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Volume I · MMXXVI AP Physics 2: Algebra-Based
Library› AP Physics 2: Algebra-Based› Unit 13: Geometric Optics
⁂   AP Physics 2: Algebra-Based · Unit 13

13. Geometric Optics

12–15% of the AP exam. 10 key topics. 4 common errors analyzed.

12–15% exam weight

Unit 13: Geometric Optics

Unit 13 (Geometric Optics) extends the AP Physics 2 curriculum. Emphasize conceptual justification: FRQ scoring rubrics award points for physical reasoning stated in precise sentences.

Key topics in this unit

  1. Law of reflection and plane mirrors
  2. Snell's law of refraction (n₁ sinθ₁ = n₂ sinθ₂)
  3. Total internal reflection and critical angle
  4. Ray diagrams for concave and convex mirrors
  5. Ray diagrams for converging (convex) and diverging (concave) lenses
  6. Thin-lens and mirror equation (1/f = 1/d_o + 1/d_i)
  7. Magnification (m = −d_i/d_o = h_i/h_o)
  8. Real vs. virtual images: location and orientation
  9. Focal length sign conventions for converging vs. diverging optical elements
  10. Multi-lens systems (qualitative and quantitative)

Common errors — what the rubric penalizes

These are the most frequently penalized mistakes on AP exam FRQs and MCQs. Review each before your next practice session.

Common error

Sign convention inconsistency — students mix up when d_o, d_i, and f are positive or negative for mirrors vs. lenses, and for real vs. virtual images; a single internally inconsistent sign choice propagates errors through the thin-lens equation and magnification

Common error

Ray diagram construction errors — drawing only one principal ray instead of two, using incorrect slopes for rays parallel to the principal axis or through the focal point, and confusing the three standard rays for concave vs. convex mirrors with those for converging vs. diverging lenses

Common error

Confusing total internal reflection conditions — students apply the critical angle formula in the wrong medium direction (light must travel from higher to lower index) and fail to identify that TIR requires the angle to exceed the critical angle, not just equal it

Common error

Magnification sign interpretation — correctly calculating a negative magnification value but then describing the image as 'smaller' rather than 'inverted'; m = −2 means the image is twice as tall AND inverted, and FRQ rubrics award separate points for each attribute

Study checklist

  • Can you list all key topics from memory without this page?
  • Have you worked through at least one FRQ-style question per topic?
  • Can you explain each common error above — and why the correct answer differs?
  • Have you reviewed the relevant College Board CED section and formula sheet entries?
← Magnetism and ElectromagnetismWaves, Sound, and Physical Optics →
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