ASTRONOMY LESSON PLAN

Name: Laura Galaviz

Title of lesson: Building Telescopes: Finding Focal Length

Date of lesson: March 29 and April 5

Length of lesson: 1 hr 30 min.

Description of the class:

Name of course: Algebra II

Grade level: High School

Honors or regular: Both

Source of the lesson:

“Building Your Own Telescope Made Easy” by Claire Hodgin and Betty

Stennett

TEKS addressed:

Algebra II:

d. 3 (A): The student analyzes situations involving quadratic functions and formulates quadratic equations or inequalities to solve problems.

Pre-Calculus

c. 5 (B) The student use properties of conic sections to describe physical phenomena such as the reflective properties of light and sound.

I. Overview

Students will learn about the focal point of a concave mirror using a flashlight beam reflected from a primary mirror and also a small mirror. Students will investigate how to find the focal length of a light ray to a concave mirror.

II. Performance or learner outcomes

Students will be able to:

1) Describe how to find the focal length of a concave mirror.

2) Report the focal length of the primary mirror and also smaller concave mirrors.

3) Apply their knowledge of focal length to the telescope.

III. Resources, materials and supplies needed

· 2 primary mirrors (Telescopes)

· 3-4 small concave mirrors

· 6 Flashlights (extra batteries)

· 5-6 sheets of white paper for reflection of flashlight beam

· 25 Data Sheets

IV. Supplementary materials, handouts.

· Attached

Five-E Organization

Teacher Does: Student Does:

Pre-Engage: Teacher will divide students up into groups by giving each of them a card. They should split up into a group depending on the number that they get. Engage: Teacher will review some of the key concepts that the students learned in their previous lessons. Teacher will draw on the board a convex and concave image and ask students to identify each one. Teacher will ask students to discuss focal points and focal lengths. Hold up a primary (concave) mirror and a small concave mirror. The teacher will ask the students if the focal point will be closer or farther away on the primary mirror than the focal point of the small concave mirror. Students should write down what they think. Good activity and an interesting question.

Students will move to their designated groups. Students will distinguish between concave and convex. Student will give a brief discussion about focal points and focal lengths. Students will write down if they think the focal point will be closer or farther away on the primary mirror.

Evaluate: Teacher will ask students to explain any answer they may give; i.e. what makes a concave mirror concave? Convex? What is a focal point?

Teacher Does: Student Does:

Explore: The teacher will explain to students that they will be working with the primary and small mirrors to find focal lengths. The teacher will tell the students that contrary to before, chalk dust cannot be used around the large mirrors because they are too expensive, so in order to find the focal length the students will use a flashlight. Since there are only 2 primary mirrors, we will have to share them. Two of the groups will use the primary mirrors while the other groups use the smaller mirrors. Since there are four students per group, one will hold a flashlight, one will hold the large mirror, one will hold a board directly next to the flashlight, and the other will measure the distance from the board to the primary mirror. The student holding the flashlight will point it directly at the mirror and the reflection of the light will shine onto the board. The teacher will ask the students to observe the reflection as the student moves the flashlight toward and from the mirror. The reflection of the flashlight should be as clear as possible. The students should record the distance between the reflection and mirror when it is most clear. After the two groups that are using the primary mirrors are done (about 10-15 min), mirrors should be rotated so that each of the groups uses the primary and small mirrors. This is an excellent idea. That way each group gets experience measuring and everyone is active. You also might compare the measurements of the different mirrors as a group. Why are there differences? You could lead a brief discussion on variability and error when measuring.

Students will listen attentively. Students will work cooperatively in their groups to find the focal lengths. Students will record their measurements. Students will share the mirrors and investigate the focal lengths of the different mirrors and compare them.

Evaluate: Teacher can ask students what each of them are supposed to be doing. Teacher will walk around to help the groups and also to make sure each student is on task. Teacher will make sure each group gets to work with the primary mirror.

Teacher Does: Student Does:

Explain: Teacher will ask students what happened with the reflection as you moved close to and away from the mirror. A student from each group will be chosen to discuss the measurements of the primary and small mirrors. All of the measurements will be written on the board or overhead in a table format. If there are any discrepancies between the measurements, students should discuss why that is and give justification for their measurement. Ok, so you’ll be doing the group discussion of measurements. You might actually have them average measurements if you need to take up a little more time. The teacher will explain that the distance from the mirror to the point where the reflection is most clear is the radius of the circle created by the mirror. The focal length is half of the radius. Why is this? Ok, but really why do you really need to divide by two. Sure they can know the fact from the previous lesson, but do they understand? Students should then make sure that they have the correct focal lengths.

Students will share what happens when you move close to and away from the mirror. Students will share their measurements to the rest of the class. Students should be able to recall from the previous lesson that PC=2xPF where PC is the radius and PF is the focal length, so then the actual focal length is one half the measured length.

Evaluate: Teacher will ask students to explain what happens when you move closer to or away from the mirror. Teacher will call on a random student from each of the groups to make sure they each have the measurements.

Teacher Does: Student Does:

Extend/ Elaborate: Students should write down if their original predictions of the focal points for the primary and small mirrors were correct. Nice tie-in to the earlier activity. Students will be asked to think about how this information relates to a telescope. Students should think about all the parts of a telescope and how it works.

Students will write this down. Students think about how this information relates to a telescope; they should write this down and also share with the class. Students should talk about the different parts of telescopes: primary mirror, tube, another mirror, lens, eyepiece, etc…

Evaluate: Teacher will collect their writings on primary and small mirrors. Teacher will ask students about the different parts of telescope and write them on the board.

Name:

Date:

Mini-Journal Entries

1. Is the focal point of the primary mirror closer to or farther away than the focal point of the smaller (concave) mirror?

2. Was your prediction correct? Explain. If not, modify your prediction and make it into a valid conclusion.

Name:

Date:

Data Sheet

Write down your measurements for the focal lengths of each of the mirrors.

Type of Mirror	Measurement from mirror to reflection	Actual Focal Length
Primary Mirror
Small Concave Mirror (1)
Small Concave Mirror (2)