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An aquatic photosynthetic organism can provide a measurable response at different distances from a lamp. Keep the organism amount and condition, measurement interval and carbon dioxide availability comparable. Allow an appropriate adjustment time at each setting before recording the response.
Video by Malmesbury Education. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Design a controlled light-distance investigation
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Why control temperature when changing lamp distance?
The lamp can change temperature as well as illumination. A second changing factor would weaken the comparison.
After watching 2: Why are bubble counts an imperfect volume measurement?
Bubble sizes can vary. A count is a proxy unless bubble volume is justified.
What you'll be able to do
An aquatic photosynthetic organism can provide a measurable response at different distances from a lamp. Keep the organism amount and condition, measurement interval and carbon dioxide availability comparable. Allow an appropriate adjustment time at each setting before recording the response.
Changing lamp distance can also change temperature, so monitor and control temperature using the approved arrangement. Repeat measurements and consider treatment order. A heat shield or water bath helps only when it actually controls the relevant temperature; its presence alone does not prove that every condition remained constant.
Counting bubbles over a defined time gives a convenient proxy, but bubbles can differ in size or merge. Measuring an appropriate collected gas volume per time can improve the comparison. Record the units and explain what is directly measured. A short observation or an escaping bubble may contribute substantial uncertainty.
The observation reflects the organism’s net gas response under the conditions, not necessarily gross photosynthetic production with respiration absent. A rate can plateau because another factor limits the process. Do not infer that a plateau automatically means the plant has died or stopped respiring.
For a suitable point-source approximation, light intensity is proportional to 1/distance². This is a model, not an assertion that every lamp, reflector and surrounding surface produces that exact pattern. A light sensor can test the actual conditions. Distinguish a graph against distance from a graph against the modelled intensity.
Explain a trend using light as an input to photosynthesis while checking temperature and other constraints. Use independent repeats and avoid extrapolating an unlimited straight-line increase. For assessments, apply tier-specific mathematical requirements as indicated by the specification.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Under the stated inverse-square model, how does intensity change when lamp distance rises from 10 cm to 20 cm?
It falls to one quarter under that model, not one half.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.