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by Natasha Cooke-Nieves
In this lesson, students will be able to explain how convex lenses cause rays of light to converge (come together) at a single point and how concave lenses cause rays of light to diverge (spread apart). Students will also be able to explain how a lens is used to produce an image. At the end, students will be able to explain the following terms: refraction, lens, convex (convergence), and concave (divergence)
__Grade Level__: four
__Duration__: 45 minutes
__Lesson Description:__ Students learn about the differences between concave and convex lenses, along with practical uses for lenses.
__Goals:__ Students will be able to identify differences between concave and convex lenses.
**Problem:** How is light reflected?
**I** **Refraction**- Pencil through water vs. oil?
- Differences in how light is reflects at an angle through liquids with different viscosities.
**II Concave vs. Convex lenses**
**Materials:**
- clear plastic wrap
- newspaper
- water
- cups for the water
- eyedroppers
- lenses (convex and concave)
- paper/pencils
**Vocabulary:**
__convex__ – A lens which is thicker in the middle than on the ends which causes light rays to focus (converge).
__concave__ – A lens which is thinner in the middle than on the ends which causes light rays to spread apart (divergence).
__refraction__ – The bending of light
**Procedure:**
*Lesson Introduction:*
Give students the following directions: "Each of you will be receiving a piece of newspaper, a piece of clear plastic wrap, an eyedropper, and a cup of water. You will lay the piece of newspaper on your desktop. Then you will lay a piece of clear plastic wrap on top of the newspaper. Using an eyedropper, you will place a drop of water on the plastic wrap. Before we begin, take out a piece of paper and write a prediction of what you think you are going to observe."
Pass out the materials needed for the activity. While the activity is being conducted, monitor the class to make sure everyone is on task. Ask the students what they are observing. What does the newspaper print look like? Encourage students to try different techniques such as adding more drops of water or folding the plastic wrap in half. Ask students to share what they think is occurring with the “water lens.”
*Lesson Focus:*
Discuss the term, *refraction* (the bending of light as it moves from one substance to another). Explain how a lens is used to refract light (a lens is a piece of curved material which light is able to pass through, such as clear plastic or glass). Explain that there are two different types of lenses, convex and concave. Ask, "Does anyone know the difference between a convex and a concave lens?"
Describe **convex** lenses -- A convex lens is thicker in the middle than on the ends, which causes light rays to focus (converge). Show a demonstration with a convex lens. Explain how the lens refracts parallel light so that the rays come together at a single point. Ask students, "When light rays come together at a point, what is that called?" (Answer: convergence) Explain that the point that light focuses on is known as the focal point, and the distance from the center of the lens to the focal point is called the focal length. Explain that if the lens is thicker in the middle, the focal length will be shorter.
Describe **concave **lenses -- A concave lens is thinner in the middle than on the ends which causes light rays to spread apart (divergence). Explain how the lens refracts the rays so that they spread apart. Demonstrate with a concave lens. Share an easy way to remember what a concave lens looks like. Focus on the word *cave* ; the opening of a cave looks similar to a concave lens. Ask students, "When light rays spread apart, what is that called?" (Answer: divergence)
Ask students if they can name any practical uses of lenses. Examples might include the following: cameras (convex); magnifying glass (convex); glasses (near sighted - concave, far sighted - convex); telescopes (at least 2 convex lenses); and microscopes (at least 2 convex lenses).
*Closure:*
Remind students of the “water lens” that they made at the beginning of class and ask them, "Was that lens convex or concave? Why or why not?" (Answer: convex) Review the terms learned today: refraction, lens, convex, and concave. Give each student time to reflect in their Science Notebook or Journal what they learned today by answering the original problem of the day.
Questions or comments? E-mail me. |