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AP Physics 2: Algebra-Based · Unit 5 of 7
Geometric Optics
12–15% of the exam4 lessons · 53 min20 terms
What this unit covers
The topics below follow the published Physics 2 course framework for Unit 5. This unit is worth 12–15% of the exam, so budget your time against that rather than against how long the unit takes to teach.
Reflection & refractionLenses & mirrorsRay diagramsTotal internal reflection
Lessons in this unit
- Reflection & Plane Mirrors11 min · 3 objectivesState and apply the law of reflection using angles measured from the normal · Distinguish specular from diffuse reflection · Describe the properties and location of an image in a plane mirror
- Refraction, Snell’s Law & Total Internal Reflection14 min · 3 objectivesRelate the index of refraction to the speed of light in a medium · Apply Snell’s law to find the direction of a refracted ray · Explain the condition for total internal reflection
- Curved Mirrors & the Mirror Equation14 min · 3 objectivesDistinguish concave (converging) from convex (diverging) mirrors and their focal points · Use the mirror equation and sign conventions to locate an image · Predict whether an image is real or virtual, upright or inverted, enlarged or reduced
- Thin Lenses & Image Formation14 min · 3 objectivesDistinguish converging from diverging lenses and their focal lengths · Apply the thin-lens equation and magnification to locate and describe images · Predict the image formed by a converging lens for different object positions
Formulas in Unit 5
Law of reflection
θ_incidence = θ_reflection (both measured from the normal)
A ray hitting the mirror at angle θ to the surface makes an angle (90° − θ) to the normal, and reflects at that same angle on the other side of the normal.
Index of refraction and Snell’s law
n = c / v · n₁ · sinθ₁ = n₂ · sinθ₂
All angles are measured from the normal. The product n·sinθ is the same on both sides of the boundary, so a bigger n forces a smaller angle.
Mirror equation and magnification
1/f = 1/d_o + 1/d_i · m = − d_i / d_o
Sign conventions: d_i is positive for a real image (in front of the mirror), negative for a virtual image (behind). A positive m is upright; negative m is inverted; |m| > 1 is enlarged.
Thin-lens equation and magnification
1/f = 1/d_o + 1/d_i · m = − d_i / d_o
For a converging lens f > 0; for a diverging lens f < 0. A positive d_i is a real image on the far side of the lens; a negative d_i is a virtual image on the same side as the object.
Every term in Unit 5
All 20 terms we publish for Geometric Optics, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Index of refraction
- n = c/v, always at least 1. Light slows in a denser medium while its frequency stays the same, so wavelength shortens.
- Snell's law
- n₁sin θ₁ = n₂sin θ₂. Light bends toward the normal entering a denser medium and away entering a less dense one.
- Total internal reflection
- Occurs when light travels toward a less dense medium beyond the critical angle sin θc = n₂/n₁. The principle of fiber optics.
- Law of reflection
- The angle of incidence equals the angle of reflection, both measured from the normal.
- Dispersion
- Index of refraction varies slightly with wavelength, so violet bends more than red — which is why a prism separates white light and why rainbows form.
- Converging vs diverging lens
- A convex lens has positive focal length and can form real images; a concave lens has negative focal length and forms only virtual, upright, reduced images.
- Thin lens equation
- 1/f = 1/d_o + 1/d_i. Positive d_i means a real image on the far side; negative means a virtual image on the same side as the object.
- Magnification
- M = −d_i/d_o = h_i/h_o. Negative M means inverted, and |M| > 1 means enlarged.
- Real vs virtual images
- Real images form where light actually converges and can be projected on a screen; virtual images only appear to come from a location and cannot.
- Concave mirror
- Converging, with positive focal length f = R/2. Produces a real inverted image when the object is beyond the focal point.
- Convex mirror
- Diverging, with negative focal length. Always produces a virtual, upright, reduced image — the reason it is used for wide-angle safety mirrors.
- Ray diagram rules for a lens
- A ray parallel to the axis refracts through the focal point; a ray through the center continues straight; a ray through the near focal point emerges parallel.
- Sign conventions for lenses and mirrors
- Object distance positive on the incoming side, image distance positive for a real image, focal length positive for converging. Most errors are sign errors.
- Predicting image type without calculating
- For a converging lens: object beyond 2f gives a real, inverted, reduced image; between f and 2f real, inverted, enlarged; inside f virtual, upright, enlarged.
- Why a magnifying glass must be held close
- Only when the object is inside the focal length does a converging lens produce an upright, enlarged virtual image.
- Apparent depth
- Refraction makes a submerged object appear shallower than it is, because rays bend away from the normal on leaving the water.
- Fiber optics
- Light entering at a shallow angle exceeds the critical angle at the core-cladding boundary and totally internally reflects along the fiber.
- Chromatic aberration
- Dispersion makes different wavelengths focus at slightly different points, blurring a lens image with colored fringes.
- Why the image of a half-covered lens is dimmer, not halved
- Every point on the lens contributes rays to every image point, so blocking half reduces intensity but leaves the whole image visible.
- Real image on a screen
- Only real images can be projected. A virtual image cannot, because no light actually converges at its apparent location.
What examiners penalize here
- Memorize the plane-mirror image: virtual, upright, same size, equal distance behind. It is the reference point you compare curved-mirror and lens images against later in the unit.
- Total internal reflection has two requirements, and both must hold: (1) light moving from high n to low n, and (2) an angle of incidence beyond the critical angle. If either fails, some light refracts through and it is not total.
- Convex mirrors and diverging lenses share a signature: they always make virtual, upright, diminished images, no matter where the object is. Concave mirrors and converging lenses are the versatile ones whose image type depends on object position.
- Learn the converging-lens cases by object position (beyond 2f, at 2f, between f and 2f, inside f) — the exam tests all four. And remember the shortcut: diverging lenses always give virtual, upright, diminished images, no computation needed.
Practice Physics 2
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Physics 2: Algebra-Based exam is Unit 5?
Unit 5, Geometric Optics, is worth 12–15% of the Physics 2 multiple-choice section according to the published course framework. Across all 7 units that makes it a substantial share — heavier than an even split would give it.
What topics are covered in Physics 2 Unit 5?
Geometric Optics covers Reflection & refraction, Lenses & mirrors, Ray diagrams and Total internal reflection. We publish 20 terms with definitions for this unit, all of them on this page.
How should I study Physics 2 Unit 5?
Read the 4 lessons below first — about 55 minutes — then drill the 20 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 7 units of AP Physics 2: Algebra-Based
Unit names, topics and exam weights follow the published College Board course framework for AP Physics 2: Algebra-Based. AP® is a trademark registered by the College Board, which does not endorse this site.