The law of reflection states that when light hits a surface, the angle of incidence equals the angle of reflection, with both angles measured from the normal (the line perpendicular to the surface), not from the surface itself.
The law of reflection is one of the simplest rules in all of AP Physics 2, and one of the easiest to mess up. When a light ray (the incident ray) strikes a surface, it bounces off (the reflected ray) at the same angle it came in. The catch is where you measure those angles from. Both the angle of incidence and the angle of reflection are measured from the normal line, an imaginary line perpendicular to the surface at the point where the ray hits. Not from the surface. If a ray comes in at 30° from the normal, it leaves at 30° from the normal, on the other side.
One more piece that diagrams quietly assume: the incident ray, the reflected ray, and the normal all lie in the same plane. Unlike refraction, reflection doesn't care what the light's speed is in either material, and the angle rule works the same whether the surface is a flat plane mirror or a curved one. For curved mirrors, the normal just points along the radius at each point, which is exactly why curved mirrors can focus light.
The law of reflection lives in Topic 6.4 (Refraction, Reflection, and Absorption) and is the engine behind Topic 6.5 (Images from Lenses and Mirrors). In 6.4, it's one of the three things light can do at a boundary, alongside refraction and absorption. In 6.5, it's the rule that makes every mirror ray diagram work. Plane mirror images, concave mirror images, convex mirror images, all of them are just the law of reflection applied ray by ray. When you draw a ray hitting a concave mirror and bending toward the focal point, that's not a separate rule. It's the law of reflection applied to a surface whose normal changes direction from point to point. If you understand that, mirror diagrams stop being memorized recipes and start being predictable geometry.
Keep studying AP Physics 2 Unit 6
Normal Line (Unit 6)
The normal is the reference line for the entire law. Every angle in reflection AND refraction is measured from the normal, never from the surface. Internalizing this once saves you on both topics.
Refraction and Snell's Law (Topic 6.4)
At a boundary between two materials, light usually does both things at once. Part of the beam reflects following the law of reflection, and part refracts following Snell's law. Same incident ray, same normal, two different fates.
Images from Mirrors (Topic 6.5)
Plane, concave, and convex mirror images are all built from the law of reflection. A plane mirror's virtual image appears as far behind the mirror as the object is in front of it, and that result falls straight out of equal angles plus geometry.
Electromagnetic Radiation (Unit 6)
Light is an electromagnetic wave, and the law of reflection applies across the EM spectrum, not just visible light. Radio dishes and satellite receivers are curved mirrors using the exact same equal-angle rule.
No released FRQ has tested the law of reflection by itself in a starring role, but it's baked into anything involving mirrors or light at a boundary. Multiple-choice questions love the normal-line trap, where the problem gives you an angle measured from the surface and the wrong answer choices are waiting for you to forget to subtract from 90°. You may also see rotating-mirror questions (rotate a mirror by θ and the reflected ray rotates by 2θ) or be asked to draw or interpret a ray diagram for a plane or curved mirror. On FRQs, the law of reflection shows up as a justification step. If you claim an image forms at a certain location, your ray diagram has to obey equal angles at every reflection point, and graders check for that.
Reflection is light bouncing off a surface; refraction is light bending as it passes through into a new material. The law of reflection says the angles are equal and needs no information about the materials. Snell's law governs refraction, and the angle changes because light travels at different speeds in different media (that's what index of refraction measures). Quick check: same side of the boundary means reflection, crossing the boundary means refraction. On the exam, both can happen at the same surface simultaneously.
The angle of incidence equals the angle of reflection, and both are always measured from the normal, the line perpendicular to the surface.
If a problem gives you an angle measured from the surface, subtract it from 90° before applying the law; this is the most common trap.
The incident ray, reflected ray, and normal all lie in the same plane.
The law of reflection works for curved mirrors too, because the normal at any point on a curve points along the radius, which is why concave mirrors can focus parallel rays to a focal point.
Reflection doesn't depend on the materials involved, unlike refraction, which depends on the indices of refraction on both sides of the boundary.
At a real boundary, light typically reflects, refracts, and absorbs at the same time, and AP questions can ask about any combination.
It states that when light reflects off a surface, the angle of incidence equals the angle of reflection, with both angles measured from the normal (the perpendicular to the surface). It appears in Topics 6.4 and 6.5.
From the normal, always. If a question says a ray hits a mirror at 25° to the surface, the angle of incidence is actually 65°, and so is the angle of reflection. This setup is a classic MCQ trap.
Yes. The equal-angle rule holds at every point on a curved mirror; the normal just changes direction along the curve since it points along the radius. That's exactly how concave mirrors focus parallel rays to a focal point.
The law of reflection covers light bouncing back into the same medium and needs no material properties, just equal angles. Snell's law covers refraction, where light crosses into a new medium and bends because its speed changes. Both can happen at the same boundary at once.
No. It holds for all electromagnetic radiation, from radio waves to X-rays, and for any frequency of visible light. The reflected angle equals the incident angle regardless of wavelength, which is different from refraction, where bending can vary with frequency.
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