Lenses Refract Light

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Lenses Refract Light

This lesson aligns with NGSS PS4.B

Introduction

Have you ever wondered how your eyeglasses help you see more clearly or how a magnifying glass can make tiny text appear larger? The answer lies in the remarkable property of lenses to refract light. Light, often described as electromagnetic waves, travels in straight lines until it interacts with an object. When it encounters an obstacle or a different medium, such as air, glass, or water, it can change direction through a process called refraction. In this educational article, we will delve into the science behind lenses and discover how they work to bend and focus light, providing us with a clearer view of the world around us.

What Are Lenses?

Lenses are transparent objects, typically made of glass and plastic, with curved surfaces. These curved surfaces are what make lenses unique in their ability to manipulate light. There are two primary types of lenses: convex and concave.

Convex Lenses

Convex lenses are thicker at the center than at the edges, giving them a curved-outward shape. This type of lens is commonly known as a converging lens because it converges or focuses light rays that pass through it. When parallel light rays (rays traveling in the same direction) enter a convex lens, they bend toward each other and converge at a single point called the focal point. This is where the magic of magnification happens.

The distance from the center of the lens to the focal point is known as the focal length.

Refraction at a Convex Lens

When parallel rays of light pass through a convex lens, they are refracted (bent) toward the center. The amount of bending depends on the shape of the lens and the angle at which the light enters. The rays of light that pass through the center of a lens are not refracted.

As the rays converge at the focal point on the opposite side of the lens, they cross each other and form a real image. The image produced can be larger or smaller than the object, depending on the focal length of the lens and the position of the object. Movie projectors use convex lenses to concentrate real images on a screen. Cameras use convex lenses to concentrate real images on a piece of film.

If the object is placed between one and two focal lengths away from the lens, a real, enlarged image is produced far away from the lens. This is how movie projectors form images on large screens.
If the object is placed more than two focal lengths away from the lens, a real, reduced image is formed close to the lens. This is how the lens of a camera produces images on the film.

Convex lenses can also form virtual images. For instance, whenever you use a magnifying glass, you are using a convex lens to form an enlarged, virtual image.

If an object is less than one focal length away from a convex lens, a virtual image is produced. The image is larger than the object and can be seen only by looking into the lens.

Concave Lenses

Concave lenses, on the other hand, are thinner at the center and thicker at the edges, giving them a curved-inward shape. These lenses are known as diverging lenses because they cause parallel light rays to spread apart after passing through them. Unlike convex lenses, concave lenses have a focal point on the same side as the incoming light rays, but they do not converge; instead, they appear to diverge from a virtual focal point.

Refraction at a Concave Lens

When light rays enter a concave lens parallel to the optical axis, they consistently diverge away from each other toward the edges of the lens. As a result, these rays do not converge to a focal point. Therefore, concave lenses can never form real images. Instead, they give rise to virtual images.

Concave lenses produce reduced virtual images. If you extend the refracted rays in a straight line behind a concave lens, you can determine where the virtual image is formed.

Summary

Lenses are transparent objects, typically made of glass or plastic, with curved surfaces.
A convex lens is thicker in the center than at the edges. When light rays enter a convex lens, they bend toward the center.
Convex lenses form a virtual image as well as the real image.
The concave lens is thinner in the center than at the edges. Light rays enter a concave lens parallel to the optical axis, always bend away from each other toward the edges of the lens, these rays never meet.
Consequently, concave lenses can never form a real image. Instead, they form virtual images.

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