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A well-developed lesson on Convex and Concave lenses

Subject:

Science  

Grades:

6, 7, 8, 9  

Title – Convex/Concave Lenses
By – Rick Mazey
Subject – Science
Grade Level – 6-9

I. Goals: The students will see how both convex and concave lenses work, some of the different uses of both types of lenses and do an experiment showing how these lenses work. Also, they will be able to describe terms related to lenses.

II. Objectives:
1) TSSBAT visually recognize the difference between concave and convex lenses.
2) TSSBAT explain that convex lenses cause rays of light to converge (come together) at a single point, and that concave lenses cause rays of light to diverge (spread apart).
3) TSSBAT explain how a lens is used to produce an image.

III. Materials:
For set- 10 plastic cups- 10 pennies or spoons
For coop. learning- pieces of window, glass, small magnifying lens, white cardboard, paper, ruler.

IV. Motivational Activity:
1) Explain how the last few days we have been talking about light rays.
2) A simple experiment that I will have them each do. Have a small empty plastic cup with a penny placed in the bottom of it. Have the students back up until they cannot see the penny. Then go around and add water to the cups until they are able to see the penny in the bottom. Or if more plausible put a spoon in a cup of water.
V. Sequence of Procedures:
After doing this experiment with the students I will ask them if they know why they were able to see the penny with the water in the cup? Or, why the spoon appeared to be “broken?” After they answer, or after a few tries explain why. It is because of refraction of the light.

Discussion:
1) Refraction- The bending of light as it moves from one substance to another.
Ask students for any more examples they can think of where refraction takes place? After students start into different kinds of lenses start with this… Then, start on how lenses, a piece of material which light is able to pass through, are used to refract light. Also, explain that there are different types of lenses.
1) Convex
2) Concave
Ask the students if they know the difference between a convex and a concave lens?
1) Convex- A lens which is thicker in the middle than on the ends.
2) Concave- A lens which is thinner in the middle than on the ends.
Way to remember the difference: A concave lens looks like the opening to a cave, therefore you can remember that it curves inward.

Convex Lenses
1) Refracts parallel light rays so they come together at a single point. Does anyone know what it is called when the light rays are made to come together?
-Known as convergence.
2) Point is know as focal point.
3) Distance from center of lens to focal point = focal length.
4) The thicker the middle of the lens the shorter the focal length.
5) Example of a convex lens is a magnifier.
6) Show overhead showing parts of convex lens explained above.
7) Image can be seen without actually looking through the lens. This type of image is known as a real image.
8) When held close to objects, inside of its focal length, it produces an image known as a virtual image. To see a virtual image you must look through the lens.

Concave Lenses
1) Refracts the rays so they come apart. . Does anyone know what it is called when the light rays are made to come apart from each other?
-Known as divergence.
2) Always forms a virtual image.
3) Show overhead of concave lens.

Practical Uses Of Lenses
1) Who knows of some uses of lenses?
2) Cameras (normally convex), glasses (near= concave; far= convex); telescopes (at least 2 convex), microscopes (at least 2 convex).
3) Ask for questions on material covered.

Closure:
Earlier in the chapter we learned about how mirrors are used to reflect light rays, today we learned how light can also be refracted across an object. Think about how these two phenomenon are different and also how they are related to each other.
1) What does a lens do to light form an image?
2) What are the difference between a convex and a concave lens?
3) What are some of the uses of lenses in everyday life?
Tomorrow we will learn how these three things (light, refraction, and reflection) work to give us the colors that we see.

Out-of-class Activity:
Have students read next section on color, and have them list the colors of the spectrum in decreasing order of frequency.
Tomorrow we will learn how these three things (light, refraction, and reflection) work to give us the colors that we see.

VI. Evaluation/Problems:
Problems with students understanding material (Make sure to ask plenty of questions).

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