Colored Light and Radish Germination: What Does the Evidence Support?
In a seventh-grade class, four lab groups test whether light color affects radish seed germination. Each group places 10 seeds on moist paper in a petri dish. Group A covers its dish with blue cellophane, Group B with red cellophane, Group C with clear plastic, and Group D puts its dish in a closed cabinet. All dishes except D sit on the same windowsill. After five days, germination counts are blue 9/10, red 7/10, clear 6/10, and dark 2/10. Group A concludes that blue light causes the best germination. The teacher notes that the blue dish averaged 21°C, red 24°C, clear 28°C, and cabinet 20°C; the cellophane sheets differ in thickness; and no group repeated a trial. The class has one week and can borrow four thermometers and a light meter. Students ask whether the conclusion is valid and how to improve the investigation.
1. Identify at least two confounding variables in the original design and explain how each could have produced the observed ranking. Then state whether Group A's causal claim is justified.
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The ranking is confounded by temperature and by light intensity/spectrum. The clear dish was 7°C warmer than the blue dish; if radish germination is inhibited near 28°C, clear would underperform even if color had no effect. Cellophane thickness changes irradiance, so 'blue' also means 'dimmer' and 'cooler' in this setup. The cabinet differs in light, humidity, and airflow, so it is not a clean dark control. With one dish per treatment, there is no replication; a single contaminated dish or uneven moisture could produce the pattern. Group A's claim is therefore not justified as causal. A pattern consistent with the hypothesis is not enough when plausible alternatives were not controlled. The class should treat the result as tentative and redesign before concluding that blue light is best.
2. Using the available equipment and one week, outline a redesigned procedure that would provide stronger evidence about light color. Include controls, replication, and a measurement plan.
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Use identical boxes and identical LED lamps at a fixed distance. Put blue, red, green, and white filters over separate dishes, plus a dark box, with three dishes per treatment (30 seeds each). Measure light with the meter at dish level and adjust distance so each colored treatment has the same intensity, or at least record intensity. Keep all dishes at 24°C in the same location, rotate positions daily, and use the same seed lot, water volume, and paper. Code dishes so the counter does not know treatments. Count germination daily and report percent germination per dish, treatment means, and ranges. Include a dark control to separate light presence from color. This design controls temperature and intensity better, provides replication, and allows an estimate of variability; it still cannot perfectly match spectrum, so the conclusion should be limited.
3. Explain how this episode could be used to teach two ideas about the nature of science, using specific events from the scenario.
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Two NOS ideas: (1) Scientific claims are tentative and revised in light of evidence. Group A announced a causal conclusion after one trial; the teacher's temperature and thickness data show why the claim must be weakened. (2) Controlled experiments and replication are central because alternative explanations must be ruled out. The single-dish treatments and the cabinet's multiple differences illustrate why a fair test requires isolating variables. A third possible idea is peer review/argumentation: classmates question the conclusion and propose a redesign. The episode also shows the difference between observation (counts, temperatures) and inference (blue light causes better germination). Students can practice writing a claim-evidence-reasoning statement that separates what was measured from what was concluded.