Describe a controlled infrared absorption experiment
Compare temperature rises under matched source and sample conditions and separate absorption from emission.
Extended only (Supplement); Physics 0625, 2026–2028, section 2.3.3. These lessons cover steady and unequal rates, Earth radiation balance, emitter/absorber experiments and temperature/area effects. Written calculations and described methods are self-checked
Before you begin
- Distinguish absorption from emission.
- Use temperature change and identify thermal-property differences.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which arrangement best isolates a surface-finish absorption comparison?
What you will learn
- Describe an experiment distinguishing good and bad infrared absorbers.
- Identify source, geometry, sample-property and outgoing-transfer controls.
Match source exposure and sample response
In a teacher-supervised method, use suitable comparable objects with different finishes, such as dull black and shiny light surfaces. Match material, mass, exposed area and initial temperature as closely as the method allows. Place them at the same distance and orientation to a suitable infrared source and expose them for the same time, with comparable surrounding airflow and other conditions.
Measure representative initial and final temperatures using an appropriate method. A greater rise for the dull black sample under a suitable controlled comparison supports stronger absorption in the standard school model. Do not put one surface closer to the source or give it a smaller thermal mass and then attribute the whole response to the finish.
Net temperature rise is not absorption alone
The samples can also emit, convect and conduct during the trial. A temperature rise records net energy gain; the initial or short-time comparison with matched properties can help investigate absorption, but exact absorbed power needs those other transfers and thermal properties accounted for. A larger rise is not a direct detector reading of total incident radiation.
For supplied comparable samples, rises of 6 K and 2 K support a different net response under the stated controls; they do not alone establish a universal three-to-one absorption property. Repeat suitable measurements, control position and monitor conditions. This is a written experiment plan and supplied-data exercise, not a completed laboratory assessment.
Pause and explain
Samples warm from 20°C to 26°C and 22°C. What are their temperature rises?
Worked example
Comparable samples begin at 20°C and reach 26°C and 22°C after matched infrared exposure. Interpret the supplied rises.
Show the worked solution
- Rises are 6 K and 2 K.
- The first has the greater net heating response under the stated controls.
- With suitable matched conditions this supports stronger absorption, but an exact universal absorbed-power ratio is not established.
Answer Rises 6 K and 2 K; the first has the greater controlled net response.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the different-finish absorption experiment and its temperature measurements.
Give me a hint
Match exposure and thermal properties before comparing rises.
Compare my reasoning
- Use comparable samples at matched initial temperatures and source geometry.
- Expose for the same time and measure representative final temperatures.
- Compare rises while considering outgoing transfers and repeats.
A controlled source-exposure and temperature-rise comparison supports the absorption investigation.
Look for these in your work
- Name the controls.
- Retain the net-response limit.
Compute rises for samples starting at 20°C and ending at 26°C and 22°C. What response comparison is supported?
Give me a hint
Use final minus initial without converting the intervals by an offset.
Compare my reasoning
- The rises are 6 K and 2 K.
- The first has the greater net temperature response.
- A universal exact absorption ratio needs more energy-account evidence.
6 K and 2 K; greater first-sample net response.
Look for these in your work
- Use intervals correctly.
- Limit the absorption claim.
The black sample warms more but has half the mass and is closer to the source. Can the result isolate finish? Improve the method.
Give me a hint
Mass and source geometry are alternative causes of the result.
Compare my reasoning
- Match sample material, mass, area and initial temperature.
- Match source distance, orientation, duration and surrounding conditions.
- Repeat and evaluate other outgoing transfers before a finish-only conclusion.
No; match sample thermal properties and source exposure before attributing the difference to finish.
Look for these in your work
- Identify both confounders.
- Keep the measurement limitations.
Common mistakes
- Match mass, material, start temperature, distance, area and time.
- A temperature rise measures net gain, not absorption alone.
- Keep emission and absorption experiments distinct.
What can you explain now?
Why does unequal sample mass confound a temperature-rise absorption comparison?
Compare with the explanation
Temperature response depends on thermal mass as well as energy gain.
Different masses can give different rises even if the absorbed energy were equal.
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Make it stick
Describe absorption testing with matched exposure and thermal properties, then distinguish net rise from absorbed power alone.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.