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Building a Telescope

Determining the Focal Length and Magnification of Lenses	Building a Telescope with a Paper Towel Roll

Table of Contents:

1. Overview
2. Introduction
3. Activity 1: Determining the Focal Length and Magnification of Lenses
   • Materials
   • Step by Step Instructions
     • Section 1: Determining the Focal Length of a Convex Lens
     • Section 2: Magnification of Two Lenses
   • Worksheet Questions
4. Activity 2: Building a Telescope
   • PCV Pipe Telescope
   • Paper Towel Roll Telescope
5. Assessment
6. Suggestions for expanding this activity
7. Illinois State and National Learning Standards
   • Early Elementary
   • Late Elementary
   • National Standards

Overview

During these hands-on, minds-on activities, students will be asked to explore the properties of lenses. Students will determine the focal lengths of two different lenses, and the magnification that the two lenses produce when used together. With the help of this information, the students will build their own telescopes.

The activities are simple and inexpensive, yet they provide endless possiblities for expanding lesson plans and meeting learning standards. The minimum amount of time needed for this lesson is 90 minutes.

Introduction

The introduction to this lesson could include a discussion about optics, including:

• What is a telescope?
• What is a telescope used for?
• What is a lens?
• Name a way that people use lenses.
• A magnifying glass is a lens. What is a magnifying glass used for?
• Why would people want to magnify objects?

Introductory Lens Exercise

The following exercise is an optional introduction to convex lenses. If students are unfamiliar with the properties of lenses, it is advised that this exercise is completed before beginning the activities that follow.

• Look at the two lenses.
  • How are they similar?
  • How are they different?
• Using one of the lenses, look at letters on a piece of paper.
  • What happened to the letters?
• Now look at the same paper with the other lens.
  • What happened to the letters this time?
  • How does this differ from using the other lens?
• Look through one of the lenses at an object across the room. Move the lens closer and farther from your eye until you can see clearly.
  • Does the object look bigger or smaller through the lens?
  • Is the object upside-down or right-side up?

Activity 1: Determining the Focal Length and Magnification of Lenses

To complete this activity you will need the following materials:

• 38mm thick double convex lens (Nasco catalog #P10208)
• 38mm thin double convex lens (Nasco catalog #P10204)
• ruler (with centimeter scale)
• flashlight or florescent ceiling light
• sheet of white paper with text

The lenses that we used were purchased from the Nasco Science Catalog. Each lens costs $1.50. For more information about Nasco:
URL: www.nascofa.com
E-mail: info@nascofa.com
U.S. Mail: Nasco, 901 Janesville Rd. Fort Atkinson, WI 53538-0901

Step by Step Instructions

Section 1: Determining the Focal Length of a Convex Lens

The spot where all the light rays come together after going through the lens is called the focus. The focal length is the distance between this spot of light and the lens. There are two methods used to determine the focal length of a convex lens. Both methods are accurate. It is suggested that students work in pairs during this activity.

Method 1: Using a flashlight

1. Have one student hold the flashlight about three feet above a blank sheet of white paper.
2. Turn on the flashlight and point the light towards the piece of paper.
   
3. The other student should then bring the lens between the paper and the flashlight. A bright dot of light should appear on the paper.
4. With the other hand, place a ruler on the sheet of paper, with the 0 cm side down.
5. Move the lens up and down until the bright dot of light is the smallest and brightest it can be. The bright dot is the focus. The distance from the lens to the paper is the focal point of the lens.
6. Repeat for the other lens.

Method 2: Using Flourescent Ceiling Lights

1. Place a piece of blank paper underneath a flourescent ceiling light.
2. Position the lens above the piece of paper. A bright spot of light should appear on the paper.
3. Place on the sheet of paper, 0 cm side down.
4. Move the lens up and down until you see the clearest picture of the light fixture you can. The picture of the light fixture is the focus. The distance from the lens to the paper is the focal point of the lens.
5. Repeat for the other lens.
   Note: The focal point of the thick lens (Nasco catalog# P10204) should be 5cm, and the focal point of the thin lens (Nasco catalog# P10208) should be 30cm.

Section 2: Magnification of Two Lenses

In this section, students will determine the distance bewteen the two lenses that result in the clearest magnification. (Note: This distance should equal the sum of the two focal points. For the above example, the distance would be 35cm.) Students will also learn how to calculate the magnification the two lenses produce.

Finding the Clearest Magnification

Hold the thick lens about 3 inches (7 cm) from your eye.

With your other hand, slowly move the thin lens away from the thick lens until you can clearly see an object from across the room.

The other person should hold up a meter stick and note the distance between the lenses.

Calculating the Magnification

The magnification produced when two lenses are used together can be calculated by the following equation:

magnification = focal length of thin lens / focal length of thick lens

For the lenses used above, the magnification would be:

Focal length of thin lens = 30 cm
Focal length of thick lens = 5 cm
magnification= 6 times (30 cm / 5 cm)

Using the two lenses above magnifies objects by 6 times!

Worksheet Questions

Focal Length

Answer the following questions concerning the Focal Length of each lens.

Using the Flashlight

1. Predict what the distance between each lens and the paper will be when the brightest dot of light will be visible.
   • Thin Lens: Predicted distance _______________cm or _______________inches
   • Thick Lens: Predicted distance _______________cm or _______________inches
2. Measure the distance from the lens to the paper when the brightest dot of light is visible on the paper.
   • Thin Lens: Actual distance _______________cm or _______________inches
   • Thick Lens: Actual distance _______________cm or _______________inches
3. The distance from the paper to the lens is called the Focal Length. Why do you think each lens has a different focal length?

Using the Florescent Ceiling Lights

1. Predict the distance between each lens and the paper when a clear picture of the light fixture will be visible.
   • Thin Lens: Predicted distance_______________ (cm or inches)
   • Thick Lens: Predicted distance_______________ (cm or inches)
2. Measure the distance from the lens to the paper when the brightest dot of light was visible on the paper.
   • Thin Lens: Actual distance_______________ (cm or inches)
   • Thick Lens: Actual distance_______________ (cm or inches)
3. The distance from the paper to the lens is called the Focal Length. Why do you think each lens has a different focal length?

Extra: Using Both Methods

• Did you get the same or different measurements using the flashlight and using the florescent ceiling light?
• Does this surprise you? Why or why not?

Magnification

Lens Observations

1. Look through the thin lens at a piece of paper with words on it. Draw and describe what the letters look like.
2. Look through the thick lens at a piece of paper with words on it. Draw and describe what the letters look like.
3. Hold the thin lens in front of your eye. Look at an object across the room. Describe what you see.
   • Is the object bigger or smaller?
   • Up-side down or right-side up?
4. Hold the thick lens in front of your eye. Look at an object across the room. Describe what you see.
   • Is the object bigger or smaller?
   • Up-side down or right-side up?

Finding the Clearest Magnification

In this section, you held two lenses in front of your eye to see an object. How far apart were the lenses when you could see the object clearly?

Calculating Magnification

What is the magnification that these two lenses produce? Note: Magnification = focal length of thin lens / focal length of thick lense

Activity 2: Building a Telescope

All telescopes have two primary lenses. One lens is the objective, which collects light from the area you want to magnify. In this activity, the thin lens is the objective. The second lens is the eyepiece, which magnifies the light collected by the objective. This makes the image appear bigger to your eye!

PVC Pipe Telescope

For each telescope you will need the following materials:

• one 38mm thick double convex lens (Nasco catalog# P10208)
• one 38mm thin double convex lens (Nasco catalog# P10204)
• 2 pipe adapters (1" to 1 1/4")
• 2 pipe couples (1 1/4")
• 35 cm length of 1 1/4" diameter PVC pipe (schedule 40)

Step 1: Assembling the End Pieces

1. Press a pipe adapter into the pipe couple. Make sure that the adapter goes all the way into the couple. You may need to push the couple against a table or the floor.
   
2. Repeat this procedure for the other pipe adapter and couple.

Step 2:Assembling the Telescope

1. Place one lens through the opening of the couple. The lens should rest inside the couple, against the rim of the adapter.
   
2. Place the couple on the floor with the adapter side down. Be sure the lens is resting flat against the adapter inside the couple.
3. Push one end of the PVC pipe into the couple. Keep pushing until it will not go any further.
   
4. Gently shake the pipe. If you hear or feel a rattling, you need to push the pipe further into the couple to secure the lens.
5. Place the second lens in the other couple as described above.
6. Push the other end of the PVC pipe into the second couple and check for rattling.
   
7. You are now ready to use your telescope!
   

Step 3: Using the Telescope

1. The thick lens of the telescope is the eyepiece, the lens you look through. Look through the thick lens to magnify objects!
   
2. You can use the telescope during the day or at night. (Note: Never look at the sun with a telescope! It can damage your eye!)

Paper Towel Roll Telescope

For each telescope you will need the following materials:

• one 38mm thick double convex lens (Nasco catalog# P10208)
• one 38mm thin double convex lens (Nasco catalog# P10204)
• clear polyvinyl tape
• one 5" x 9" sheet of black construction paper (15 x 22 cm)
• paper towel roll
• sticker (or bright colored crayon or marker)

Step 1: Securing the Thin Lens to the Paper Towel Roll

1. Place the thin lens inside one end of the paper towel roll. The lens should be even with the end of the paper towel roll. Note: All paper towel rolls do not have the same diameter. If the diameter of the roll is larger than the diameter of the lens, you may need to roll a piece of construction paper and insert it into the paper towel roll.
2. Tear off a few inches of polyvinyl tape. Cut small tabs into the tape so that it curves with the lens and paper towel roll.
3. Place the tape on the paper towel roll and on the lens. Try to cover the surface of the lens as little as possible for optimum visibility.
   

Step 2: Preparing the Construction Paper Tube

1. Roll the 5" x 9" piece of construction paper into a tube that is just slightly smaller than the paper towel roll.
2. Insert the construction paper tube into the paper towel roll. Be sure that the construction paper tube and can be moved in and out of the paper towel roll.
3. Tape the construction paper tube so that it stays rolled.
   

Step 3: Securing the Thick Lens

1. Place the thick lens into one end of the construction paper tube you just made.
2. Tape the lens to the construction paper tube the same way you secured the other lens.
   
3. Place the open end of the construction paper roll into the open end of the paper towel roll.
4. A tab must be added to the construction paper tube so it does not completely slide into the paper towel roll. To make this tab, tape two pieces of polyvinyl tape to each other. Fasten this tab to the construction paper tube.
   

Step 4: Using the Telescope

1. Look through the thick lens of the scope. To adjust the focus of the telescope, slide the construction paper tube in and out of the paper towel roll.
   
2. When you have found the best focus (which should be about 35 cm, with the lenses used above) place a sticker or make a mark on the construction paper roll where the construction paper enters the paper towel roll. This will indicate the best position to view objects.
   
   

Assessment

Our assessment methods were the same for Illinois and National Standards, and for Early and late Elementary School students. What changed was the level of the questions asked and the discussion. We used informal assessment methods in the form of verbal questions and discussion. Formal methods of assessment included drawings, and written answers to questions, and the development of articles for the BOAST Kids Newsletter.

During the Lesson

The participants used inquiry strategies to successfully determine the focal length and magnification of convex lenses, and during the construction of the telescope. Then they tested the device and reported on its use. Throughout the lesson, the participants discussed the inter-relationships between science, mathematics, technology, and society. A sampling of student writings concerning this activity can be found in the Reactions to this Activity section.

At the End of the Lesson

At the conclusion of the lesson, a discussion was held to determine the level of participant understanding during the lesson. Here is a sample of questions from that discussion

• How can you determine the focal length of a convex lens?
• How do you calculate the magnification produced by two lenses?
• When building a telescope, how do you figure out how far apart the two lenses should be?
• How does the image look through a telescope? Why does this happen?

Each of the children who participated in the lesson was able to answer these questions at his/her own level.

Suggestions for expanding this activity

Visiting an Observatory

A trip to the university observatory was planned as an extension to this hands-on lesson. Actitivities that took place at the observatory included

• learning about how telescopes work,
• learning about the history of telescopes,
• learning about the objects in the night sky,
• looking through the university telescope at objects in the night sky, and
• looking through the "BOAST" telescope at objects in the night sky,
• drawing pictures of objects in the night sky with the naked eye, the "BOAST" telescope and the university telescope.

Visiting a Planetarium

A trip to a planetarium might offer students the opportunity to learn more about objects in the night sky.

Illinois State and National Learning Standards

Early Elementary

Late Elementary

National Science Education Standards that Apply to this Lesson

Take a look at the complete National Science Education Standards.