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### Fun With Optics

#### by Teachers Talking Science

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Created more than 400 years ago, the telescope is a tool that has allowed both amateurs and scientists alike to explore the sky in much more detail than they ever could with their naked eyes. All telescopes work in more or less the same way: they collect a large amount of light and focus it into our eyes --or in some cases, into a camera. Because objects that are very far away, like stars, appear very dim to an observer, a lot of light needs to be collected before they can be seen clearly.

In this activity, students will perform several experiments, using simple materials to explore the properties of reflection and refraction and how they work in telescopes. Students also will observe how light can be amplified and focused using different methods, and measure the focal point of a given magnifying glass.

Subject Matter: Physical Science
National Standards: NS.5-8.1, NS.5-8.2
The year 2009 marked the 400th anniversary of the invention of the telescope. To celebrate its birthday, Science Friday hit the streets of New York City to find out what people know about the telescope and its history. Can you explain how a telescope works?

Activity Materials
Magnifying glasses, one for each student or group of students. If possible, use a variety of magnifying glasses of different sizes with different types of lenses.
Small mirrors, one for each student
Rulers, one per pair of students
Index cards, one per pair of students
Light bulb and lamp fixture
One Permanent marker
Pencils

Vocabulary
Reflection: the process in which light bounces off a surface, such as a mirror.
Refraction: the process in which light passes through a material, like glass or plastic, and bends so that it changes direction.
Focal Point: the single place where all the light that is passing through a lens or bouncing off a curved mirror converges.
Focal Length: the distance between a lens or curved mirror to the focal point.

What To Do

1. Begin the lesson by having students watch the Science Friday video “Happy Birthday, Telescope.” Begin a discussion with the students on what they know about telescopes. What do we use telescopes for? How do they work? Why are they important?

2. Tell students that they will conduct a series of experiments on how light interacts with mirrors and lenses, the key components of telescopes. Inform students that at no time should they ever use mirrors or lenses to shine light directly into their own eyes or the eyes of another person -- particularly when the light source is the sun.

Experiment 1 – Reflection

1. Assign students to work in pairs. Hand out one mirror and one index card to each pair of students.

2. Challenge students to use the mirror and ambient light in the room to create a spot on their index card that is as bright as possible. Students must place their index card on the table, but can move the mirror into different positions or angles.

3. After a few minutes of exploration, start a discussion reviewing their results. Were they able to create a spot on their index card? Ask students to come up with ideas on how they could make the spot brighter. Have students think about using multiple mirrors instead of just one mirror. Have three students use two and then three mirrors, to see what happens to the spot on the index card.

4. Assign students to groups of three or four, and hand out a mirror to each student. Challenge each group to use the mirrors in their group together to create a bright spot on one index card that is on the table. What method yielded the best results? Ask students to explain why they were able to create a spot on the index card. Did the number of light sources in the room change? How does light reflection relate to this activity?

Experiment 2 - Refraction

Note: When dealing with magnifying glasses, do not use sunlight as the source; it may cause a fire. Close the shades if the sun is shining brightly into the room.

1. Hand out a magnifying glass to each pair of students. Challenge them to use the magnifying glass and ambient light in the room to create a bright spot on the index card that is on the table.

2. Allow students to explore various ways to focus light onto the index card. After a few minutes, have a class discussion to review their results. How did they have to hold the magnifying glass? How far was the magnifying glass from the page? What did the spot look like?

3. Ask students to explain if the magnifying glass reflected or refracted the light. What are the differences between the two?

4. Using a ruler, have students measure the distance that the magnifying glass must be placed from the index card in order to yield the sharpest dot of light. This length is known as the focal length and is the distance from the lens to the focal point, the point where all light passing through the lens converges on one spot.

Experiment 3 - Focal Point

Prep: Take a regular, opaque white incandescent bulb, and use a permanent marker to draw small shapes directly on the bulb. Simple shapes with sharp details, like triangles, stars or thick arrows, are preferable.

1. Place the white bulb in the lamp fixture (without a lamp shade or other obstruction) and center it in the room. Turn the lamp on and turn off any other lights and close the shades, if any. Stand a few inches away from the light and hold an index card up. Ask students to describe how the light looks against the index card. Is it bright or dim?

2. Now take a few steps back and hold up the same index card. How does the light look against the card now? Walk as far back from the light source as possible and hold up the index card. How bright or dim is the light on the card now, compared to what it was before? Ask students to explain why the index card is more dimly lit in the back of the room. Explain how light spreads out in all directions from a light source, and that an index card far away from the source is collecting a much smaller percentage of light than one that is close to the source.

3. Have students work in groups of three or four, and use their mirrors to collectively create a bright spot on an index card that has been placed flat on their tables. How does the light on the index card look? How does it look compared to using the ambient light?

4. Distribute the magnifying glasses. Have each group of students position a magnifying glass and index card so that the shape drawn on the bulb is clearly projected onto their cards. For this step, it would be best to have one student hold up the index card as still as possible and face the light source, while another student changes the position of the magnifying glass with respect to the card. How does the light look? Can any of the shapes be seen clearly? How are they oriented?

5. Using a ruler, have students measure the distance between their magnifying glass and the index card when the image formed is clearest. This distance is known as the focal length of the lens in each magnifying glass. How does the focal length compare among different magnifying glasses, or for other groups of students with the same magnifying glass? How do you think this relates to the way telescopes form images of distant objects?

What's Happening?
As light rays from a light source spread out, the light that is received at any point is greatly decreased as it travels farther away from the source. An object twice as far from a light source only receives one-fourth of the original light. An object ten times farther away receives one-hundredth of the original light! This is why even bright stars that are far away can be very difficult to see. In order to see stars more clearly, telescopes use mirrors or lenses to gather and focus as much of this dim light as possible. There are many different types of telescopes, but they all fall into three different categories: reflecting, refracting, and combination telescopes.

In the first experiment, students used a mirror to reflect light from a light source and focus it on a specific point. Reflecting telescopes work in a similar manner by using large mirrors or many small ones to gather large amounts of light and focus that light into a small area, which magnifies the image greatly. The Hubble Space Telescope, for example, is a reflecting telescope.

In the second experiment, students used the lens of a magnifying glass to refract or bend light and focus it on a specific point as well. Refracting telescopes function in the same manner, except they use lenses to bend the light in towards a point.

Curved mirrors and lenses have a point in which all light rays that have passed through them will converge. This is known as the focal point. Flat mirrors have focal points that are infinitely far away, meaning that the light will never converge on a single point. In the second and third experiments, students were able to find the focal point of their magnifying glasses by seeing where the light converged to a single point, or where the light came together to form the best image.

Today, combination telescopes are the most popular, because they use the best features of reflection and refraction to get the best quality image with fewer distortions.

Topics for Science Class Discussion
• What are some other science tools that use mirrors and/or lenses to magnify?
• Explain the relationship between refraction and rainbows.
• How do modern telescopes differ from telescopes built 100, 200 or 300 years ago?
• Name some examples in day-to-day life where reflection and refraction is used or seen. Are there any ways that these properties are used to make our lives easier?

Investigate the law of reflection using a mirror, protractor and low-cost laser level (available at any hardware store) by having students measure and compare the angle at which a beam of light hits a flat mirror and the angle at which it bounces off.

Cut thick Mylar plastic sheets (available at hardware or art supply stores) into 4” x 4” squares and distribute to students. Give students flashlights and have them investigate how the focal point changes as the Mylar is curved into a U shape. What happens to the light when it is shone into a semicircle of Mylar and angled onto a page? What happens when it is shone onto the back of the semicircle?

Learn more about the Hubble Space Telescope, view amazing images and find educational activities on the NASA website:
http://www.nasa.gov/audience/foreducators/hubble-index.html

http://www.jwst.nasa.gov/

This lesson plan was created by the New York Hall of Science in collaboration with Science Friday as part of Teachers Talking Science, an online resource for teachers, homeschoolers, and parents to produce free materials based on very popular SciFri Videos to help in the classroom or around the kitchen table.

The New York Hall of Science is a science museum located in the New York City borough of Queens. NYSCI is New York City's only hands-on science and technology center, with more than 400 hands-on exhibits explore biology, chemistry, and physics.

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Teacher's Talking Science was a collaboration between the New York Hall of Science and Science Friday to create free, multimedia-based science resources for teachers, homeschoolers, and parents.

The views expressed are those of the author and are not necessarily those of Science Friday.