What it covers: How light bounces and bends — reflection, refraction, Snell's law, total internal reflection, mirrors, and lenses.
Exam weight: About 12–15% of the AP Physics 2 exam.
The big question: How can simple geometric rules — angle in equals angle out, n₁sinθ₁ = n₂sinθ₂ — predict exactly where mirrors and lenses will form images?
In the College Board CED: Unit 13: Geometric Optics (Topics 13.1–13.4) — the revised CED numbers Physics 2 units 9–15.
Key topics at a glance
Law of Reflection
Angle of incidence = angle of reflection, both measured from the normal. The simplest, most reliable rule in this unit.
Snell's Law
n₁ sinθ₁ = n₂ sinθ₂. Light bends toward the normal entering a denser medium, away from the normal entering a less dense medium.
Total Internal Reflection
Occurs only going from higher n to lower n, beyond the critical angle. The basis of fiber optics.
Plane & Curved Mirrors
Flat mirrors always form virtual, upright, same-size images. Curved mirrors use the mirror equation: 1/f = 1/d_o + 1/d_i.
Thin Lenses
1/f = 1/d_o + 1/d_i (same form as the mirror equation). Converging lenses focus light; diverging lenses spread it.
Magnification
m = -d_i/d_o. Negative magnification means inverted image; |m| > 1 means enlarged.
Ray Diagrams
Trace principal rays to locate images geometrically — a visual cross-check for the algebraic mirror/lens equation results.
Real vs. Virtual Images
Real images form where light actually converges (can be projected). Virtual images only appear to come from a point (cannot be projected).
The key terms you must know
Law of reflection — angle of incidence equals angle of reflection.
Index of refraction (n = c/v) — how much a medium slows light relative to vacuum.
Snell's law (n₁sinθ₁ = n₂sinθ₂) — describes how light bends at a boundary between media.
Critical angle / total internal reflection — the angle beyond which light is completely reflected rather than refracted.
Magnification (m = -d_i/d_o) — the ratio of image size to object size, including orientation (sign).
Real vs. virtual image — whether light rays actually converge (real) or only appear to (virtual).
Key themes to remember
Mirrors and lenses share the same governing equation. 1/f = 1/d_o + 1/d_i applies to both — only the sign conventions and ray-tracing rules differ slightly.
Refraction is about light changing speed, not "wanting" to bend. The bending is a geometric consequence of one side of the wavefront slowing down before the other.
Algebra and ray diagrams should always agree. If your calculated image distance and your ray-traced image don't match in sign/location, you've made an error somewhere.
Convex/diverging always virtual; concave/converging can be either. Object position relative to the focal point determines real vs. virtual for converging elements.
Total internal reflection only works one direction. Going from low n to high n, light always refracts through — it never totally internally reflects.
Common exam traps
Angles are always measured from the normal, never from the surface itself. A common source of careless errors.
Don't confuse "bends toward normal" with "bends toward higher angle." Bending toward the normal means the angle gets SMALLER, not larger.
A negative image distance means virtual, not "negative size." Sign conventions track real/virtual and orientation, not literal negative quantities.
Total internal reflection requires going from higher n to lower n AND exceeding the critical angle — both conditions are necessary, not just one.
A diverging lens or convex mirror can NEVER form a real image from a real object — don't try to force a positive image distance out of the equation for these.
Magnification sign matters as much as magnitude. m = +2 means upright and doubled; m = −2 means inverted and doubled — don't drop the sign when interpreting results.