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Physics 11–12 · Year 12

Black body radiation and Wien's law: a dimmed filament and the solar spectrum peak

Module 7: The Nature of Light (Light: Quantum Model)

Practical, model not builtLow risk

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The idea

A hot body radiates a continuous spectrum whose peak wavelength shifts to shorter values as temperature rises, in inverse proportion to the temperature.

What you need

  • 12 V filament lamp on a variable supply with a voltmeter and ammeter, hand spectroscope
  • Infrared thermometer for a warm surface, and the NASA Sun fact sheet effective temperature 5772 K for the calculation

How to do it

  1. View the lamp through the spectroscope at 3, 6, 9 and 12 V; sketch how far the visible band extends towards the blue at each setting.
  2. Compute the filament resistance from V and I at each setting and rank the settings by temperature (resistance rises with temperature).
  3. Use Wien's law to compute the peak wavelength for a filament at 2700 K and for the Sun at 5772 K.
  4. In the simulation, sweep the temperature and watch the curve's peak move and its area grow.

What you should see

At low voltage only the red end of the band shows; at 12 V the band reaches into the blue. A 2700 K filament peaks at 1073 nm (in the infrared, so most of its output is invisible), a 3000 K filament at 966 nm, and the Sun at 502 nm, in the middle of the visible band. Doubling the temperature halves the peak wavelength and multiplies the radiated power by 16.

What changes

What you change
filament temperature (supply voltage)
What you measure
extent and peak of the spectrum
What you keep the same
  • same lamp
  • same spectroscope

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Hot objects glow only in one colour; they radiate a whole spectrum with a peak that shifts.
  • Red-hot is hotter than white-hot; white-hot is hotter because the peak has moved towards the blue.

Safety card

Low riskLearners carry it out

Hazards

  • hot lamp

Controls

  • handle by the base after cooling

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH12-14PH12-15PH11/12-4
  • Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-03PY-12WS-06
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/sites/default/files/noindex/2025-03/physics-stage-6-syllabus-2017.docx
  2. education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/physics/12Physics_-module-7-guide.docx
  3. phet.colorado.edu/en/simulations/blackbody-spectrum
  4. pdg.lbl.gov/2024/reviews/rpp2024-rev-phys-constants.pdf
  5. nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html
  6. curriculum.nsw.edu.au/learning-areas/science/physics-11-12-2025/outcomes

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