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Physics Notes: 1. Introduction to the surface absorption/emission of EMR

GCSE level Physics exam revision notes on radiation

Absorption & emission of EM radiation: Part 1.

Introduction to the temperature effect on absorbing and emitting radiation by materials across the electromagnetic spectrum

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[KEY POINTS and learning objectives for this page, after initial notes]

EMR shorthand for electromagnetic radiation

INDEX of physics notes: Absorption & emission of EM radiation by materials - temperature and surface factors

1. Introduction to absorbing and emitting radiation across the electromagnetic spectrum

All objects are constantly emitting electromagnetic (EM) radiation over a range of frequencies depending on the temperature of the material.

At the same time, the same objects are constantly absorbing EM radiation.

 

At low temperatures, most of this absorbed or emitted radiation is in the infrared EM waves range.

 

At higher temperatures objects may:

glow red e.g. the hot elements of an electric fire (> 550oC), the red glow gets more intense up to ~950oC.

at higher temperatures emit visible light (orange - violet), e.g. hot blue flame (~1000oC, plus lots of infrared, you reach what is called 'white heat' at ~1350oC)

and at very high temperatures objects will emit ultraviolet light e.g. burning magnesium ribbon flame (~2200oC, plus lots of infrared and obviously visible light too!).

 

The EM radiation emitted or absorbed depends on the material and its temperature.

 

Three possible situations in terms of what the material is experiencing as regards EM radiation and temperature

When the rate of an object's emitted radiation > absorbed radiation, it means the object is cooling

(also means: the average power the object is absorbing < average power object is emitting)

The temperature of the object is decreasing.

A hot cup of tea on the table will radiate more infrared than it absorbs, it will give out a net transfer of heat until, on cooling, it reaches the ambient room temperature. The heat transfer still involves conduction and convection but the statement as regards EM radiation is still valid.

 

When the rate of an object's emitted radiation = absorbed radiation, it means the object is at the same constant temperature as its surroundings.

(also means: the average power the object is absorbing = average power object is emitting)

The input and output radiation balanced, no increase or decrease in temperature, stays constant.

 

When the rate of emitted radiation < absorbed radiation, it means the object is heating up

(also means: the average power the object is absorbing > average power object is emitting)

The temperature of the object is increasing.

A piece of bread when placed in a toaster is cooked as the temperature rises by infrared heat absorption. Other cases might involve heat transfer by conduction and convection but the statement as regards EM radiation is still valid.

 

So the rule is - when an object that is hotter (higher temperature) than its surroundings, it will emit more radiation than it absorbs, and, an object that is cooler than its surroundings will absorb more radiation than it emits.

 

For more on this, now read  part 2.

Relating temperature to the intensity, frequency and wavelength of emitted radiation

INDEX physics notes: Absorption and emission of EM radiation by materials

Key points for Physics - Introduction to absorbing and emitting radiation across the electromagnetic spectrum

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA & CIE GCSE physics 9-1 level science examinations.

Here's a syllabus-aligned summary of the topic Temperature Effects on Absorbing and Emitting Radiation across the electromagnetic spectrum, tailored for students preparing for GCSE/IGCSE Physics under WJEC, CCEA, CIE, AQA, Edexcel, and OCR.

Temperature Effects on Absorption and Emission of Radiation

Core Concepts

  • All objects emit and absorb electromagnetic radiation.
  • The rate of emission and absorption depends on:
    • The temperature of the object
    • The surface properties (color, texture, material)
    • The type of radiation (e.g. infrared, visible, UV)

Temperature and Radiation

Temperature Effect on Radiation
Higher Emits more radiation overall
Lower Emits less radiation
Constant Absorbed = Emitted radiation (thermal equilibrium)
  • Hotter objects emit radiation at shorter wavelengths (e.g. visible or UV).
  • Cooler objects emit longer wavelengths, mainly infrared.

Across the Electromagnetic Spectrum

Radiation Type Typical Source Temperature Range
Infrared (IR) Warm objects, humans ~300 K
Visible Light Hot objects (e.g. filament bulbs) ~1000–6000 K
Ultraviolet (UV) Very hot stars, welding arcs >10,000 K

Surface Properties and Radiation

Surface Type Absorption Emission
Black, matte Excellent Excellent
White, shiny Poor Poor
  • Black surfaces absorb and emit radiation more effectively.
  • Shiny surfaces reflect radiation, reducing both absorption and emission.

Real-World Examples

  • Earth’s temperature is regulated by the balance between absorbed solar radiation and emitted infrared radiation.
  • Emergency blankets use shiny surfaces to reduce heat loss by radiation.
  • Solar panels are black to maximize absorption of sunlight.

Exam Board Focus

See 2. Relating temperature to the intensity, frequency and wavelength of emitted radiation

Key Emphases
Absorption/emission of IR, surface effects, thermal equilibrium
Radiation balance, surface color and texture, Earth’s temperature
Radiation curves, energy transfer, practical applications
Infrared radiation, surface properties, temperature effects
Emission/absorption, energy transfer, Earth’s radiation balance
Thermal radiation, surface effects, temperature and emission

Student Tips

  • Sketch radiation curves to visualize how temperature affects emission.

  • See 2. Relating temperature to the intensity, frequency and wavelength of emitted radiation

  • Memorize surface effects: black = good absorber/emitter, shiny = poor.
  • Use analogies: e.g. black t-shirt on a sunny day vs. white one.
  • Practice explaining how radiation affects temperature in different contexts (e.g. greenhouse effect, insulation).
  • Link to experiments: Leslie cube, infrared camera observations.

Keywords, phrases and learning objectives for absorption and emission of radiation

Appreciate that materials, particularly surfaces, are constantly  absorbing and emitting radiation from various regions of the electromagnetic spectrum

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INDEX physics notes: Absorption and emission of EM radiation by materials

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