The Anatomy of a Lens
The Anatomy of a Lens
Refraction and the Ray Model of Light - Lesson 5 - Image Formation by Lenses
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If a piece of glass or another transparent material is shaped correctly, it can cause parallel incoming rays to either converge at a point or appear to diverge from a point. A piece of glass with this shape is known as a lens.
A lens is a carefully shaped or molded piece of transparent material that refracts light rays to form an image. Lenses can be considered as a collection of tiny refracting prisms, each refracting light to create its own image. When these prisms work together, they produce a bright image focused at a single point.
Types of Lenses
Various types of lenses exist, differing in shape and the materials used in their construction. We will focus on lenses that are symmetrical across their horizontal axis—referred to as the principal axis. In this unit, lenses can be categorized as converging or diverging.
A converging lens gathers rays of light traveling parallel to its principal axis. The shape of such lenses is identifiable; they are thicker in the center and thinner at the edges. Conversely, a diverging lens spreads rays of light traveling parallel to its principal axis. Diverging lenses have a thinner middle and thicker edges.
A double convex lens is symmetrical across both its horizontal and vertical axes. Each face can be conceptualized as part of a sphere, and its thickness in the center indicates that it converges light rays parallel to its principal axis. A double convex lens is thus a converging lens. In contrast, a double concave lens is also symmetrical across both axes. Its thinner center suggests that it diverges rays of light traveling parallel to its principal axis. A double concave lens serves as a diverging lens. These two lens types will be the focus of this unit in The Physics Classroom Tutorial.
The Language of Lenses
As we delve into the refraction of light rays and image formation with these lens types, we must familiarize ourselves with specific terms. Many of these should already be known to you from Unit 13. If any terms are unclear, take the time to review them thoroughly. Understanding this vocabulary is crucial as we progress through Lesson 5. Important terms include:
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- Principal axis
- Vertical Plane
- Focal Point
- Focal Length
Imagining a symmetrical lens as a slice of a sphere allows us to visualize a line passing through the sphere's center and connecting to the lens' midpoint. This imaginary line is called the principal axis. There is also an imaginary vertical axis that bisects the lens into two halves. Rays of light incident on either lens face and traveling parallel to the principal axis will either converge or diverge. If the rays converge (as in a converging lens), they converge at a specific point known as the focal point. If the rays diverge (as in a diverging lens), they can be traced backward to intersect at a point called the focal point of the diverging lens. The letter F denotes the focal point in the diagrams below. Each lens possesses two focal points—one on either side. Unlike mirrors, lenses allow light to pass through either face, depending on the direction of incoming rays. Therefore, every lens has two potential focal points. The distance from the lens to the focal point is called the focal length (abbreviated to f). A lens does not have a significant center of curvature related to this discussion, but it does have an imaginary point referred to as the 2F point, which lies on the principal axis and is twice the distance from the vertical axis to the focal point.
As we examine the image characteristics produced by converging and diverging lenses, these terms will become increasingly relevant. Remember to refer back to this page whenever necessary.
We Would Like to Suggest ...
Why simply read about it when you could interact with it? That's precisely what you do when using one of The Physics Classroom's Interactives. We recommend combining the reading of this page with our Optics Bench Interactive. You can find this in the Physics Interactives section of our website. The Optics Bench Interactive offers an engaging environment for exploring image formation through lenses and mirrors, akin to having a complete optics toolkit on your screen.