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Unit 5 · Geometric Optics Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Physics 2 Unit 5 Essentials

The must-know terms and big ideas for Unit 5: Geometric Optics. Every vocabulary word and concept you need to master.

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Big Idea 1
Two simple angle rules explain almost everything in this unit
The law of reflection (angle in = angle out) and Snell's law (n₁sinθ₁ = n₂sinθ₂) are the entire foundation of geometric optics. Every mirror, lens, prism, and fiber-optic cable in this unit is just these two rules applied repeatedly to different geometries. If you fully understand these two relationships and can apply them confidently, the rest of the unit is mostly bookkeeping.
Reflection Refraction Snell's Law
Big Idea 2
Mirrors and lenses are governed by the exact same equation
1/f = 1/d_o + 1/d_i applies identically to mirrors (reflection-based) and lenses (refraction-based) — the physical mechanism differs, but the geometric relationship between focal length, object distance, and image distance is the same. Once you've mastered solving this equation and interpreting its sign conventions for one type of optical device, you've effectively mastered both.
Mirror Equation Lens Equation Image Formation
Big Idea 3
Real vs. virtual is about where light rays actually go
The distinction between a real image (light rays physically converge there — could be projected on a screen) and a virtual image (light rays only appear to come from that point — cannot be projected) is the single most conceptually important distinction in image formation. Diverging lenses and convex mirrors can only ever produce virtual images; converging lenses and concave mirrors can produce either, depending on where the object sits relative to the focal point.
Real Images Virtual Images Ray Diagrams
Law of reflection
The angle of incidence equals the angle of reflection, both measured from the normal to the surface.
Reflection
Normal
An imaginary line perpendicular to a surface at the point of contact; all angles in optics are measured from the normal.
Reflection
Specular vs. diffuse reflection
Specular reflection occurs off smooth surfaces (like a mirror), producing a clear reflected image. Diffuse reflection occurs off rough surfaces, scattering light in many directions.
Reflection
Refraction
The bending of light as it passes between media with different indices of refraction, due to a change in light's speed.
Refraction
Index of refraction (n)
n = c/v — the ratio of light's speed in vacuum to its speed in a given medium; describes how strongly a medium slows and bends light.
Refraction
Snell's law
n₁ sinθ₁ = n₂ sinθ₂ — relates the angles of incidence and refraction to the indices of refraction of the two media.
Refraction
Critical angle
The angle of incidence (in a higher-index medium) at which the refracted ray travels exactly along the boundary surface (refraction angle = 90°).
Total Internal Reflection
Total internal reflection
Complete reflection of light at a boundary when traveling from a higher-index to a lower-index medium at an angle beyond the critical angle.
Total Internal Reflection
Plane mirror
A flat mirror that always forms a virtual, upright, same-size image located as far behind the mirror as the object is in front of it.
Mirrors
Concave mirror
A mirror that curves inward (toward the observer); can produce real or virtual images depending on object position relative to the focal point.
Mirrors
Convex mirror
A mirror that curves outward (away from the observer); always produces a virtual, upright, reduced image.
Mirrors
Mirror equation
1/f = 1/d_o + 1/d_i — relates the focal length, object distance, and image distance for a curved mirror.
Mirrors
Converging (convex) lens
A lens that bends parallel light rays inward toward a focal point on the far side.
Lenses
Diverging (concave) lens
A lens that bends parallel light rays outward, away from each other; always produces a virtual, reduced image.
Lenses
Thin lens equation
1/f = 1/d_o + 1/d_i — the same mathematical form as the mirror equation, applied to refraction through a lens.
Lenses
Focal length (f)
The distance from a mirror or lens to its focal point, where parallel incoming rays converge (or appear to diverge from).
Lenses
Magnification (m)
m = -d_i/d_o = h_i/h_o — the ratio of image height to object height; negative values indicate an inverted image.
Image Formation
Real image
An image formed where light rays actually converge; can be projected onto a screen. Corresponds to a positive image distance.
Image Formation
Virtual image
An image formed where light rays only appear to diverge from, without actually converging there; cannot be projected onto a screen. Corresponds to a negative image distance.
Image Formation
Ray diagram
A geometric diagram tracing principal rays of light through an optical system to locate and characterize the resulting image.
Image Formation