There are two basic kinds of lenses: convex and concave. Looking through both types of lenses reveals some of their primary characteristics. For instance, convex lenses cause light rays to converge or come together because they are thicker in the middle than on the edges. Their design enables convex lenses to focus light and project an image. Also, things examined through a convex lens look bigger and brighter because the lens is capable of concentrating a beam of light on a surface. Concave lenses, on the other hand, are thicker on the edges than in the middle. Thus, they seem to cave in toward the center and things look smaller through them. Instead of concentrating light, concave lenses make light rays bend outward or diverge, spreading them across a surface.

Required Materials

• Science notebooks
• Plano-convex lenses
• Plano-concave lenses
• Flashlights
• White paper

What will the students do?

Students will explore and discover the properties of the two basic types of lenses. Students can work individually, in pairs, or in small groups. The activity can be guided by the teacher or students can work independently using the student page as a guide. After distributing the materials, allow students ample time to explore the materials freely.

Ask students to look closely at the lenses and describe how the lenses are shaped. Introduce the terms convex and concave for the two different types of lenses. As students discover new things about how we look at objects, have them write their discoveries in their science notebooks. Encourage students to look through the lenses at the pages of a book, a picture, their hands, and other objects. Then, they should shine a flashlight through the lenses on to a white piece of paper. Have them look through and shine the flashlight through different combinations of lenses and then explain the differences in what they see. Afterwards, introduce the terms focus, converge, and diverge for describing the properties of the lenses.

	Interactive Java Tutorial
Simple Magnification Explore how a magnifying glass works to project an enlarged image on the retina of the eye.

Activity Extensions

Research - Using a variety of print and electronic media students can research and write about the different types of lenses and why they have different effects on images that are seen through them. This could include answering the following questions: Why do some people need to wear glasses? What is the difference between being nearsighted and farsighted?

Designing Instruments - After researching how various optical devices work, students can make simple telescopes and microscopes using the lenses. Telescopes and microscopes can be made by using the convex and concave lenses, and cardboard tubes (an empty paper towel tube and a plastic wrap tube work well since one will fit inside the other). Rather than supplying step-by-step instructions, after students have completed this activity see if they can design their own instruments. Many texts, such as The Optics Book (ISBN 0806999470), contain science activities that can be completed using everyday items.

Guest Speaker - Have students study how the lenses of our eyes enable us to see. Challenge students to determine what causes nearsightedness and farsightedness. Then, invite an optician to be guest speaker so students can learn how the lenses in eyeglasses are used to correct vision. Making a model of the eye and how it works would also be an appropriate activity.

Expository Writing - Use the following as an expository writing prompt: Movie and slide projectors use lenses to project images on a screen. Think about your experiences with the lenses and how the shapes of lenses bend light in different ways. Now write a description of how you think lenses are used to project images on a screen.

Narrative Writing - Use the following as a narrative writing prompt: Magnifying glasses and microscopes are sometimes used to investigate crime scenes. Pretend that an unknown thief has burglarized your classroom. Think of an unusual piece of evidence that could be discovered using a magnifying glass that would identify the thief. Now write a short mystery story about how the evidence helped solve the case.

Historical Vignette: Augustin Fresnel

In 1822 a French scientist named Augustin Fresnel invented a lens that is now used in lighthouses around the world. The lens looks like a giant glass beehive with a lamp in the center. The lens is made out of rings of glass prisms above and below the lamp that bend and concentrate the light into a bright beam. The Fresnel lighthouse lens work so well that the light can be seen from a distance of 20 or more miles. Before Fresnel's invention, lighthouses used mirrors to reflect the light. These lights could be seen at only short distances and hardly at all during foggy or stormy days. Lighthouses equipped with Fresnel's lenses have saved many ships from going aground or crashing into rocky coasts.

Activity - Lighthouses have always fascinated people. There are a number of books depicting the colorful history of lighthouses. Students can research the history of nearby lighthouses or historic lighthouses. They could also research other uses of Fresnel lenses.

Reading - Students can read Women of the Lights by Candace Fleming (ISBN 0807591653). The book is a compilation of the adventures of some of the 250 women who have served as lighthouse keepers in America.

Reading - Students will also enjoy The Wreckers by Iain Lawrence (ISBN 0440415454), which is a fast-paced adventure involving shipwrecks, pirates, kidnapping, mystery, ghosts, and courage. The story is historically based upon a community on the coast of Cornwall, England during the early nineteenth century. Wreckers used lights to lure ships to crash on their rocky shores and lived on the loot salvaged from the wreckage.

Florida History - Key West, Florida, was settled originally by wreckers. A wreckers' museum exists there and there are many documented stories about how the area was settled. Thus, here is an opportunity to work Florida history and culture into your science curriculum.

Artist Vignette: Michael Davidson

Michael Davidson is a scientist at the National High Magnetic Field Laboratory in Tallahassee, Florida, who has developed techniques for creating artistic photographs by taking pictures through a microscope. This is called photomicrography. Using polarizers, multiple exposures, and other techniques, Davidson has created unusual and colorful pictures that look like dream landscapes or alien worlds. He calls his creations microscapes.

Davidson uses microscopic images of real moon rock slices, recrystallized vitamin C, proteins, DNA, and other substances to create his microscapes. He combines these images into artistic creations with titles such as Black Hole in Space, Crystal Forest, Macdonald's on Mars, Mount Meatloaf, and Star Wars. You can see Davidson's creations at The Microscapes Collection website.

Activity - After viewing Davidson's microscapes on the Internet, students can create their own microscapes using school microscopes or convex magnifying lenses. Have students examine everyday items such as fingerprints, leaves, a basketball, bark, or food items through the lens. If available, students can view the items through polarizers or a diffraction grating. They can then draw and color various patterns and textures that they see using crayons, markers or paints. Encourage students to be creative in the use of color and names for their microscapes.