Lesson overview
Thermal radiation: Why are houses in hot countries often painted white?
Predict which side of a Leslie cube gives out most infrared, check the empty cube, then fill it with hot water and turn each side to a radiation detector; refill it hotter and read every side again. Then decide where a lamp must stand between a matt black and a matt white can, switch it on and watch both temperatures rise and level off, then switch it off.
Suggested sequence
- Lesson 1 · Emitting infrared. Thermal radiation as infrared, given out by every object and needing no medium; good and bad emitters; Supplement: an experiment to tell them apart, and how the rate depends on surface temperature and area. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Absorbing infrared. Good and bad absorbers and reflectors, with an experiment to tell them apart; Supplement: constant temperature as a balance of energy in and out, the Earth’s temperature, a fire and a car radiator. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Emitting infrared
- Know that thermal radiation is infrared radiation and that all objects emit this radiation; know that thermal energy transfer by thermal radiation does not require a medium (2.3.3.1, 2.3.3.2)
- Describe the effect of surface colour (black or white) and texture (dull or shiny) on the emission of infrared radiation (2.3.3.3)
- Supplement: Describe experiments to distinguish between good and bad emitters of infrared radiation; describe how the rate of emission of radiation depends on the surface temperature and surface area of an object (2.3.3.7, 2.3.3.9)
Part 2: Absorbing infrared
- Describe the effect of surface colour (black or white) and texture (dull or shiny) on the absorption and reflection of infrared radiation (2.3.3.3)
- Supplement: Describe experiments to distinguish between good and bad absorbers of infrared radiation; know that for an object to be at a constant temperature it needs to transfer energy away from the object at the same rate that it receives energy, and what happens if the rate at which it receives energy is less or more than the rate at which it transfers energy away (2.3.3.8, 2.3.3.4, 2.3.3.5)
- Supplement: Know how the temperature of the Earth is affected by factors controlling the balance between incoming radiation and radiation emitted from the Earth’s surface; explain complex applications where more than one type of thermal energy transfer is significant, including a fire burning wood or coal and a radiator in a car (2.3.3.6, 2.3.4.2)
Misconceptions to listen for
- “Only hot objects give out infrared.” — Every object emits infrared. A cold object emits less than a hot one, but it still emits.
- “An object gives out infrared only when it glows.” — Infrared is invisible. Your hand emits infrared all the time without glowing.
- “The campfire warms your face by convection.” — Convection carries the hot air and smoke upwards. Energy reaches your face sideways, by radiation.
- “Energy from the Sun reaches us by convection through space.” — Convection needs a liquid or gas that can flow, and space has almost no particles.
- “Your hand does not emit infrared because it is cooler than the mug.” — Every object emits infrared. Your hand emits less than the hot mug, so it gains energy overall, but it still emits.
- “Radiation needs air to travel through, like sound.” — Thermal radiation needs no medium: it crosses empty space, which is how sunlight reaches the Earth.
- “In the cold shadow the suit stops emitting infrared.” — All objects emit infrared. A cooler suit emits less, but it still emits.
- “The shiny can emits most, because it looks bright.” — Looking bright in light does not mean emitting infrared. Shiny surfaces are poor emitters of infrared.
- “Matt white hardly emits any infrared.” — Matt white paint emits almost as well as matt black; it is shiny metal that emits very little.
- “Colour matters more than texture: white surfaces hardly emit.” — Matt white emits almost as well as matt black. Shiny surfaces are the poor emitters.
Scope
Thermal radiation as infrared radiation, which all objects emit (2.3.3.1), and which needs no medium (2.3.3.2); the effect of surface colour (black or white) and texture (dull or shiny) on emitting, absorbing and reflecting infrared (2.3.3.3); Supplement: the energy balance of an object at constant temperature (2.3.3.4), and what happens when it receives energy faster or slower than it transfers energy away (2.3.3.5); the Earth’s temperature and the factors that control its balance of incoming and emitted radiation, such as greenhouse gases and ice (2.3.3.6); experiments to tell good and bad emitters (2.3.3.7) and absorbers (2.3.3.8) of infrared; how the rate of emission depends on surface temperature and surface area (2.3.3.9); applications where more than one transfer matters: a fire burning wood or coal and a radiator in a car (2.3.4.2), and also a vacuum flask. Conduction and convection are in Conduction and convection; the electromagnetic spectrum and wavelengths of infrared are in a later module.