CIE iGCSE Co-ordinated Sciences-P2.3.3 Radiation- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-P2.3.3 Radiation – Study Notes
CIE iGCSE Co-ordinated Sciences-P2.3.3 Radiation – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Know that thermal energy transfer by thermal radiation does not require a medium and is mainly due to infrared radiation
- Describe the effect of surface colour (black or white) and texture (dull or shiny) on the emission, absorption and reflection of thermal radiation
Supplement
- Know that the temperature of the Earth is affected by the radiation absorbed by the Earth and the radiation emitted by the Earth
- Describe experiments to distinguish between good and bad emitters of thermal radiation
- Describe experiments to distinguish between good and bad absorbers of thermal radiation
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Thermal Radiation
Thermal radiation is the transfer of heat energy by infrared (IR) radiation. Unlike conduction and convection, it does not require a medium and can travel through a vacuum.
Key Features:
- Thermal radiation is a form of electromagnetic radiation (mainly infrared waves).
- It can travel at the speed of light and does not need particles to transfer energy.
- All objects emit and absorb infrared radiation, but hotter objects emit more.
- Shiny, light-coloured surfaces are poor emitters and absorbers but good reflectors.
- Dull, dark surfaces are good emitters and absorbers of thermal radiation.
Why It Does Not Require a Medium:
- Conduction and convection rely on particle collisions and movements.
- Thermal radiation is carried by electromagnetic waves, so it can travel through empty space.
- Example: Heat from the Sun reaches the Earth through the vacuum of space by thermal radiation.
Examples of Thermal Radiation:
- Feeling heat from the Sun on your skin.
- Heat from a fire or electric heater warming your hands at a distance.
- Use of thermal imaging cameras that detect infrared radiation.
Example :
Explain why a black car feels hotter than a white car when parked in the sun.
▶️ Answer/Explanation
Step 1: The Sun transfers heat to the car mainly by thermal radiation (infrared).
Step 2: Black surfaces are good absorbers of infrared radiation, while white surfaces are poor absorbers (good reflectors).
Final Answer: The black car absorbs more radiation and heats up more than the white car.
Effect of Surface Colour and Texture on Thermal Radiation
Surfaces affect how much thermal (infrared) radiation they emit, absorb, and reflect.
Key Points:
- Dull, black surfaces → best absorbers and best emitters of radiation; poor reflectors.
- Shiny, white surfaces → poor absorbers and emitters; best reflectors.
- Dull, dark surfaces → radiate and absorb heat more effectively.
- Shiny, light surfaces → reflect most of the heat away.
Summary Table
| Surface Type | Absorption of Radiation | Emission of Radiation | Reflection of Radiation |
|---|---|---|---|
| Dull Black | Best absorber | Best emitter | Poor reflector |
| Dull White | Moderate absorber | Moderate emitter | Moderate reflector |
| Shiny Black | Absorbs less than dull black | Emits less than dull black | Good reflector (better than dull surfaces) |
| Shiny White / Metallic | Poor absorber | Poor emitter | Best reflector |
Everyday Examples:
- Solar panels: Painted black to absorb maximum heat.
- Thermos flasks: Shiny inner surfaces reflect heat, reducing energy loss.
- Clothing: Black clothes feel hotter in sunlight; white clothes keep cooler.
Example :
Why are the cooling fins at the back of a refrigerator painted black?
▶️ Answer/Explanation
Step 1: Dull black surfaces are the best emitters of thermal radiation.
Step 2: The refrigerator fins need to release heat quickly to keep the inside cool.
Final Answer: They are painted black so they emit thermal radiation efficiently, speeding up cooling.
Earth’s Temperature and Radiation Balance
The temperature of the Earth depends on the balance between the radiation absorbed from the Sun and the radiation emitted back into space.
1. Radiation Absorbed by the Earth
- The Sun emits shortwave radiation (mainly visible light and ultraviolet).
- Some of this radiation is reflected by clouds, ice, and bright surfaces (albedo effect).
- The rest is absorbed by the Earth’s surface (land and oceans), warming the planet.
2. Radiation Emitted by the Earth
- The Earth re-emits energy as longwave infrared radiation.
- If emission equals absorption, the Earth’s temperature remains constant.
- If more energy is absorbed than emitted, the Earth’s temperature rises.
- If more energy is emitted than absorbed, the Earth’s temperature falls.
3. Influence of the Atmosphere (Greenhouse Effect)
- Greenhouse gases (carbon dioxide, methane, water vapor) absorb and re-radiate some of the infrared radiation.
- This traps heat in the atmosphere, keeping Earth warmer than it would be otherwise.
- This process is natural and essential, but human activity is increasing greenhouse gas levels, enhancing the effect.
Summary Table
| Process | Radiation Type | Effect on Earth |
|---|---|---|
| Absorption | Shortwave (visible + UV) from Sun | Heats Earth’s surface |
| Emission | Longwave infrared from Earth | Cools Earth, energy lost to space |
| Greenhouse Effect | Infrared trapped by gases | Keeps Earth warmer than without atmosphere |
Example :
Why does the Earth cool down at night, and why do cloudy nights stay warmer than clear nights?
▶️ Answer/Explanation
Step 1: At night, the Earth emits infrared radiation back into space, cooling the surface.
Step 2: On a clear night, radiation escapes easily → surface cools more quickly.
Step 3: On a cloudy night, clouds absorb and re-radiate some infrared back to the ground.
Final Answer: The Earth cools at night due to emission of radiation; clouds slow this process by trapping heat.
Experiment 1 — Leslie’s Cube (Classic)
Apparatus:
- Leslie’s cube (metal cube with four differently finished vertical faces: dull black, dull white, shiny metal, and painted/varnished).
- Hot water (to fill the cube).
- Thermometer or thermopile / infrared detector (mounted on a stand) to measure radiation intensity at a fixed distance from each face.
- Ruler or fixed stand to keep detector at same distance.
Method:
- Fill the Leslie cube with hot water at a known temperature (ensure all faces reach nearly the same surface temperature).
- Place the detector at a fixed distance (e.g. 10 cm) perpendicular to one face and record the reading after it stabilises.
- Repeat the reading for each face keeping distance and detector orientation identical.
- Repeat readings to reduce random error and take averages.
Expected Observations:
- The dull black face gives the highest detector reading (strongest emission).
- The shiny metal face gives the lowest detector reading (weakest emission).
- The dull white and painted faces give intermediate readings (dull surfaces emit more than shiny ones; dark > light).
Explanation:
- Dull black surfaces are good emitters and good absorbers of infrared radiation — they emit more thermal radiation at the same temperature. Shiny/metallic surfaces reflect more and emit less.
- The detector measures emitted infrared intensity; higher intensities show better emission.
Precautions & Tips:
- Ensure all faces are at the same temperature (wait after filling until thermal equilibrium).
- Keep detector distance and alignment identical for every face.
- Avoid drafts and direct sunlight which change readings.
- Use averages of repeated measurements for reliability.
Experiment 1 — Sunlight / Lamp Plate Comparison (Simple Classroom Demo)
Apparatus:
- Two identical flat metal plates (or spoons) — one painted dull black, the other left shiny.
- Sunlight (outdoor) or a strong lamp / IR heat lamp indoors.
- Contact thermometers or IR thermometer (same emissivity setting) and stopwatch.
- Ruler / stand to keep distance constant.
Method:
- Place both plates side by side so they receive the same incident radiation from the sun/lamp.
- Measure and record the initial temperature of each plate.
- Expose for a fixed time (e.g., 3–5 minutes) then record surface temperatures immediately.
- Compare temperature rises (ΔT) for the two plates.
Expected Observations:
- The dull black plate will show a larger temperature rise than the shiny plate — indicating it is a better absorber.
Explanation:
- Dull black surfaces absorb more incident radiation (higher absorptivity), converting it to internal energy → larger ΔT. Shiny surfaces reflect more → smaller ΔT.
Low-tech demo alternative:
- Heat two identical spoons (one blackened) and, at a safe distance, let students feel which radiates more heat (black will feel warmer). Emphasise safety and limited use of hands near hot objects.
Precautions & Tips:
- Ensure identical exposure to radiation (same angle, distance, time).
- If using IR thermometers, remember they depend on surface emissivity — use the same device and treat results as relative comparisons.
- Avoid reflective backgrounds that redirect radiation onto surfaces.
- Wear heat-resistant gloves when handling hot items.