Aim for FivePhysics 2

Qualitative/quantitative translation (QQT)

Colors and black bands in a soap film

  • Unit 14
  • 8 points
  • About 18 minutes

You can use a calculator on this question, just like on exam day.

A shorter multipart problem that connects reasoning in words with an equation. You make and justify a claim without equations, derive an equation from a physics principle, and then explain whether the two agree or use them to predict what happens in a changed situation. On the exam: Question 4 of 4. New for May 2027: the free-response section is 95 minutes, down from 100, for all 4 questions (50% of your score), one of each type in this order. Calculator and equation sheet allowed. The CED suggests 15–20 minutes. This question type started in May 2025, so Physics 2 free-response questions from 2024 and earlier are built differently.

The question

A soap film (index of refraction 1.33) is held vertically in a wire loop, with air on both sides. The film drains under gravity, so it is thinnest at the top and gets gradually thicker toward the bottom. The film is viewed in reflected white light, at normal incidence.

Suggested time: 18 minutes

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Part (a)

3 points

Near the very top, where the film is much thinner than the wavelength of visible light, the film looks black: almost no light is reflected. Explain why. Do not use equations in this part; reason in words.

Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).

0 / 2,500 characters

Part (b)

3 points

Derive an expression for the smallest film thickness t that strongly reflects light of wavelength λ (measured in air), in terms of λ and the film's index of refraction n. Then calculate t for λ = 600 nm.

Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).

0 / 2,500 characters

Part (c)

2 points

A layer of the same soap solution, 113 nm thick, is spread on a flat sheet of glass (index of refraction 1.50) and viewed in reflected light at normal incidence. Predict whether 600 nm light is now reflected strongly or weakly, and explain how this follows from the reasoning you used in part (a).

Type math plainly, like x^2, sqrt(x) or (x+1)/(x−1).

0 / 2,500 characters

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