photon has only about 4 × 10−7 eV, so it cannot excite atoms
Gamma radiation
- MeV photons can break nuclei apart
Thermal energy
- Thermal energy kT at room temperature is about 0.025 eV, so thermal collisions rarely excite atoms
Section 8 of 11Checking answers
Wavelength units
- Wavelength units convert nm to m before using hc in J m, or use 1240 eV nm directly
Visible light
- Visible light answers should be around 2–3 eV, or 3–5 × 10−19 J
Unit conversion
- Unit conversion convert eV to J before using SI values of h
Section 9 of 11More from the specification
More from the specification
Photon Energy Quantization
- Electromagnetic radiation is emitted and absorbed in discrete packets called photons,
- where photon energy is calculated using the Planck equation E = hf = hc/λ, corresponding to transitions between specific atomic energy states.
Rutherford Alpha Particle Scattering Experiment
- In the gold foil scattering experiment conducted by Rutherford, Geiger,
- and Marsden, positively charged alpha particles were fired at thin foil;
- while most passed undeflected, a small fraction scattered at large angles (> 90°), proving that atomic mass and positive charge reside in a tiny, dense nucleus.
Atomic Absorption and Emission Spectra
- When electrons absorb photons with specific energies, they transition to higher energy levels;
- (absorption spectrum)
- when relaxing back down, photons of identical specific frequencies are emitted.
- (emission spectrum)
Photon Energy Quantization
- Electromagnetic radiation is emitted and absorbed in discrete packets called photons,
- where photon energy is calculated using the Planck equation E = hf = hc/λ, corresponding to transitions between specific atomic energy states.
Rutherford Alpha Particle Scattering Experiment
- In the gold foil scattering experiment conducted by Rutherford, Geiger,
- and Marsden, positively charged alpha particles were fired at thin foil;
- while most passed undeflected, a small fraction scattered at large angles (> 90°), proving that atomic mass and positive charge reside in a tiny, dense nucleus.
Atomic Absorption and Emission Spectra
- When electrons absorb photons with specific energies, they transition to higher energy levels;
- (absorption spectrum)
- when relaxing back down, photons of identical specific frequencies are emitted.
- (emission spectrum)
Absorption
- The conversion of wave energy to internal energy of the medium.
Scattering
- Deflection of particles from their original path.
Alpha particle
- Helium nucleus, charge +2.
Absorption spectrum
- A continuous spectrum crossed by dark lines where a cool gas absorbed specific wavelengths.
Nuclear notation
- Symbol showing mass number and atomic number.
Rutherford scattering
- Scattering of charged particles by a nucleus through electrostatic repulsion alone.
Subatomic particle
- A particle smaller than an atom, e.g. the electron.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Photon | Quantum of electromagnetic radiation, energy hf. |
| Planck constant h | 6.63 × 10⁻³⁴ J s. |
| Electronvolt | 1.60 × 10⁻¹⁹ J. |
| Photon flux | Number of photons per second (per unit area). |
| Excitation | Raising an electron to a higher energy level by absorbing a photon. |
| Ionisation | Removal of an electron from an atom. |
Section 11 of 11Exam technique
1
Thinking a brighter beam has more energetic photons — brightness changes photon number, not energy.
2
Using eV in hf = E without conversion to joules when using SI values of h.
3
Mixing up wavelength and frequency in E = hf.
4
Forgetting that photons are neutral with no rest mass but still carry energy and momentum.
5
Thinking radio photons have large energies because radio waves can carry large power — power comes from huge numbers of photons.
Marked practiceQuizzes for this lesson
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