Investigating Science 11–12 · Year 12

Testing a sunscreen claim: how much UV gets through a thin layer?

Module 7: Fact or Fallacy?

Practical, model not builtLow risk

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The idea

An SPF is measured at a set thickness of sunscreen; a thinner layer lets through far more UV, so testing the claim shows why the amount applied matters as much as the number on the tube.

What you need

broad-spectrum SPF 50+ sunscreen, balance to 0.01 g, disposable gloves
polyethylene film stretched over four identical frames with a 10 cm × 10 cm window (100 cm²)
a handheld UV index or UV meter, UV-sensitive beads for a qualitative check
a clear sunny day near midday; hats and sunscreen for learners

How to do it

Measure the UV reading in full sun through an uncoated film; this is the baseline.
Weigh 50 mg, 100 mg and 200 mg of sunscreen onto three films (0.5, 1.0 and 2.0 mg/cm²) and spread each evenly with a gloved finger.
Take three readings under each film, alternating with the baseline, and calculate the fraction transmitted.
Place UV beads under each film and record the colour after 1 minute.
Compare the transmitted fraction with the model and discuss the validity of the test (meter spectral response, evenness of the film, sample size).

What you should see

SPF defines the fraction of sunburning UV transmitted at the test thickness: 1/50 = 2.0 % for SPF 50. For a uniform film obeying the Beer–Lambert law the transmission at a fraction f of the test thickness is SPF^(−f), so a half layer transmits 14 % (effective SPF 7.1) and a quarter layer 38 % (effective SPF 2.7). The model predicts that transmission rises steeply as the layer thins, and the learner tests that trend; the meter’s spectral response differs from the sunburn weighting, so the readings test the trend rather than the exact SPF. Cancer Council advises about 35 mL for an adult’s whole body; spread over 1.82 m² (Mosteller body surface area for 170 cm and 70 kg) that is 1.9 mg/cm², assuming 1 g/mL.

What changes

What you change: amount of sunscreen per area
What you measure: fraction of UV transmitted
What you keep the same: same film and frame
same product
readings alternated with baseline within minutes
meter held at the same angle

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

SPF 50 stops all UV.
A thin layer gives half the protection of a full layer.
Sunscreen is only needed on hot days.

Safety card

Low riskLearners carry it out

Hazards

UV exposure outdoors
skin sensitivity to sunscreen ingredients

Controls

hats, sunscreen and shade between readings; keep sessions short
wear gloves to spread the sunscreen

Note

No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS12-14 INS11/12-4 INS11/12-5 INS11/12-6 INS11/12-7
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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