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13.6 Required Practical: Infrared Radiation

GCSE Physics · AQA · Chapter 13: Electromagnetic Waves

A free GCSE Physics lesson for AQA (8463) from OBA School: how the type of surface affects the infrared radiation it emits and absorbs. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Plan and carry out an investigation into how the type of surface affects the infrared radiation emitted
  • Describe a method to compare how well different surfaces absorb infrared radiation
  • Identify the independent, dependent and control variables, and the hazards and how to control them
  • Present results with a bar chart or line graph, calculate means and rates, and draw conclusions
  • Evaluate the method: sources of error, resolution and improvements

Key words

  • Leslie cube: A hollow metal cube whose four vertical faces have different surfaces. It is filled with hot water.
  • Infrared detector: A sensor (e.g. a thermopile) that gives a reading that increases with the infrared radiation reaching it.
  • Independent variable: The variable you change on purpose – here, the type of surface.
  • Dependent variable: The variable you measure – here, the detector reading or the temperature change.
  • Control variable: A variable kept the same so that the test is fair, e.g. distance from the surface.
  • Resolution: The smallest change an instrument can show, e.g. 0.1 °C on a digital thermometer.

The idea behind the practical

Every object emits and absorbs infrared radiation. How much it emits at a given temperature depends on its surface. A good emitter of infrared is also a good absorber; a poor emitter is a good reflector.

  • Matt (dull) black surfaces are the best emitters and absorbers.
  • Shiny silver (light, shiny metallic) surfaces are the worst emitters and absorbers – they reflect most infrared.

The aim is to compare surfaces in a fair test: first how much infrared each emits (Part A), then how well two surfaces absorb it (Part B).

Part A: emission from a Leslie cube

Apparatus: Leslie cube, kettle, heatproof mat, infrared detector and meter (or a thermometer with a blackened bulb), metre rule, stopwatch.

Method

1. Stand the Leslie cube on a heatproof mat. Fill it carefully with hot water from a kettle and put on the lid.

2. Place the infrared detector 10.0 cm from the centre of the first face, pointing straight at it. Use the rule to check the distance.

3. Wait for the reading to settle, then record it.

4. Turn the cube so the next face points at the detector, at the same distance. Record the reading. Repeat for all four faces, working quickly so the water does not cool much.

5. Empty, refill with fresh hot water and repeat the whole set twice more. Calculate a mean for each face.

  • Independent; In this practical type of surface (matt black, matt white, shiny black, shiny silver) – a categoric variable
  • Dependent; In this practical infrared detector reading (or temperature reading)
  • Control; In this practical distance from detector to face; temperature of the water; same detector and meter; detector pointing straight at the face; time allowed before reading; no other heat sources or draughts nearby

Hazard: Hot water can scald. Fill the cube while it stands on the heatproof mat, do not carry it when full, mop up spills straight away and do not touch the hot faces. Let the water cool before emptying.

Part A: results and analysis

  • matt black; Reading 1 4.6; Reading 2 4.8; Reading 3 4.7; Mean 4.7
  • matt white; Reading 1 4.1; Reading 2 4.3; Reading 3 4.2; Mean 4.2
  • shiny black; Reading 1 3.6; Reading 2 4.0; Reading 3 3.8; Mean 3.8
  • shiny silver; Reading 1 0.7; Reading 2 0.6; Reading 3 0.8; Mean 0.7

The independent variable is categoric (names, not numbers), so the results are plotted as a bar chart, not a line graph. The pink bars show the range of the three repeats.

Worked example: mean and uncertainty for the shiny black face

mean = (3.6 + 4.0 + 3.8) ÷ 3 = 11.4 ÷ 3 = 3.8
range = 4.0 − 3.6 = 0.4, so uncertainty ≈ ± half the range = ± 0.2

Answer: 3.8 ± 0.2

Conclusion: the matt black face emitted the most infrared and the shiny silver face by far the least. The matt white face emitted nearly as much as the matt black one, which shows that how shiny the surface is matters more than its colour for infrared.

Part B: comparing absorption

Method 1 – temperature rise. Paint one can matt black and leave another shiny silver. Put the same volume of cold water, at the same starting temperature, in each. Place both the same distance from a radiant heater (or in the same sunlight). Record the temperature of each every 2 minutes for 10 minutes and plot both on one graph.

Worked example: rate of temperature rise

matt black: (27.5 − 15.0) ÷ 10 = 1.25 °C per minute
shiny silver: (19.0 − 15.0) ÷ 10 = 0.40 °C per minute

Answer: the black can heats about 3 times faster

Method 2 – wax and drawing pins. Stick a drawing pin with a little wax on the back of a matt black plate and a shiny silver plate. Stand the plates the same distance either side of a radiant heater and time how long each pin takes to fall. The pin on the matt black plate falls first because that plate absorbs infrared faster, so its wax melts first.

Evaluating the method

  • Water cools while the faces are measured one after another; Effect later faces get lower readings (a systematic error); Improvement measure quickly; change the order of the faces in each repeat; or keep the water topped up at a fixed temperature
  • Distance or angle of detector not exactly the same; Effect readings vary (random error); Improvement use a clamp and a fixed ruler; repeat and take a mean
  • Thermometer with 1 °C resolution; Effect small differences are missed; Improvement use an infrared detector or digital thermometer (resolution 0.1 °C)
  • Infrared from people or lamps nearby; Effect extra readings; Improvement keep other sources away; check the zero reading first

Exam tip: If you are asked why results are repeatable, say the repeats were close together (a small range). Repeating and calculating a mean reduces the effect of random errors, not systematic ones.

Common mistakes

  • Drawing a line graph for a categoric independent variable. Use a bar chart.
  • Saying “black surfaces reflect infrared”. Matt black absorbs it; shiny silver reflects it.
  • Giving “use the same amount of water” as a control for Part A without mentioning the distance and water temperature, which matter most.
  • Saying repeats make results more accurate. Repeats improve repeatability and reduce random error; accuracy needs a correctly calibrated instrument.
  • Forgetting the hazard: the hazard is scalding from hot water, not “infrared burns”.

Revision questions

Try these, then check your answers below each one.

Question 1. In the Leslie cube investigation, what is the independent variable? (Choose from: The type of surface, The distance from the detector, The temperature of the water, The detector reading.)

Answer: The type of surface

Working: The type of surface is changed. The detector reading is the dependent variable; distance and water temperature are control variables.

Question 2. Which surface is the best emitter of infrared radiation? (Choose from: Matt black, Shiny black, Matt white, Shiny silver.)

Answer: Matt black

Working: Matt black is the best emitter (and absorber). Shiny silver is the worst – it reflects infrared.

Question 3. True or false? A good absorber of infrared radiation is also a good emitter.

Answer: True

Working: True. Matt black surfaces are the best absorbers and the best emitters; shiny silver surfaces are poor at both.

Related lessons

Continue in the full course

This lesson is section 13.6 (AGCPH1306) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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