Week 16 - Light Behaves in Predictable Ways

Lesson 6

Lesson 6: Lenses
Science in Action 8
Pages 204-208

Background Information
In previous lessons, you learned that a light wave traveling from one medium (air) to another medium (water) bends. The bending of a light wave due to change in speed when the wave moves from one medium to another is called refraction. The larger the change in speed, the more the light wave is refracted.

Do you like taking photographs of your friends and family? Have you ever watched a bird through binoculars or viewed microscopic plants and animals through a microscope? All of these involve the uses of lenses. A lens is a transparent object with at least one curved side that causes light to bend. The amount of bending can be controlled by making the sides of the lens more or less curved. The more curved the sides, the more a ray of light entering the lens is bent.

Convex Lenses

A lens that is thicker in the center than at the edges is called a convex lens. Convex lenses are also called converging lenses. Light rays traveling parallel to the optical axis are bent so that they meet at the focal point. A convex lens forms images on the side of the lens opposite the source. Since we can form an image on a piece of paper, we call this kind of image a real image. You can also see real images.

If the surface is highly curved, the focal point is close to the lens and the focal point is short. If the surface is not highly curved, the focal point is further away from the lens and the focal point is longer.

A double convex lens is symmetrical across both its horizontal and vertical axis. The fact that a double convex lens is thicker across its middle is an indicator that it will converge rays of light that travel parallel to its optical axis.

Watch this animation to see a double convex lens in action. You can move the yellow candle forward and backward to see its image projected onto the wall.


Convex Lenses and Images

The image formed by a convex lens depends on how far the object is from the focal point. If the object is farther than two focal lengths from the lens, the image is inverted and smaller than the object.

Watch this animation to see a convex lens and a concave lens being compared. One is a converging lens and one is a diverging lens.


If the object is closer to the lens than one focal length, then the rays coming form points on the object diverge after passing through the lens. The image formed is right-side up and larger than the object and is called a virtual image. This is what occurs when you use a magnifying glass. The image formed is right-side up and larger than the object.

This animation shows a convex lens and the image of a boy. The line is marked with points so you can see how many focal lengths it takes for the image to change.


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