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Thin Lenses: Types & Measurement

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  • 0:05 Thin Lenses
  • 1:17 Lens Measurements
  • 2:21 Converging Lenses
  • 3:00 Diverging Lenses
  • 4:03 Other Lenses
  • 5:06 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

We are around many thin lenses throughout our daily lives, but most of us don't really know what this means. In this lesson, we'll explore the measurements of lenses, the common types of thin lenses, and the different ways they affect light.

Thin Lenses

In 1871, Lewis Carroll told the world a story of a girl named Alice who looked through a looking glass and found another world on the other side. When I look at a mirror, I just see myself. Not nearly as exciting. But throughout our lives, we may find ourselves looking through all kinds of surfaces, and how those surfaces are formed can actually have a great impact on what we see on the other side. Don't believe me? Well, then let's take a look at a lens, a piece of transparent material with two curved surfaces shaped to alter the direction of light.

This is a lens. Like most lenses, it's circular in shape, and if we look at how light passes through it, we can see that the light has changed direction, something light doesn't generally do on its own. Most of the lenses we deal with on a daily basis are thin lenses, defined as having a width that is minimal in relation to the circumference of the lens. Want to get a closer look? Well, then I think you know what you have to do.

Lens Measurements

Alright, now that we're on the other side of the looking glass, we can take a closer look at these thin lenses. I mentioned that a thin lens is one with minimal width compared to the height and circumference of the lens, but how exactly do we measure this? Lenses are measured in diopters, which is a unit of measurement equal to the reciprocal of the focal length in meters. Okay, so what does this mean? The focal length of a lens indicates how strongly it alters the direction of light.

Look at this lens again. See how the two beams of light end up crossing? That spot where they meet is the focal point. The distance between the center of lens and the focal point is the focal length. The diopter, the unit we use to measure a lens, is a reciprocal of that focal length. So, if a lens focuses light 1/2 meters from the center of the lens, the focal length is 1/2 meters, and the diopter is 2/1. It is a 2-diopter lens.

Converging Lenses

So, that's how lenses are measured. Now, how do they actually work? Well, for that we're going to have to look at the kinds of lenses. There are two main kinds, the first being convex, or converging. A converging lens has two surfaces that both curve outwards that bend rays of light to converge at a single point. See how the light converges in this converging lens? That's what we've been looking at so far. In the real world, magnifying glasses, microscopes, and many cameras use converging lenses to focus light onto a single point, bringing it into sharper focus.

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