The shape of the lesson
By the end of the lesson, learners can
- Feel it across the gap: Compare surfaces and temperatures and watch how much infrared radiation each one emits.
- Surface colour and texture: The same surface property controls both how well an object emits radiation and how well it absorbs it.
- Radiation is infrared (electromagnetic) and needs no medium, so it crosses a vacuum.
- Hotter objects emit more radiation per second.
- Dull black is the best emitter and absorber; shiny light is the poorest (it reflects).
One picture every idea returns to
Thermal radiation is the transfer of energy by infrared electromagnetic waves and needs no medium, so it can travel through a vacuum. Dull black surfaces are the best emitters and absorbers; shiny, light surfaces are poor emitters and absorbers but good reflectors.
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, Heat that crosses empty space.. The warmth of the Sun reaches us across the vacuum of space, where there is nothing to conduct or carry it. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Feel it across the gap | Compare surfaces and temperatures and watch how much infrared radiation each one emits. Project tab 6 of the lab (Radiation) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Surface colour and texture | The same surface property controls both how well an object emits radiation and how well it absorbs it. | Corners |
| 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, 14 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 Surface colour and texture, 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.
Corners
Put one answer or viewpoint in each corner of the room. Students move to the corner they agree with, justify the choice to a partner there, then hear one voice from each corner before the reveal.
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 |
|---|---|
| A good absorber of radiation is also a good emitter. | A dull black surface both takes in and gives out infrared well, while a shiny silver surface does both poorly. The same property governs absorbing and emitting. |
Thermal radiation: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
State what thermal radiation is, what type of wave it consists of, and whether it can travel through a vacuum.
State which kind of surface is the best emitter of thermal radiation, and which kind is the best absorber.
A flask designed to keep drinks hot has shiny silvered inner walls. Explain how this helps to keep the drink hot.
The panel of a solar water heater is usually painted dull black. Explain why this colour is chosen.
State the main way in which thermal radiation differs from conduction and convection.
Two identical cans are filled with hot water at the same temperature. One has a dull black surface, the other a shiny surface. Predict which cools faster and explain your answer.
Answer key and marking notes
- It is the transfer of energy by waves (without needing a material). ✓
- The waves are infrared (electromagnetic) waves. ✓
- It can travel through a vacuum (no medium is needed). ✓
- Best emitter: a dull (matt) black surface. ✓
- Best absorber: also a dull black surface (a good absorber is a good emitter). ✓
- A shiny silvered surface is a poor emitter of radiation. ✓
- So the hot drink loses energy by radiation only very slowly, staying hot for longer. ✓
- A dull black surface is a good absorber of radiation. ✓
- So it absorbs as much of the Sun's radiation as possible, heating the water more effectively. ✓
- Radiation does not need a medium (any material) to travel, so it works through a vacuum. ✓
- Conduction and convection both need particles of a material, so they cannot occur in a vacuum. ✓
- The dull black can cools faster. ✓
- A dull black surface is a good emitter, so it radiates energy quickly. ✓
- The shiny surface is a poor emitter, so it radiates energy slowly and stays hot longer. ✓
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.
Through which of the following can thermal energy travel by radiation alone?
- Aa vacuum
- Ba copper bar
- Cstill water
- Da brick wall
Two identical cans of hot water, one with a matt black surface and one with a shiny silver surface, are left to cool in the same room. Which cools faster and why?
- AThe matt black can, because dark matt surfaces are better emitters of radiation.
- BThe shiny silver can, because shiny surfaces are better emitters.
- CBoth cool at exactly the same rate.
- DNeither cools, because the room is warm.
Quiz key
Thermal radiation is an electromagnetic wave (infrared) and needs no medium, so it is the only way energy crosses a vacuum, such as from the Sun to the Earth. The other options transfer energy mainly by conduction.
Matt black surfaces are the best emitters of thermal radiation, so the black can loses energy faster and cools sooner. Shiny silver surfaces are poor emitters and good reflectors (ruling out B).
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.