Years 7–10 · Week 3 of 12
The Double-Slit Pattern
Learning goals
By the end, you can…
- Predict how classical particles and classical waves would behave at two slits.
- Describe the difference between one detection and an accumulated pattern.
- Compare one-slit and two-slit probability distributions.
- Explain why the complete experimental arrangement matters.
What you already know
Connect to a familiar idea
Last week you separated individual detections from the pattern formed by many trials. The double-slit experiment makes that distinction especially clear.
- Waves can interfere.
- Photodetectors record localised events.
- A distribution summarises many outcomes.
Opening story
Start with something familiar
A source sends one quantum at a time towards a barrier with two narrow openings. Each run produces one mark on a detector. The first few marks look scattered; after many carefully repeated runs, bright and dark bands emerge.
Plain-English explanation
Build the idea carefully
Three expectations to compare
Classical pellets sent through two openings would normally make two overlapping clusters, while coherent classical waves can form alternating maxima and minima. Quantum experiments register individual localised events, yet the distribution accumulated over many identically prepared trials can show interference.
Events, patterns and experimental context
Opening one slit gives a different distribution from opening both; the two-slit distribution is not generally obtained by simply adding the two one-slit probabilities. Changing the apparatus to obtain which-path information changes the physical experiment and can remove the interference pattern. Quantum theory predicts probabilities for the specified preparation and measurement arrangement; consciousness is not required to create the pattern.
Try the model
Build the pattern one event at a time
Choose one slit, two-slits-without-interference or coherent two slits, then add events singly or in batches.
Ready. Adjust a control, then run the model.
What this model shows: A seeded educational probability model compares cases; dots are simulated events rather than real laboratory measurements.
Text alternative for this interactive
Static event-count sequence Inspect accessible summaries after 1, 10, 100 and 1000 events for one-slit and coherent two-slit cases.
Expected observation: Single events do not display an interference pattern, but many two-slit events build alternating high- and low-probability regions.
Guided activity
Prediction before simulation
- Sketch a prediction for each of the three arrangements.
- Add ten events and note why the evidence is still limited.
- Add one thousand events and compare the shapes.
- Describe which change to the apparatus changed the distribution.
Evidence to collect: Annotated predictions and observed frequency plots showing that many repeated trials, not a single hit, reveal the modelled interference distribution.
Glossary
Words to know
- slit
- A narrow opening in a barrier.
- coherence
- A stable phase relationship that allows interference to be observed.
- which-path information
- Physical information that can distinguish alternative paths.
- distribution
- How outcomes are spread across possible values.
- interference minimum
- A region where combining amplitudes gives a low probability.
- repeated preparation
- Recreating the same experimental conditions for many trials.
Short recap
Keep these ideas
- Each quantum trial produces an individual detection event.
- Many repeated trials reveal a distribution that depends on the apparatus.
- The double-slit pattern is predicted from the physical experiment, not from consciousness.
Knowledge check
6 clear questions
Choose an answer for immediate feedback. You may retry, and your best submitted score is kept.
Go Further Optional extension for Years 9–10
For Years 9–10, compare adding probabilities with adding amplitudes. If alternatives are distinguishable, probabilities can be combined; when coherent alternatives are not distinguished, amplitudes combine before probabilities are calculated.
Try this
Use arrows on a plane as qualitative amplitudes: show how aligned arrows can reinforce and opposite arrows can cancel without performing complex arithmetic.
Adult support Teacher and parent notes
Discuss
- Ask students to separate the observed dots from the probability model generating them.
- Avoid language suggesting that a particle secretly splits into two classical halves.
Answer guidance
Students should mention repeated preparation, accumulated events and the apparatus. Reject consciousness-based explanations.
Offline activity
Reveal pre-generated event cards in batches and have groups update a class histogram; label the dataset as simulated.
Safety
Use the supplied model rather than attempting an improvised laser double-slit experiment without approved equipment and supervision.
Sources and further reading
Checked references for this lesson
These sources support the lesson’s main scientific claims. Links open on the source organisation’s site.
- Quantum interactive learning tutorial on the double-slit experiment to improve student understanding of quantum mechanics American Physical Society · peer-reviewed education research · checked 2026-08-02
- Wave-Particle Duality OpenStax, Rice University · open textbook · checked 2026-08-02
Content review: Reviewed on 2026-08-02.