AP Physics 2 Unit 13: Geometric Optics
Unit 13 models light as rays and uses geometry to explain reflection, mirror images, refraction, and thin-lens images. The organizing chain is ray direction → normal/boundary → focal geometry → image evidence.
What is AP Physics 2 Unit 13 about?
Geometric optics treats light as rays so reflection, refraction, and image formation can be analyzed with diagrams and algebra. A correct solution keeps the ray geometry, local normal, focal point, sign convention, and equation consistent; Unit 14 takes over when the wave nature of light becomes essential.
Four topics that build one ray-optics model
Begin with rays and reflection, add mirror focal geometry, cross a refracting boundary, then use the same ray-evidence logic with thin lenses.
The Unit 13 equations and model statements worth recognizing
Each expression belongs to a particular optical interaction and sign convention; do not use a formula before the ray geometry says it is plausible.
θ_i = θ_rReflection angles measured from the local normal.
1/f = 1/s_o + 1/s_iPlane/spherical mirror image relation with one sign convention.
M = h_i/h_o = −s_i/s_oMirror image size and orientation.
n = c/vHigher index corresponds to lower light speed.
n₁sinθ₁ = n₂sinθ₂Refraction across a boundary.
sinθ_c = n₂/n₁Critical angle only for n₁>n₂.
1/f = 1/s_o + 1/s_iThin-lens image relation.
principal rays locate the imageUse actual rays or backward extensions to classify image type.
Four diagrams that carry most Unit 13 reasoning
The diagram is part of the solution: label normals, focal points, ray directions, and where actual rays or backward extensions meet.
Ray + wavefront
Ray direction is perpendicular to the wavefront in the geometric model.
Mirror ray diagram
Trace principal rays, then classify the image from where reflected rays meet.
Refraction boundary
Measure both angles from the normal and predict toward/away behavior before calculating.
Lens ray diagram
Use parallel, central, and focal-ray behavior to locate real or virtual images.
Which optics topic should you repair first?
Rate each topic from 1 (weak) to 4 (strong). The lowest-rated topic becomes the recommended starting point and is saved locally in the browser.
Start with the lowest-rated topic, then retest it with exact-topic practice.
Keep geometric optics focused
These boundaries prevent unnecessary optical-instrument detail and keep ray optics separate from the wave behavior developed next.
Two connected geometric-optics investigations
Use angle measurements, ray diagrams, transformed graphs, repeated trials, and uncertainty to test the optical model.
Unit 13 quick questions
What is geometric optics?
It is the ray-based model of light used to analyze reflection, refraction, and image formation when wave-spreading effects can be neglected.
What is the most common angle mistake?
Measuring from the surface instead of the local normal. Reflection and refraction angles are measured from the normal.
How do I decide whether an image is real or virtual?
Real images occur where actual reflected or refracted rays meet; virtual images occur where backward extensions of diverging rays meet.
When can total internal reflection happen?
Only for light traveling from a higher-index medium toward a lower-index medium, at incidence beyond the critical angle.
What comes after Unit 13?
Unit 14 develops wave behavior, including interference, diffraction, polarization, and other effects the ray model cannot explain.