The shape of the lesson
By the end of the lesson, learners can
- Slide across the spectrum: Move along the spectrum and watch the wavelength and frequency trade off through c = fλ, with the named regions and the narrow visible band highlighted.
- From radio to gamma: Order matters: increasing frequency means decreasing wavelength.
- Same speed, different everything else: Across the spectrum, only one quantity is shared in a vacuum.
- Speed: all travel at c in a vacuum.
- Link: c = fλ.
- Visible: 400 nm to 700 nm.
One picture every idea returns to
Electromagnetic waves are transverse and all travel at the same speed in free space, c = 3.0 × 10⁸ m s⁻¹. The spectrum runs from radio waves (longest wavelength, lowest frequency) through microwaves, infrared, visible light (400 nm to 700 nm), ultraviolet and X-rays to gamma rays (shortest wavelength, highest frequency). They obey c = fλ.
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, One family, many wavelengths.. Radio waves, light and gamma rays are the same kind of wave, differing only in wavelength and frequency. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Slide across the spectrum | Move along the spectrum and watch the wavelength and frequency trade off through c = fλ, with the named regions and the narrow visible band highlighted. Project tab 4 of the lab (Electromagnetic spectrum) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: From radio to gamma | Order matters: increasing frequency means decreasing wavelength. | Pairs |
| 21 to 30 min | Teach and check: Same speed, different everything else | Across the spectrum, only one quantity is shared in a vacuum. | Anticipation / Reaction Guide |
| 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 Same speed, different everything else, 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.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
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 |
|---|---|
| Do not think shorter waves travel faster: in a vacuum all electromagnetic waves travel at the same speed c. | Keep the visible range in the right units: 400 nm to 700 nm is 4 × 10⁻⁷ to 7 × 10⁻⁷ m. And electromagnetic waves are transverse, so unlike sound they can be polarised. |
The electromagnetic spectrum: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
State two properties that are common to all electromagnetic waves.
A microwave has a frequency of 12 GHz. Taking c = 3.0 × 10⁸ m s⁻¹, find its wavelength.
Place these regions in order of increasing frequency: radio, infrared, ultraviolet, gamma.
State the approximate wavelength range of visible light in metres.
A gamma ray has a wavelength of 1.0 × 10⁻¹² m. Taking c = 3.0 × 10⁸ m s⁻¹, find its frequency.
Explain, in terms of the spectrum, why X-rays are more penetrating than visible light.
Answer key and marking notes
- They are all transverse waves ✓
- They all travel at the same speed, c = 3.0 × 10⁸ m s⁻¹, in a vacuum ✓
- λ = c / f = (3.0 × 10⁸) / (12 × 10⁹) ✓
- λ = 0.025 m ✓
- Increasing frequency means decreasing wavelength ✓
- Order: radio, infrared, ultraviolet, gamma ✓
- About 4 × 10⁻⁷ m (violet) to 7 × 10⁻⁷ m (red) ✓
- i.e. 400 nm to 700 nm ✓
- f = c / λ = (3.0 × 10⁸) / (1.0 × 10⁻¹²) ✓
- f = 3.0 × 10²⁰ Hz ✓
- X-rays have a much higher frequency, and so higher photon energy, than visible light ✓
- The higher energy lets them pass through materials that visible light cannot ✓
10 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.
In a vacuum, all electromagnetic waves travel at:
- A340 m s⁻¹
- B3.0 × 10⁸ m s⁻¹
- Cdifferent speeds
- Dthe speed of sound
Electromagnetic waves are:
- Alongitudinal
- Bboth
- Cneither
- Dtransverse
Which region has the highest frequency?
- Agamma rays
- Bradio waves
- Cinfrared
- Dvisible light
Visible light spans wavelengths of about:
- A4 nm to 7 nm
- B400 to 700 mm
- C400 nm to 700 nm
- D40 to 70 nm
A radio wave of frequency 2.0 × 10⁸ Hz has wavelength (c = 3.0 × 10⁸):
- A0.67 m
- B1.5 m
- C6.0 × 10¹⁶ m
- D150 m
Moving from gamma rays to radio waves, the wavelength:
- Adecreases
- Bstays the same
- Cdrops to zero
- Dincreases
An X-ray of frequency 3.0 × 10¹⁸ Hz has wavelength:
- A1.0 × 10⁻¹⁰ m
- B1.0 × 10⁻⁸ m
- C9.0 × 10²⁰ m
- D1.0 × 10¹⁰ m
Which list is in order of increasing wavelength?
- Aradio, microwave, infrared, visible
- Bvisible, ultraviolet, X-ray, gamma
- Cgamma, X-ray, ultraviolet, visible
- Dinfrared, visible, ultraviolet, X-ray
Two EM waves differ in frequency. In a vacuum they have the same:
- Awavelength
- Bspeed
- Cphoton energy
- Dperiod
Microwaves lie between which two regions?
- Ainfrared and visible
- Bultraviolet and X-ray
- CX-ray and gamma
- Dradio and infrared
Quiz key
All EM waves travel at c in a vacuum.
EM waves are transverse.
Gamma rays have the highest frequency and shortest wavelength.
Visible light is roughly 400 to 700 nm.
λ = c/f = (3.0 × 10⁸)/(2.0 × 10⁸) = 1.5 m.
Gamma has the shortest wavelength, radio the longest, so wavelength increases.
λ = c/f = (3.0 × 10⁸)/(3.0 × 10¹⁸) = 1.0 × 10⁻¹⁰ m.
Wavelength increases from gamma up; gamma < X-ray < ultraviolet < visible.
All EM waves share the speed c in a vacuum.
Order: radio, microwave, infrared, then visible.
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.