How does light reflect off a surface, and why does a mirror produce an image?
Topic 13.1 Reflection: apply the law of reflection and the ray model of light to plane surfaces.
A focused answer to AP Physics 2 Topic 13.1, covering the ray model of light, the law of reflection that the angle of incidence equals the angle of reflection, the distinction between specular and diffuse reflection, and image formation in a plane mirror, with full worked examples.
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What this topic is asking
The College Board (Topic 13.1) wants you to apply the ray model of light and the law of reflection to plane surfaces, distinguish specular from diffuse reflection, and describe the image formed by a plane mirror.
The ray model and the law of reflection
The ray model treats light as straight-line rays, which is all geometric optics needs. The law of reflection is its first rule, and the single most important point is that all angles are measured from the normal, not from the surface. A ray hitting at a glancing angle (large angle from the normal) reflects at the same large angle; a ray hitting head-on (zero from the normal) reflects straight back. Getting the angles from the normal is the most common slip.
Specular versus diffuse reflection
The difference is entirely about surface roughness, not a different law. On a smooth surface all the little normals point the same way, so an ordered beam stays ordered, you see a mirror image. On a rough surface the normals tilt randomly, so the same beam scatters every which way; you see the object's surface but no reflected image. This is why a polished surface shines and a matte one does not, even though every individual ray reflects the same way.
Image in a plane mirror
A plane mirror forms an image by reflecting rays so they appear to diverge from a point behind the mirror. The image is virtual (no light actually passes through it, it cannot be projected on a screen), upright, the same size as the object, and located the same distance behind the mirror as the object is in front. It is also laterally reversed (left and right appear swapped), which is why text looks backward in a mirror. The strategic role of this topic is that reflection is the foundation of all mirror optics: the law of reflection applied to curved mirrors (Topic 13.2) produces magnified or reduced, real or virtual images, and the same ray-tracing logic, with refraction added, handles lenses (Topic 13.4). Mastering "angles from the normal" and the plane-mirror image properties here is what makes the curved-mirror and lens equations make sense.
Try this
Q1. A ray hits a mirror at degrees from the normal. State the angle of reflection. [1 point]
- Cue. degrees (equal to the angle of incidence).
Q2. State two properties of the image formed by a plane mirror. [2 points]
- Cue. Virtual and upright (also: same size as the object, same distance behind the mirror).
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2024 (style)5 marksSection II (short FRQ). A ray of light strikes a plane mirror, making an angle of degrees with the mirror surface. (a) State the law of reflection, defining the angles relative to the normal. (b) Calculate the angle of reflection (measured from the normal). (c) Describe the image of an object formed by a plane mirror.Show worked answer →
A 5-point FRQ on reflection.
(a) Law (2 points): the angle of incidence equals the angle of reflection, both measured from the normal (the line perpendicular to the surface), and the incident ray, reflected ray and normal lie in one plane.
(b) Angle (2 points): the ray makes degrees with the surface, so the angle from the normal is degrees. The angle of reflection equals the angle of incidence, degrees.
(c) Image (1 point): a plane mirror forms a virtual, upright image, the same size as the object and the same distance behind the mirror as the object is in front.
Markers reward the law stated relative to the normal, the conversion from surface angle to normal angle, and the virtual upright same-size image.
AP 2023 (style)1 marksSection I (multiple choice). Light reflecting off a rough, matte wall scatters in many directions. This is best described as (A) specular reflection (B) diffuse reflection (C) refraction (D) total internal reflection. Justify your reasoning.Show worked answer →
A 1-point MCQ on types of reflection. The answer is (B).
Diffuse reflection occurs when parallel rays strike a rough surface whose normals point in many directions, so the reflected rays scatter (though each obeys the law of reflection locally). Specular reflection, on a smooth surface, keeps parallel rays parallel. The trap is (A): a matte wall scatters light, which is diffuse, not specular.
Related dot points
- Topic 13.2 Images Formed by Mirrors: apply the mirror equation and magnification to images from concave and convex mirrors.
A focused answer to AP Physics 2 Topic 13.2, covering concave and convex mirrors, the focal length and its relation to the radius, the mirror equation, the magnification equation, the sign conventions, and the characteristics of real and virtual images, with full worked examples.
- Topic 13.3 Refraction: apply Snell's law and the index of refraction, and find the critical angle for total internal reflection.
A focused answer to AP Physics 2 Topic 13.3, covering the index of refraction, Snell's law for the bending of light at a boundary, the link between index and speed, total internal reflection and the critical angle, and the direction of bending, with full worked examples.
- Topic 13.4 Images Formed by Lenses: apply the thin-lens equation and magnification to images from converging and diverging lenses.
A focused answer to AP Physics 2 Topic 13.4, covering converging and diverging lenses, the focal length sign convention, the thin-lens equation, the magnification equation, real and virtual images, and ray tracing, with full worked examples.
- Topic 14.1 Properties of Wave Pulses and Periodic Waves: describe transverse and longitudinal waves and apply v = f lambda to periodic waves.
A focused answer to AP Physics 2 Topics 14.1 and 14.2, covering wave pulses and periodic waves, the distinction between transverse and longitudinal waves, the meaning of amplitude, wavelength, frequency and period, the wave equation v = f lambda, and the fact that a medium does not travel with the wave, with full worked examples.
Sources & how we know this
- AP Physics 2: Algebra-Based Course and Exam Description — College Board (2024)