

7.1. Discrete Energy and Radioactivity
At GCSE we talked a lot about light behaving as a wave, doing wavey-type things, such as reflecting and refracting, as well as listening to the Electromagnetic Spectrum Song to death (at least, you would have done if you were in my class!).
Now light - or more generally all electromagnetic waves - have some very odd behaviour. There exists something called 'Wave-Particle duality' - basically saying that sometimes light can do wavey things (diffracting, interfering, refracting - basically all the stuff from Chapter 4), and sometimes it can behave more... particle-y (if that is a word) - and it's this particle-y behaviour that we will look at now.
This is the start of your introduction into the wonderful world of Quantum Mechanics. Crash Course Physics takes a whistle-stop tour through the first half of this section (N.B. beyond 4:15 is HL stuff only!)
I've split this section up as follows:
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The Photon - a little 'particle' of light
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Energy Levels and the Bohr model - moving beyond the 'solar system' model of the atom
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Types of decay - alpha, beta- , beta+ and gamma decay
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Half life graphs - an introduction to exponential decay behaviour
The Photon
Background : The UV Catastrophe
Here is my very oversimplified history of physics up until the 20th century:
So up until about the end of the 19th century (...also until the end of GCSE physics), most people thought light was a wave. In a nutshell, there was plenty of evidence (see this video for some of this evidence) to suggest light behaved as waves, and it mostly obeyed the mathematical rules of what we now call 'classical physics' (Newton, Galileo, Huygens and the like). However, when exploring mathematical models of something called 'black bodies' (stuff that radiates energy, explored in Section 8.2) , the rules of classical physics started to fall apart. This was termed the 'UV Catastrophe' - a rather melodramatic term for when the maths goes wrong - like when you end up dividing by zero. A fudge was found (... well, not quite, it was a bit more involved) by a guy called Max Planck to make the maths work - treating the emitted light as 'discrete' particles.
These discrete particles of light were termed 'photons' - 'photo' meaning relating to light, an -ons being a standard suffix that Physicists add when talking about particle'y things (neutrons, electrons and so on).
A much better explanation is given by Physics Girl below:
(N.B. the history of UV Catastrophe is not part of the IB syllabus, but it's quite nice for a bit of context for where our equation came from)
The Photon
A photon is a particle of light, which has a discrete 'packet' of energy. This amount of energy carried by each photon is given by the following equation:
E = hf
(or E = hc/λ)
This tells us that the energy of a photon only depends on the frequency of that light.
For example, if we have violet light with a frequency of 7.5 x 10¹⁴ Hz, each photon will have an energy of :
7.5 x 10¹⁴ x 6.63 x 10ˉ³⁴ = 4.97 x 10ˉ¹⁹ J
where:
E = Energy of photon (in Joules)
h = Planck's constant (6.63 x 10ˉ³⁴ m²kg / s)
f = frequency of light (Hz)
λ = wavelength of light (m)
c = speed of light (3 x 10⁸m/s)
The Electronvolt
A Joule is very useful for measuring energy on macroscopic scales (e.g. the GPE gained when I lift a book onto a table). The Electronvolt is a unit of energy which we use to measure stuff on atomic energy scales (i.e. much much smaller than a Joule. THe conversion between Joules and electronvolvts is:
1 eV = 1.6 x 10ˉ¹⁹ J
(N.B. This is not explicitly given in your formula book, instead it is the same value as that of the electronic charge (on an electron) 1.6 x 10ˉ¹⁹ C. For the reason why, see PhysicsOnline's video below):
Video Lessons
Khan Academy | Photon Energy | |||||
Physics Online | The Photon | The Electronvolt | ||||
Science Shorts | Energy Levels (Photon) |
Resources
IB Physics | Topic 7 Notes | |||||
IB-Physics.net | Chapter 7 Summary | IB Revision Notes | ||||
Isaac Physics | Electromagnetic Waves | |||||
Mr. G | 7.1 Teaching Notes | 7.1 Student Notes | ||||
Physics and Maths Tutor | Particles Definitions | Particles Key Points | Particles Detailed Notes | EM Radiation & Quanta Flashcards | A Level Resources - content slightly different |
Questions
Cambridge University Press | Topic 7: Add Qs | Topic 7: Add Qs MS | Topic 7: MCQs | CUP Website Link | Freely available online | |
Grade Gorilla | 7.1 (Radioactivity) MCQ | Topic 7 (Nuclear) End Quiz | Quick IB Specific Mixed MCQs | |||
Isaac Physics | Quantum Calculations | Particle wave/ momentum HL only | ||||
Mr. G | 7.1 Formative Assessment | Topic 7 Summary Qs | IB Specific Questions |
The Bohr Model and Energy Levels
Bohr Model
Photons are discrete packets of energy - otherwise known as 'quanta'. The whole field of quantum physics is based around the idea that energy exists in discrete quantised packets.
This led to the development of the Bohr Model of the atom. Click the link below to take a look at a nice simulation by Walter Fendt.
Geogebra have a lovely simulation on energy levels. First try changing the waveglength of the photon and seeing how the energy in eV changes (bottom right). Then try looking at di
Emission and Absorption Spectra
Neon lighting works by having neon (or others) gas enclosed in a glass tube, with a high potential difference across each end. The gas becomes excited, and emits light of a certain colour. We can see below a selection of gases - Neon is deep red, Argon is purple. However, what is really interesting is if we look at the different wavelengths of light emitted in each case.


Video Lessons
Chris Doner | The Atom | Atomic Spectra | IB Specific | |||
Khan Academy | Atomic Energy Levels | Absorption & Emission Spectra | Emission Spectrum of Hydrogen | |||
Physics Online | Energy Levels | Emission/ Absorption Spectra | ||||
Science Shorts | Atomic Structure Introduction | Energy Levels | ||||
Study Nova | Energy Levels |
Resources
IB Physics | Topic 7 Notes | |||||
IB-Physics.net | Chapter 7 Summary | IB Revision Notes | ||||
Mr. G | 7.1 Teaching Notes | 7.1 Student Notes | ||||
Physics and Maths Tutor | Particles Definitions | Particles Key Points | Particles Detailed Notes | EM Radiation & Quanta Flashcards | A Level Resources - content slightly different |
Questions
Cambridge University Press | Topic 7: Add Qs | Topic 7: Add Qs MS | Topic 7: MCQs | CUP Website Link | Freely available online | |
Grade Gorilla | 7.1 (Radioactivity) MCQ | Topic 7 (Nuclear) End Quiz | Quick IB Specific Mixed MCQs | |||
Isaac Physics | Energy Levels | |||||
Mr. G | 7.1 Formative Assessment | Topic 7 Summary Qs | IB Specific Questions | |||
Physics and Maths Tutor | Energy Levels & Photons (AQA 2) | Energy Levels & Photons MS (AQA 2) | Quantum Physics (AQA 1) | Quantum Physics MS (AQA 1) | A-Level Qs: overlapping content | |
Physics and Maths Tutor | MCQ EM & Quantum Phenomena (AQA 2) | MCQ EM & Quantum Phenomena MS (AQA 2) | A-Level Qs: overlapping content | |||
Physics and Maths Tutor | Energy Levels (Edexcel 1) | Energy Levels MS (Edexcel 1) | Electron Collisions (AQA 2) | Electron Collisions MS (AQA 2) | A-Level Qs: overlapping content |
Types of Decay - Alpha, Beta-, Beta+ and Gamma
Video Lessons
Chris Doner | Alpha, Beta and Gamma | Nuclear Reactions | IB Specific | |||
Khan Academy | Types of Decay | Decay Equations | ||||
Physics Online | Beta Decay | |||||
Science Shorts | Radioactive Decay Equations | |||||
Study Nova | Alpha, Beta, Gamma Decay |
Resources
IB Physics | Topic 7 Notes | |||||
IB-Physics.net | Chapter 7 Summary | IB Revision Notes | ||||
Isaac Physics | Radioactive Decay Equations | |||||
Mr. G | 7.1 Teaching Notes | 7.1 Student Notes | ||||
Physics and Maths Tutor | Particles Definitions | Particles Key Points | Particles Detailed Notes | Particles Flashcards | A Level Resources - content slightly different |
Questions
Cambridge University Press | Topic 7: Add Qs | Topic 7: Add Qs MS | Topic 7: MCQs | CUP Website Link | Freely available online | |
Grade Gorilla | 7.1 (Radioactivity) MCQ | Topic 7 (Nuclear) End Quiz | Quick IB Specific Mixed MCQs | |||
Isaac Physics | Nuclear Equations | |||||
Mr. G | 7.1 Formative Assessment | Topic 7 Summary Qs | IB Specific Questions | |||
Physics and Maths Tutor | Constituents of the Atom (AQA 2) | Constituents of the Atom MS (AQA 2) | A-Level Qs: overlapping content | |||
Physics and Maths Tutor | Nuclear Physics (AQA 1) | Nuclear Physics MS (AQA 1) | Stable & Unstable Nuclei (AQA 2) | Stable & Unstable Nuclei MS (AQA 2) | A-Level Qs: overlapping content |
Half Life Graphs
In this section, you should be able to link the above ideas of mass defect to calculate the energy produced through fission and fusion.
However, it is worth refreshing your memory on the key aspects of fission and fusion, so flick through the following resources.
Video Lessons
Khan Academy | Photon Energy | |||||
Physics Online | The Photon | The Electronvolt | ||||
Science Shorts | Energy Levels (Photon) |
Resources
IB Physics | Topic 7 Notes | |||||
IB-Physics.net | Chapter 7 Summary | IB Revision Notes | ||||
Isaac Physics | Electromagnetic Waves | |||||
Mr. G | 7.1 Teaching Notes | 7.1 Student Notes | ||||
Physics and Maths Tutor | Particles Definitions | Particles Key Points | Particles Detailed Notes | EM Radiation & Quanta Flashcards | A Level Resources - content slightly different |
Questions
Cambridge University Press | Topic 7: Add Qs | Topic 7: Add Qs MS | Topic 7: MCQs | CUP Website Link | Freely available online | |
Grade Gorilla | 7.1 (Radioactivity) MCQ | Topic 7 (Nuclear) End Quiz | Quick IB Specific Mixed MCQs | |||
Isaac Physics | Quantum Calculations | Particle wave/ momentum HL only | ||||
Mr. G | 7.1 Formative Assessment | Topic 7 Summary Qs | IB Specific Questions |
Additional Resources
IB Questions
A question by question breakdown of the IB papers by year is shown below to allow you to filter questions by topic. Hopefully you have access to many of these papers through your school system. If available, there may be some links to online sources of questions, though please be patient if the links are broken! (DrR: If you do find some broken links, please contact me through the site)
Questions on this topic (Section 7) are shown in dark purple.
Use this grid to practice past IB questions topic by topic. You can see from the colours how similar the question topic breakdown is year by year. The more you can familiarise yourself with the IB question style the better - eventually you will come to spot those tricks and types of questions that reappear each year.