Infrared Radiation Virtual Lab: Leslie Cube and Surfaces

This infrared radiation virtual lab matches the AQA GCSE Physics required practical on how the nature of a surface affects the infrared it emits and absorbs. In part A, students turn each face of a hot-water Leslie cube towards an infrared detector and record the readings. In part B, a heater warms a matt black plate and a shiny silver plate, and students time how long a wax-held pin takes to drop from each. Students finish with an emission bar chart, two absorption times and a fair-test discussion.

Curriculum links

• AQA GCSE Physics (8463) 4.6.2.2 and required practical 10: how the type of surface affects how much infrared radiation it emits and absorbs. In Combined Science: Trilogy the same content is 6.6.2.2.
• It also supports the physics-only idea that a good absorber is also a good emitter (4.6.3.1).

Simulic is not affiliated with or endorsed by AQA.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"The four faces of the cube are at the same temperature. How will the detector readings for matt black, matt white, dull silver and shiny silver compare?"

Most students expect white to emit as little as shiny silver.

Method in the simulation

1. Open tab A. Emission – Leslie cube. Leave Room temperature at 20 °C, Hot water at 80 °C, Detector distance at 10 cm and Time speed at ×5.
2. Press Zero detector. The meter label changes to "zeroed".
3. Choose Matt black. Press Refill hot water, wait about 5 seconds for a steady reading, then press Record result.
4. Repeat step 3 for Matt white, Dull silver and Shiny silver. Auto-measure 4 faces does the same for you. Every line of the fair-test panel should have a tick.
5. Open tab B. Absorption – pins and wax. Set Heater to 1500 W and Plate–heater distance to 6 cm. Choose ×20 if time is short. Press Switch on heater and wait until both pins drop. The table records both times.
6. Press New run and repeat at 2000 W with 4 cm, then at 1000 W with 6 cm.

Face (80 °C, 10 cm, zeroed)	Matt black	Matt white	Dull silver	Shiny silver
Detector reading (mV)
% of matt black	100

Heater (W)	Distance (cm)	Pin drop, matt black (s)	Pin drop, shiny silver (s)
1500	6
2000	4
1000	6

Expected results

All readings come from the simulation, whose emissivities and heater numbers are teaching values. The last digit of the meter flickers by about 0.01 mV.

• Emission at 80 °C and 10 cm: matt black 2.12 mV, matt white 2.01 mV, dull silver 0.56 mV, shiny silver 0.15–0.16 mV. Shiny silver gives about 7 % of the black reading, matt white 95 %.
• Zeroing matters. Without it every reading carries an extra 0.35 mV: black reads 2.48 and shiny 0.51, so shiny seems to give 21 % of black instead of 7 %.
• The water cools by about 0.4 °C per simulated minute at 80 °C. The fair-test panel flags a spread of more than 2 °C.
• Absorption: at 1500 W and 6 cm, the black pin drops after 23 s and the shiny pin after 243 s, about 10 times longer. At 2000 W and 4 cm: 14 s and 90 s. At 1000 W and 6 cm, the black pin drops after 32 s, but the shiny plate levels off at 54.6 °C, just below the 55 °C melting point of the wax, so its pin never drops.

Questions for students

1. (Prediction, asked again after the lab) How will the readings for the four faces compare?
2. Which quantities must stay the same while you compare the four faces?
3. At 80 °C and 10 cm, with the detector zeroed and the water refilled, what does the detector read for the matt black face?
4. At 1500 W and 6 cm, how many times longer does the shiny silver pin take to drop than the matt black pin?
5. A student records the matt black face first and the shiny silver face ten minutes later without refilling. Why is this unfair, and how would you improve it?

Answers for teachers: (1) Matt black highest, matt white nearly as high, dull silver much lower, shiny silver lowest. (2) The water temperature and the distance from the face to the detector (and the room temperature). (3) 2.12 mV (accept 2.05–2.16). (4) 243 ÷ 23 ≈ 10.6 (accept 9.5–11). (5) The water cools by about 4 °C, so the shiny reading is too low for a reason unrelated to the surface. Refill or record all faces quickly, check the water temperature each time, and keep the distance fixed.

Common misconceptions

• "White surfaces are poor emitters." At the long infrared wavelengths given off by warm water, white paint behaves almost like black paint: 2.01 mV against 2.12 mV. Shiny metal is the poor emitter.
• "Shiny silver absorbs nothing." It still absorbs about a tenth of the heater's radiation, so its pin does drop at 1500 W, just ten times later.

Extension

• Distance. With the matt black face, the reading is 4.93 mV at 5 cm, 2.12 at 10 cm and 0.65 at 20 cm. Doubling from 10 to 20 cm leaves about 31 %, not the 25 % of the inverse-square law. Ask why a large face close to the detector does not follow the law exactly.
• Threshold test. Find the lowest heater power at 6 cm that still drops the shiny pin within 1200 s (in the simulation: 1100 W, after about 527 s). Students explain why the plate temperature levels off: the infrared absorbed balances the heat lost.

FAQ

Why does the simulation have a "Zero detector" button?

A real thermopile drifts and picks up radiation from the room. Without zeroing, the shiny face looks three times better than it is.

Can students use the matt white result in an exam?

Teach the exam point first: matt black is the best emitter and absorber, shiny silver the worst. Then use the white result to discuss wavelength, as the note under the simulation does.

Does this replace the wet practical?

Not for AQA: required practicals must still be done by hand. Use the simulation to plan, rehearse or analyze the results. See virtual labs vs physical labs.

Related simulations and guides

Blackbody radiation – Planck curve, Wien's law and colour Heat Transfer and Insulation – keeping a house warm

For more physics activities, see interactive physics lesson ideas. To run the prediction step well, see predict–observe–explain with simulations.