Refraction and Lenses

Learning Objectives

Define refraction as the bending of light.

Explain that refraction is caused by a change in the speed of light as it enters a new medium.

Distinguish between a concave lens and a convex lens.

Describe how a convex lens can be used as a magnifier.

Bending Light

Have you ever noticed that a straw in a glass of water looks like it's bent at the water's surface? The straw isn't actually bent. What you are seeing is an effect called refraction.

Refraction is the bending of a wave as it passes from one medium (substance) to another. Refraction of light happens because light travels at different speeds in different media.

Light travels fastest in a vacuum.

It travels slightly slower in air.

It travels even slower in water.

It travels slower still in glass or a diamond.

When a light ray enters a new medium at an angle, the change in speed causes it to bend or change direction. The amount it bends depends on the index of refraction of the two media.

Lenses

A lens is a curved piece of glass or other transparent material that is used to refract light. A lens forms an image by bending light rays so that they either meet at a point or appear to come from a point. There are two main types of lenses.

1. Convex Lens

A convex lens is a lens that is thicker in the center than at the edges. It's a converging lens.

How it works: A convex lens bends parallel light rays so that they converge, or meet, at a single point called the focal point.

Images: A convex lens can form two types of images:

If an object is placed far from the lens, the lens forms a real image. A real image is one where the light rays actually meet. It can be projected onto a screen (like the image from a movie projector).

If an object is placed very close to the lens (closer than the focal point), it acts as a magnifying glass, forming a virtual image that is larger and upright.

Applications: Magnifying glasses, cameras, eyeglasses for farsightedness, telescopes, and microscopes.

2. Concave Lens

A concave lens is a lens that is thinner in the center than at the edges. It's a diverging lens.

How it works: A concave lens bends parallel light rays so that they diverge, or spread out, as if they were coming from a virtual focal point behind the lens.

Images: A concave lens always produces a virtual, upright, and smaller image.

Applications: Eyeglasses for nearsightedness, some types of telescopes and binoculars to spread out light.

Key Terms

**Refraction: The bending of a wave as it enters a new medium at an angle, caused by a change in its speed.
**Medium: The substance through which a wave can travel.
**Lens: A curved piece of transparent material, such as glass or plastic, that refracts light.
**Convex Lens: A lens that is thicker in the center than at the edges and converges light rays.
**Concave Lens: A lens that is thinner in the center than at the edges and diverges light rays.
**Real Image: An image formed where light rays actually converge. It can be projected onto a screen.
**Virtual Image: An image formed where light rays only appear to diverge from. It cannot be projected onto a screen.