In AP Physics 2, a lens is an optical device with curved surfaces that refracts light to form an image. A thin convex (converging) lens bends parallel rays toward a real focal point; a thin concave (diverging) lens spreads rays as if they came from a focal point on the incident side (Topic 13.4).
A lens is a piece of transparent material with curved surfaces that bends light by refraction. That bending is the whole story. When parallel rays hit a thin convex (converging) lens, they refract and meet at a single point on the far side called the focal point. When parallel rays hit a thin concave (diverging) lens, they spread out as if they all started from a focal point on the same side the light came in. The distance from the lens to that point is the focal length, and it's the one number that controls everything the lens does.
Lenses form two kinds of images. A real image happens when refracted rays actually intersect at a common point, which means you can catch it on a screen. A virtual image happens when the rays only appear to come from a common point, so your eye sees it but a screen can't catch it. Converging lenses can make either kind depending on where the object sits. Diverging lenses only ever make virtual, upright, reduced images, no matter what you do.
Lenses live in Topic 13.4 (Images Formed by Lenses) in Unit 13: Geometric Optics, under learning objective 13.4.A, which asks you to describe the image a lens forms. The essential knowledge (13.4.A.1 through 13.4.A.4) spells out exactly what you're responsible for. You need to know how converging and diverging lenses handle parallel rays, and what makes an image real versus virtual. Everything you learned about refraction and Snell's law earlier in the unit pays off here. A lens is just refraction happening twice, once at each curved surface, engineered so the bent rays form an image. If you can draw a ray diagram and reason about where the rays go, you can answer almost any lens question on the exam.
Keep studying AP® Physics 2 Unit 13
Converging lens (Unit 13)
A convex lens is the workhorse of Topic 13.4 because it's the only thin lens that can form a real image. Object position decides everything. Outside the focal length you get a real, inverted image; inside it you get a virtual, upright, magnified one, which is exactly how a magnifying glass works.
Diverging lens (Unit 13)
A concave lens is the simple sibling. Per 13.4.A.2, it spreads parallel rays as if they came from a focal point on the incident side, so it always produces a virtual, upright, reduced image. If a question says diverging lens, you already know the image type before doing any math.
Focal length (Unit 13)
Focal length is the single number that characterizes a lens. It's positive for converging lenses and negative for diverging ones, and it sits at the heart of the thin lens equation. The 2017 FRQ literally asked students to design an experiment to measure a convex lens's focal length.
Ray diagram (Unit 13)
Ray diagrams are how you actually answer lens questions. Drawing two or three principal rays through a lens tells you where the image is, whether it's real or virtual, and whether it's inverted, often faster than the algebra.
Lens questions show up in two main flavors. Multiple-choice stems give you a lens type, a focal length, and an object or screen position, then ask what image forms. For example, parallel rays hitting a concave lens with f = -25 cm won't produce a sharp image on a screen 50 cm behind it, because diverging lenses can't make real images. Other MCQs flip it around and give you image properties (real, inverted, half-size) and ask you to find the object distance, or ask how the image changes as you slide the object from beyond 2f toward the lens. Free-response questions lean experimental. The 2017 long FRQ asked students to design a procedure using a light box, lens, and screen to determine a convex lens's focal length. So be ready to do three things with lenses. Draw correct ray diagrams, apply the thin lens equation with proper sign conventions, and explain in words why an image is real or virtual.
Both lenses and mirrors form images and use nearly identical equations, but the physics is different. A lens works by refraction, so light passes through it and the image forms on the opposite side when it's real. A mirror works by reflection, so light bounces back and a real image forms on the same side as the object. The trap on the exam is sign conventions and geometry. A converging lens is convex, but the converging mirror is concave. Mixing those up is one of the most common geometric optics errors.
A lens forms images by refracting light at its curved surfaces, and its behavior is summarized by one number, the focal length.
Parallel rays through a convex (converging) lens meet at a real focal point on the far side; parallel rays through a concave (diverging) lens spread as if from a focal point on the near side.
A real image forms when refracted rays actually intersect, so it can be projected on a screen; a virtual image forms when rays only appear to come from a point, so it can't.
Diverging lenses always make virtual, upright, reduced images, no matter where the object is placed.
Converging lenses make real, inverted images when the object is outside the focal length and virtual, upright, magnified images when the object is inside it.
Lenses converge like convex mirrors don't. Remember that converging means convex for lenses but concave for mirrors.
A lens is an optical device with curved surfaces that refracts light to form an image. It's covered in Topic 13.4 of Unit 13 (Geometric Optics), where you learn how converging and diverging lenses redirect parallel rays toward or away from a focal point.
No. A thin concave (diverging) lens always spreads light rays apart, so the rays never actually intersect. Every image it forms is virtual, upright, and smaller than the object, which is why a screen placed behind a diverging lens shows no sharp image.
A lens refracts light passing through it, while a mirror reflects light back. The biggest exam trap is that a converging lens is convex but a converging mirror is concave, and real images form on opposite sides of a lens but on the same side as the object for a mirror.
Yes, it's explicit in the CED under 13.4.A. A real image forms when refracted rays actually intersect at a point, so you can project it on a screen. A virtual image forms when rays only appear to diverge from a point, so your eye sees it but a screen can't capture it.
Shine light from a distant or well-defined object through a converging lens onto a screen, measure object and image distances, and apply the thin lens equation. The 2017 AP Physics 2 long FRQ asked exactly this, having students design a procedure with a light box, lens, and screen to determine a convex lens's focal length.
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