The shape of the lesson
By the end of the lesson, learners can
- Fan out the spectrum: Send white light into the prism and watch it split into its colours, with red bent least and violet most.
- Sorted, not created: The prism separates colours that white light already contains, bending each by a different amount.
- Cause. Each colour has a different refractive index in glass, so each refracts by a different amount.
- Order. Red bends least (longest wavelength), violet bends most (shortest wavelength).
- Where. The colours start to separate at the first air-to-glass boundary, not the second.
One picture every idea returns to
White light is a mixture of colours. A prism refracts each colour by a slightly different amount, so they separate into the spectrum: red, orange, yellow, green, blue, violet. Red is refracted the least and violet the most. This separation is dispersion.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Splitting white light.. Send a beam of white sunlight through a glass prism and it fans out into a band of colour, the same spread you see in a rainbow. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Fan out the spectrum | Send white light into the prism and watch it split into its colours, with red bent least and violet most. Project tab 8 of the lab (Dispersion) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Sorted, not created | The prism separates colours that white light already contains, bending each by a different amount. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 13 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Sorted, not created, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Red light is refracted the least and violet the most, a pair that is easy to reverse. | And the prism does not create the colours: they are already present in white light, and the prism simply separates them. |
Dispersion of light: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Explain what is meant by dispersion of light, and state what it shows about white light.
A prism disperses white light into a spectrum.
(a) List the colours of the spectrum in order. [1] (b) State which colour is deviated the most and which the least. [1]Explain, in terms of speed and refraction, why red light and violet light are deviated by different amounts as they pass through a prism.
A student draws white light passing straight and undivided through the first face of a prism, with the colours only appearing as it leaves the second face. Explain why this diagram is incorrect.
A student claims that red light must bend the most through a prism because it has the highest energy. Explain what is wrong with this statement.
White light from the Sun passes through raindrops and forms a rainbow, which is dispersion in action.
(a) State the property of light that determines how much each colour is deviated. [1] (b) A prism produces a spectrum with red at the top. State where violet appears, and why. [1]Answer key and marking notes
- Dispersion is the splitting of white light into its separate colours (a spectrum) when it refracts through a prism. ✓
- It shows that white light is a mixture of many colours. ✓
(a) Red, orange, yellow, green, blue, indigo, violet. ✓
(b) Violet is deviated the most, red the least. ✓
- Different colours travel at slightly different speeds in glass, so glass has a slightly different refractive index for each. ✓
- Violet is slowed the most, so it refracts and deviates the most. ✓
- Red is slowed the least, so it refracts and deviates the least. ✓
- The colours begin to separate at the first air-to-glass boundary, because each colour refracts by a different amount as it enters the glass. ✓
- So the light is already dispersing inside the prism. The diagram should show the colours fanning apart from the first surface, separating further at the second. ✓
- Red light has the longest wavelength and is slowed the least by the glass, so it actually bends the least. ✓
- It is violet, with the shortest wavelength, that is slowed the most and bends the most. ✓
(a) Its wavelength (which sets how much the glass slows and refracts it). ✓
(b) Violet appears at the bottom, because it is deviated the most while red is deviated the least. ✓
2 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
White light passes through a glass prism and spreads out into a band of colours. What is this process called?
- Adispersion
- Breflection
- Cdiffraction
- Dtotal internal reflection
When white light is dispersed by a prism, which colour is refracted (bent) the most?
- Aviolet
- Bred
- Cyellow
- Dgreen
Quiz key
Splitting white light into its colours by a prism is dispersion: each colour is refracted by a different amount. Reflection and diffraction do not separate white light into a spectrum like this.
Violet light has the shortest wavelength and the highest refractive index in glass, so it is bent the most. Red, with the longest wavelength, is bent the least.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.