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⚡ Infrared Emission, Absorption and Black Bodies

Spec 6.6.5 (physics only) 📙 Higher
📖 In-Depth Theory

Emission and Absorption of Radiation

All objects emit and absorb THERMAL RADIATION (infrared radiation) continuously.
EMISSION:
All objects above absolute zero emit infrared radiation.
Hotter objects emit MORE radiation and at SHORTER wavelengths (higher frequency, more energetic).
ABSORPTION:
Objects absorb radiation from their surroundings.
Dark, matt surfaces are better ABSORBERS than light, shiny surfaces.
Dark matt surfaces are also better EMITTERS.
Shiny, light surfaces are better REFLECTORS and poorer emitters/absorbers.
EMISSION vs ABSORPTION RATES:
If emission rate > absorption rate → object cools.
If absorption rate > emission rate → object heats up.
If rates are equal → temperature is constant (thermal equilibrium).

Perfect Black Bodies

A PERFECT BLACK BODY is a theoretical object that:
ABSORBS all radiation that falls on it (reflects none).
EMITS the maximum amount of radiation for its temperature.
Perfect black bodies emit the most radiation of any object at the same temperature.
The emission spectrum of a black body depends ONLY on temperature — not on the material.
BLACK BODY RADIATION CURVES:
As temperature increases:
Peak of emission shifts to SHORTER wavelengths (higher frequency).
Total energy emitted per second INCREASES dramatically.
EXAMPLE:
A cool object: emits mainly low-frequency infrared — invisible to the eye.
Hot iron (600°C): glows dull red — emitting visible red light.
Very hot iron (1000°C): orange-white — peak moves into visible range.
The Sun (~5500°C surface): peak emission in visible light (yellow-green).
A cavity with a small hole acts as an approximate black body.

Applications

STARS AS BLACK BODIES:
Stars approximately behave as black bodies.
The colour of a star indicates its surface temperature:
Red stars (~3000 K) → cool → peak in infrared/red.
Yellow stars (like the Sun, ~5500 K) → medium → peak in visible.
Blue-white stars (~30,000 K) → very hot → peak in UV/blue.
EARTH'S TEMPERATURE:
The Earth absorbs solar radiation and re-emits as infrared (longer wavelength — Earth is cooler than Sun).
Greenhouse gases absorb Earth's infrared emission → trap energy → warming effect.
THERMAL CAMERAS:
Detect infrared emitted by warm objects — used in: night vision, firefighting, medical diagnosis (detecting hot spots), building inspection (heat loss).
SPACE TELESCOPES:
Infrared telescopes detect emission from cool objects like dust clouds in space.
Hubble detects visible light; James Webb Space Telescope detects infrared.
SELECTIVE EMISSION AND ABSORPTION:
Solar panels: dark, matt surfaces to maximise solar absorption.
Solar water heaters: black tubes to maximise absorption.
Thermos flasks: silvered walls to minimise emission and absorption (reflect radiation back).
⚠️ Common Mistake

A PERFECT BLACK BODY is black because it absorbs ALL radiation — it also emits MAXIMUM radiation for its temperature. Being a 'perfect emitter' and 'perfect absorber' go together. Hotter objects emit at SHORTER wavelengths — this seems counterintuitive but is why glowing objects change colour from red to white-hot as they get hotter.

📌 Key Note

All objects emit and absorb IR. Dark matt = best emitter and absorber. Shiny = best reflector. Black body: absorbs all radiation, emits maximum for its temperature. Hotter → more emission at shorter wavelength. Stars: colour indicates temperature (red=cool, blue=hot). Earth absorbs solar, re-emits IR — greenhouse effect.

🎯 Matching Activity — Infrared and Black Bodies

Match each statement to the correct concept. — drag the symbols on the right to match the component names on the left.

Dark matt surface
Drop here
Perfect black body
Drop here
Hotter object
Drop here
Blue-white star
Drop here
Absorbs all incident radiation — emits maximum radiation for its temperature
Best emitter and absorber of infrared radiation
Emits more radiation AND at shorter wavelengths — peak shifts towards visible/UV
Very hot (~30,000 K) — peak emission in UV/blue visible range
⭐ Higher Tier Only

Explain black body radiation curves — how peak wavelength depends on temperature. Describe how Earth's radiation balance determines its temperature. Explain the greenhouse effect mechanistically. Evaluate how changes in albedo or greenhouse gas concentration affect Earth's temperature.

🔬 Triple Science Only

Infrared emission/absorption and black bodies (physics only) — not in Combined Science.

🎯 Test Yourself
Question 1 of 2
1. A metal rod is heated in a furnace. First it glows red, then orange, then white. What does this tell us?
2. Why are the walls of a thermos flask silvered?
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