Hot things glow: a dimmed lamp, the Sun and the cosmic microwave background

The idea

Any hot object gives out a continuous spectrum whose peak wavelength is inversely proportional to its temperature, which is why a dimmed lamp glows red, the Sun peaks in visible light and the 2.7 K glow left from the early universe peaks in microwaves.

Safety card

What you need

• 1 filament lamp rated 12 V (ray-box or car-type, 24 to 36 W) with a variable low-voltage DC supply, 0 to 12 V
• 1 ammeter and 1 voltmeter with leads
• diffraction grating slides, 600 lines per mm, or a hand spectroscope
• a white card and a darkened room

How to do it

1. Connect the lamp to the supply with the ammeter in series and the voltmeter across the lamp, and darken the room.
2. Raise the voltage in 1 V steps from 2 V to 12 V; at each step record voltage, current and the colour of the filament.
3. At each step look at the filament through the grating and note the colour at the violet end of its spectrum.
4. Calculate the resistance V / I at each step and plot it against voltage.
5. Use the live model to find the peak wavelength for 1,500 K, 3,000 K, the Sun (5,772 K) and the cosmic microwave background (2.725 K).
6. Explain how a black-body glow at 2.725 K filling the sky supports the big bang model.

What you should see

At low voltage the filament glows dull red and its spectrum stops in the orange; as the voltage rises the spectrum extends through green to violet and the lamp turns yellow-white, while V / I climbs steadily as the filament gets hotter. From Wien's law (b = 2.897771955 x 10^-3 m K, CODATA), a filament at 1,500 K peaks at 1.93 micrometres and at 3,000 K at 0.966 micrometres, both in the infrared, so even a bright filament gives out most of its energy as infrared. The Sun, at 5,772 K (NASA), peaks at 502 nm in visible light and emits 6.29 x 10^7 W from each square metre. The cosmic microwave background, at 2.72548 K (Fixsen 2009), peaks at 1.063 mm, a microwave; the ATNF gives its black-body spectrum at 2.725 K as one of the main lines of evidence for the big bang. The learner knows it worked when the violet end of the spectrum appears only at the higher voltages and V / I rises at every step.

What changes

What you change

voltage across the lamp (filament temperature)

What you measure

colour of the glow, the shortest wavelength seen, and resistance

What you keep the same

• same lamp
• viewing distance
• room darkness

Common misconceptions

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

• Red-hot is hotter than white-hot; white-hot is hotter.
• The big bang left nothing that can be measured today; its glow is measured as the cosmic microwave background at 2.725 K.
• Cold objects give out no radiation; everything above absolute zero radiates, colder objects at longer wavelengths.

Curriculum references

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

• Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC5-WAM-01SC5-WS-01SC5-WS-05
• Australian Curriculum v9AC9S10U03AC9S10I03AC9S10I05

Sources

The pages the author read to write this activity.

1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
2. vocabulary.curriculum.edu.au/MRAC/2024/04/LA/SCI/export/MRAC/2024/04/LA/SCI.jsonld
3. www.atnf.csiro.au/resources/education/senior-cosmicengine/cosmology/bigbang
4. arxiv.org/abs/0911.1955
5. physics.nist.gov/cgi-bin/cuu/Value?bwien
6. physics.nist.gov/cgi-bin/cuu/Value?sigma
7. nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html