AP Physics 2: Algebra-Based, AP exams, 12–15%

Unit 13: Geometric Optics

Unit 13 treats light as rays. Reflection, refraction, total internal reflection, and image formation by mirrors and lenses are the content, and ray diagrams plus the thin-lens relationship are the tools. Sign conventions for object distance, image distance, and focal length are the usual source of lost points.

Position: 5 of 7 units
Published weighting: 12–15% in the official framework
Topics covered: 5 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Physics 2: Algebra-Based. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Reflection and mirrors: The law of reflection and image formation by plane, concave, and convex mirrors.
Refraction and Snell's law: Index of refraction, bending at an interface, and how wavelength and speed change across a boundary.
Total internal reflection: The critical angle condition and applications such as optical fibers.
Lenses and image formation: Converging and diverging lenses, ray diagrams, and real versus virtual images.
Thin lens and magnification relationships: Relating object distance, image distance, and focal length, and computing magnification and orientation.

Study tips

How to practice Geometric Optics.

Section-specific guidance that follows from how this material is usually tested.

Write the sign conventions at the top of your work every time until they are memorized.
Draw the ray diagram before calculating; it predicts the answer and catches sign errors.
Practice describing an image with all four attributes: real or virtual, upright or inverted, magnified or reduced, and located where.

Where this fits

Unit 13: Geometric Optics is 5 of 7 units in the AP Physics 2: Algebra-Based guide, and it carries a published weighting of 12–15% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Physics 2: Algebra-Based guide Official exam information

Sample Geometric Optics questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
Light travels from air into a transparent medium whose index of refraction is 1.5. Using 3.0 x 10^8 m/s for the speed of light in vacuum, what is the speed of the light inside the medium?
1. A1.5 x 10^8 m/s
2. B2.0 x 10^8 m/s
3. C3.0 x 10^8 m/s
4. D4.5 x 10^8 m/s
Show answer
Answer: B. 2.0 x 10^8 m/s
The index of refraction is defined by n = c/v, so v = c/n = (3.0 x 10^8)/1.5 = 2.0 x 10^8 m/s. The 4.5 x 10^8 m/s choice multiplies by n instead of dividing and would exceed the speed of light in vacuum.
Key insight: A larger index means slower light and more bending at an interface.
Question 2Foundational
A person stands 2.0 m in front of a flat plane mirror. What is the distance between the person and the image the mirror forms?
1. A0 m
2. B1.0 m
3. C2.0 m
4. D4.0 m
Show answer
Answer: D. 4.0 m
A plane mirror forms a virtual image as far behind the mirror as the object is in front, so the image is 2.0 m behind the surface and 4.0 m from the person. The 2.0 m choice reports the object-to-mirror distance instead of the object-to-image distance.
Key insight: Plane mirror images are virtual, upright, the same size, and equally distant behind the mirror.
Question 3Intermediate
An object is placed 15 cm from a thin converging lens of focal length 10 cm. How far from the lens is the image formed?
1. A6.0 cm
2. B25 cm
3. C30 cm
4. D150 cm
Show answer
Answer: C. 30 cm
The thin lens equation 1/f = 1/do + 1/di gives 1/di = 1/10 - 1/15 = 1/30, so di = 30 cm on the far side of the lens, and the image is real and inverted. The 6.0 cm choice adds the reciprocals instead of subtracting.
Key insight: An object between f and 2f of a converging lens always produces a real, inverted, magnified image.
Question 4Intermediate
Light inside a transparent material of index of refraction 2.0 strikes the boundary with air, whose index is 1.0. What is the critical angle for total internal reflection at that boundary?
1. A15 degrees
2. B30 degrees
3. C45 degrees
4. D60 degrees
Show answer
Answer: B. 30 degrees
Total internal reflection begins when the refracted ray grazes the surface, so sin(critical angle) = n2/n1 = 1.0/2.0 = 0.50 and the angle is 30 degrees. The 60 degrees choice takes the arccosine of 0.50 instead of the arcsine.
Key insight: Total internal reflection is possible only when light travels from a higher-index medium toward a lower-index one.
Question 5Advanced
An object is placed 8.0 cm from a thin converging lens whose focal length is 12 cm. What kind of image does the lens form?
1. AA real, inverted image three times as large as the object
2. BA real, inverted image the same size as the object
3. CA virtual, upright image one third as large as the object
4. DA virtual, upright image three times as large as the object
Show answer
Answer: D. A virtual, upright image three times as large as the object
With the object inside the focal point, 1/di = 1/12 - 1/8 = -1/24 gives di = -24 cm, and the negative image distance means the image is virtual and on the same side as the object. The magnification is -di/do = +3, so the image is upright and three times as large, which is how a magnifying glass works.
Key insight: A converging lens makes a magnified virtual image only when the object sits closer than one focal length.

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Unit 13: Geometric Optics