A concave mirror is a converging mirror whose curved surface reflects incident rays parallel to the principal axis toward a real focal point in front of the mirror, located approximately halfway between the mirror's surface and its center of curvature (f = R/2).
A concave mirror curves inward like the inside of a bowl, and that shape makes it a converging mirror. Send in light rays parallel to the principal axis, and they all reflect through one common spot in front of the mirror called the focal point. For a spherical mirror, the AP CED lets you approximate that focal point as sitting on the principal axis halfway between the mirror's surface and its center of curvature, which gives you the handy relationship f = R/2.
What makes concave mirrors interesting (and testable) is that the image depends entirely on where the object sits. Put the object farther from the mirror than the focal point and you get a real, inverted image, an image actual light rays pass through, so you could project it on a screen. Slide the object inside the focal length and the reflected rays diverge, producing a virtual, upright, magnified image, which is exactly why makeup and shaving mirrors are concave. Same mirror, totally different image, just from moving the object.
Concave mirrors live in Topic 13.2 (Images Formed by Mirrors) in Unit 13: Geometric Optics, and they directly support learning objective 13.2.A, which asks you to describe the image formed by a mirror. The CED's essential knowledge spells out the core idea you need: parallel incident rays reflect toward a common focal point in front of a concave mirror, in contrast to a convex mirror where reflected rays only appear to come from a focal point behind the glass. Concave mirrors are also where image-description vocabulary gets real. Real versus virtual, upright versus inverted, magnified versus reduced all show up in one device depending on object position, so if you can fully analyze a concave mirror, you've basically mastered the language of geometric optics.
Keep studying AP® Physics 2 Unit 13
Focal length (Unit 13)
The focal length is the single number that defines a concave mirror's behavior. The CED lets you treat it as half the radius of curvature, so f = R/2, and a concave mirror's focal length is positive because its focal point is real and in front of the mirror.
Ray diagrams and principal rays (Unit 13)
Concave mirrors are where ray diagrams earn their keep. You only need two principal rays to locate an image, and the rest of the rays are redundant. That's why blocking the parallel rays from an object doesn't make the image disappear; it just gets dimmer.
Inverted image and magnification (Unit 13)
When the object is beyond the focal point, a concave mirror flips the image upside down and the magnification comes out negative. Inside the focal point, the image is upright and magnified. Memorize that switch at f; it's the most common thing the exam probes.
Plane mirror (Unit 13)
A plane mirror is the limiting case of a concave mirror as the curvature flattens out. The CED says a plane mirror's focal point is infinitely far away, which is why it can only ever make an upright, virtual image the same size as the object.
Expect concave mirrors in multiple-choice questions that mix geometry with physics. A classic stem gives you a focal length and asks where a reflected ray goes, like a 15 cm focal length mirror struck by a parallel ray 5 cm off-axis, where you find the reflection angle by tracing the ray through the focal point. Conceptual stems test whether you understand convergence as the mechanism, such as explaining why a solar furnace gets blisteringly hot at the focal point (every parallel ray gets concentrated there). Trickier questions probe whether you really get ray diagrams, like asking what happens to a real image if all the parallel rays from the object are blocked. The answer is that the image still forms at the same location from the unblocked rays, just dimmer. No released FRQ has used this term verbatim, but describing the image formed by a mirror is exactly what LO 13.2.A demands, so be ready to sketch a ray diagram and justify whether the image is real or virtual, upright or inverted, enlarged or reduced.
Both are spherical mirrors, but they bend light in opposite directions. A concave mirror converges parallel rays to a real focal point in front of the mirror, so it can form real, inverted images (or virtual magnified ones when the object is inside f). A convex mirror diverges parallel rays so they only appear to come from a focal point behind the mirror, which means it can only ever form virtual, upright, reduced images. Quick check: concave caves inward like a bowl and concentrates light; convex bulges outward and spreads it.
A concave mirror is a converging mirror, meaning incident rays parallel to the principal axis reflect through a real focal point in front of the mirror.
The focal point of a spherical concave mirror sits approximately halfway between the mirror's surface and its center of curvature, so f = R/2.
If the object is farther from the mirror than the focal point, the image is real and inverted; if the object is inside the focal point, the image is virtual, upright, and magnified.
Blocking some rays from an object makes a real image dimmer but does not change its location or shape, because any two rays are enough to locate the image.
A concave mirror concentrates parallel light at its focal point, which is why it works in solar furnaces and reflecting telescopes.
It's a mirror curved inward that converges parallel light rays to a real focal point in front of the mirror. It appears in Topic 13.2 (Images Formed by Mirrors) in Unit 13: Geometric Optics, where LO 13.2.A asks you to describe the images mirrors form.
No. It forms a real, inverted image only when the object is beyond the focal point. If the object sits inside the focal length, the reflected rays diverge and the image is virtual, upright, and magnified, which is exactly how a makeup mirror works.
A concave mirror converges parallel rays to a real focal point in front of the mirror and can form either real or virtual images. A convex mirror diverges rays so they only appear to come from a focal point behind the mirror, and it can only form virtual, upright, reduced images.
On the principal axis, approximately halfway between the mirror's surface and its center of curvature. That gives you f = R/2, a formula the AP exam expects you to use for spherical mirrors.
Every incident ray parallel to the principal axis reflects through the same focal point, so the mirror concentrates light energy from its entire surface into one small spot. That convergence is why concave mirrors power solar furnaces.
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