Physics 2 · Geometric and Physical Optics, Unit 6 · 14 min read · Updated 2026-05-11
Images — AP Physics 2
AP Physics 2 · Geometric and Physical Optics, Unit 6 · 14 min read
1. Core Concepts of Image Formation★★☆☆☆⏱ 2 min
An image forms when light rays emitted or reflected from an object either converge to a single point (real image) or appear to diverge from a single point (virtual image).
Per the AP Physics 2 CED, Unit 6 (Geometric and Physical Optics) makes up 20-25% of the total exam score, and this subtopic accounts for roughly 7-10% of total exam weight, appearing in both MCQ and FRQ sections. This guide uses the standard Cartesian (real-is-positive) sign convention adopted by College Board for all AP Physics 2 problems.
2. Image Formation by Mirrors★★★☆☆⏱ 3 min
Mirrors form images via reflection, and are categorized as plane (flat) or spherical (curved). For plane mirrors, the object distance $d_o$ (always positive for real objects, the distance from the object to the mirror) relates to image distance $d_i$ (distance from the mirror to the image) as:
d_i = -d_o, \quad m = +1
Magnification $m$, the ratio of image height to object height, is $m=+1$ for plane mirrors, meaning the image is upright, same size as the object, and always virtual. For spherical mirrors with constant radius of curvature $R$, the focal length (distance from the mirror surface to the focal point, where parallel rays converge) is $f = R/2$. The mirror equation relating $f$, $d_o$, and $d_i$ is:
\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i}
Sign convention for mirrors: $f$ is positive for concave (converging, curved inward toward the object) mirrors and negative for convex (diverging, curved outward away from the object) mirrors. A positive $d_i$ means a real image in front of the mirror; negative $d_i$ means a virtual image behind the mirror. Magnification for all mirrors follows:
m = -\frac{d_i}{d_o}
Positive $m$ indicates an upright image, negative $m$ indicates an inverted image, and $|m|$ gives the size ratio relative to the object.
Exam tip: Always confirm the sign of $f$ before starting any calculation. For mirrors, curved inward = concave (positive f), curved outward = convex (negative f) — this never changes for real objects.
3. Image Formation by Thin Lenses★★★☆☆⏱ 3 min
Thin lenses form images via refraction, and follow nearly the same mathematics as mirrors, with only a small change to the sign interpretation of $d_i$. Lenses are divided into converging (convex, thicker at the center), which have positive $f$, and diverging (concave, thinner at the center), which have negative $f$, matching the sign convention for mirrors. The thin lens equation is identical in form to the mirror equation:
\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i}
Magnification is also identical to that for mirrors:
m = -\frac{d_i}{d_o}
The key difference from mirrors is the interpretation of $d_i$: for lenses, a positive $d_i$ means the image is on the opposite side of the lens from the object (the side where light exits after refraction), which is a real image. A negative $d_i$ means the image is on the same side as the object, which is virtual. Converging lenses can form both real and virtual images depending on whether the object is outside or inside the focal length, while diverging lenses always form virtual, upright, diminished images for any real object.
Exam tip: When asked to describe an image, AP graders require all three properties to earn full credit: 1) real or virtual, 2) upright or inverted, 3) magnified/diminished/ same size. Never leave out any of these three.
4. Ray Tracing for Images★★★★☆⏱ 3 min
Ray tracing is a graphical method to locate images and confirm their properties, and is frequently required on AP Physics 2 FRQs. The method relies on three principal rays that follow simple rules; their intersection (or the intersection of their extensions) marks the top of the image. For both mirrors and lenses, the three principal rays are:
**Parallel ray**: Leaves the top of the object traveling parallel to the principal axis. After reflection/refraction, it passes through (or appears to diverge from) the focal point.
**Focal ray**: Leaves the top of the object passing through (or heading toward) the focal point, and exits parallel to the principal axis after reflection/refraction.
**Center ray**: Leaves the top of the object heading for the center of the mirror/lens, and travels straight through without changing direction.
If the actual rays intersect, the image is real; if only the extensions of the rays intersect, the image is virtual. Ray tracing can be used to check calculations from the lens/mirror equation, or to answer conceptual questions about image properties.
Exam tip: Always draw arrows on your rays pointing in the direction of light travel. AP graders consistently deduct points for missing or reversed arrows on ray diagrams.
5. AP Style Worked Practice Problems★★★★☆⏱ 3 min
Common Pitfalls
Why: Students mix up the sign rules for converging/diverging elements across lenses and mirrors
Why: Students carry over the mirror sign interpretation to lenses
Why: Students only remember the magnitude of magnification and drop the negative sign during calculation
Why: Students assume all parallel rays pass through a focal point, regardless of lens type
Why: Students rush and reverse the relationship between focal length and radius of curvature