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Light, Color, and MirrorsBrian Tiburzi , Laurie Tamborino & Gordo ParkerPublished online: 02 Apr 2010.

To cite this article: Brian Tiburzi , Laurie Tamborino & Gordo Parker (2000) Light, Color, and Mirrors, Science Activities:Classroom Projects and Curriculum Ideas, 36:4, 25-28, DOI: 10.1080/00368120009601062

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BRIAN TIBURZI, LAURIE TAMBORINO, and GOR

he following exercise (activity for developing the mind or body) incorporates several areas of study into a unified activity. Using flashlights, mirrors, and

colored paper, students can discover scientific principles firsthand, which the teacher should then reinforce during class discussions. These include the property of reflection from a mirrored surface, measurement of incident (striking) and reflected angles, the concept that white light is com- posed of all colors, and the idea that a mirror image differs from a directly observed object. The teacher should also emphasize to students that they need to make careful mea- surements if they want to draw valid conclusions from experimental data. Specifically, correct angle measurement shows that the angle of incidence of a light beam equals the angle of reflection of that beam from a mirror surface.

Background and Concepts Developed Students are familiar with mirrors and viewing objects in

mirrors. However, they may not understand the concepts of reflection from mirrored surfaces-that is, the relationship between the angle of incidence (light striking a mirror) and the angle of reflection (light reflected from the mirror sur- face). Part A of this exercise has the students develop ray diagrams, which teaches them the law of reflection. Teach- ers should discuss with the students the principle that the angle of incidence equals the angle of reflection. For exam- ple, equal angles of incidence and reflection are used to advantage when bouncing a basketball. The angle at which

BRIAN TlBURZl is a graduate student in physics at the Univer- sity of Washington, Seattle. He also teaches labs for the introduc- tory physics sequence at the University.

LAURIE TAMBORINO teaches jifth-grade classes at Edison Oakland Public School Academy in Ferndale, Michigan.

GORDON A. PARKER is currently coordinator of laboratory courses for nonscience majors in the College of Arts, Science, and Literature at the University of Michigan-Dearborn. During the summei; he does science enrichment programs with junior high school students.

the ball is projected toward the floor is the same as the angle at which the ball bounces away from the floor. In billiards, the angle at which a struck ball rebounds from the side of the table equals the angle at which it strikes the table. If teachers decide to deemphasize these scientific concepts, they should emphasize the reality of experimental error and the need for careful attention to experimental details. Values from student measurements of angles of incidence and reflection should be equal. Actual student measurements can be inaccurate, however, and may not always produce the expected results. If desired, teachers can have the stu- dents repeat their angle measurements, paying even greater attention to detail.

By doing the exercises in Part B the students learn that light is composed of many colors and that a spectrum (rain- bow) of light is easily generated from a white light source. An object’s color is determined by the absorption and reflection of selected colors when white light is directed onto it. For example, a ripe apple appears red because its surface reflects red light. The apple absorbs or blocks the other colors. Simi- larly, a dandelion appears yellow because it reflects yellow light, and it absorbs all the other colors. To illustrate this, stu- dents should observe a red reflected ray from red construc- tion paper or a yellow reflected ray from yellow paper.

Part C teaches students about mirror images. Students may have difficulty with the perception of mirror images because the reflection observed is backward from the con- figuration of the actual object. When the students place one hand on top of the other, they will observe that no amount of twisting will allow identical placement. Because of the effect of mirror images, some ambulances have intentional- ly painted the word ambulance in reverse order on the front of their units. When drivers ahead of them view the ambu- lance through their rearview mirror, they can read the word correctly and react appropriately in an emergency situation.

Materials (per student pair) Small handheld mirror

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26 SCIENCE ACTIVITIES Vol. 36, No. 4

Meter stick Flashlight Sheet of black posterboard Chalk and eraser Protractor Construction paper (white and colored sheets) Compact disk (to illustrate the breakup of white light into a

spectrum. If a single disk is used, the teacher can demon- strate this principle.)

A darkened room

Preparation for Part A Cover the flashlight with a disk of black posterboard (or

thick construction paper) into which a slit has been made to allow the passage of a narrow beam of light (See Figure 1). To achieve the best results, make the slit quite small and use

Figure 1. Paper mask fitted over flashlight lens.

a flashlight that has a powerful beam. If available, use a Ray Box, which projects light through a screen with small slits.

Part A-Light

Procedure

1. Divide students into two-person teams. 2. Darken the room by pulling down the window shades

or blinds. 3. Light emitted from a light bulb represents straight

lines of light projected in all directions. Keep the flashlight on the table and direct the light beam toward a sheet of black construction paper. What effect does the black paper have on the light emitted from the flashlight? The black paper absorbs the light, although a minute amount of light is reflected.

4. Light tends to travel in a straight line direction unless it hits another object. Observe this property of light by pointing the flashlight beam toward a distant object. As the beam broadens, the light rays will disperse. Draw a picture of what is visible when the flashlight is on without its light encountering any object.

5. A ray is both a line and an arrow indicating the direc- tion light takes. A diagram that represents light as a ray is

Incident 1E;;ted \ /Angle

\ Flashlight I

Normal

Figure 2, Light paths incident to and reflected from a mirror.

called a ray diagram. Transform the picture drawn in step 4 into a ray diagram.

6. Next, interrupt the light’s direction by placing a mirror against its beam. Instead of showing a straight line path, the mirror will reflect the beam. Draw a ray diagram showing this reflection.

7. The ray of light from the flashlight that strikes the mir- ror is called the incident ray. The ray of light the mirror reflects is called the reflected ray. Vary the position of the flashlight so that the incident ray strikes the mirror at a dif- ferent incident angle than previously (See Figure 2). Does the reflected angle change as the incident angle changes?

8. It can be concluded that the incident angle affects a reflected angle, but to discover the relationship between the two, more measured data are necessary. Place a mirror on a dark sheet of paper on the table and then outline the mirror’s position with chalk. Put the flashlight on the paper in front of the mirror and shine it at the mirror while marking the position of its beam on the paper. Trace the incident ray as well as the reflected ray with chalk and label the incident angle and the reflected angle. Measure each of these angles using a protractor.

9. Make two more diagrams using different incident angles and record the data in Table 1. Is there a relationship

Table 1. Angles from a Mirror

Trial Number Incident Angle Reflected Angle

1. degrees degrees 2. degrees degrees 3. degrees degrees

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Winter 2000

between the incident and reflected angles? What is the rela- tionship?

10. Using the protractor, on a separate sheet of dark paper, draw and label a ray diagram similar to those mea- sured but with an incident angle of 35". To draw the reflect- ed angle, refer to the labeled chalk outlines done in step 8.

I 1. Align the flashlight to produce a beam along the inci- dent ray in the drawing. Put the mirror on the spot. Does the drawn reflected ray match the reflected light from the flash- light?

Part B-Color

Procedure 1. To separate white light into its various colors, pass the

light from the flashlight through a prism. As an alternative, direct the light beam from the flashlight onto the surface of the compact disk. The disk serves the same function as the prism. Place a sheet of white paper where it will catch the reflected ray to show the rainbow of colors that make up white light.

2. To investigate the interaction of light with nonmirror surfaces, choose different colored sheets of construction paper. Select a color and place it so it will interrupt the direction of light coming from the flashlight. Is there a reflected beam? How does the interaction of light with the colored paper differ from its interaction with the mirror?

3. Put a sheet of white paper where you would expect the reflected ray from the colored sheet to be. Does the colored paper reflect anything onto the white sheet? If so, what does it reflect? Record observations in Table 2. How does the reflected color relate to the choice of paper color?

4. Generalize on how light interacts with nonmirror sur- faces.

5. Why do objects lose their colors at night? 6. From what has been shown about light and reflection,

explain why a red ball is red.

Part C-Mirrors

Procedure 1. This part of the exercise requires only a mirror. Look

into the mirror and see your image. Although mirrors show images of the objects in front of them, the mirror's version is distorted because the objects are seen backward. Mirror images are called virtual images because they appear real but they are not.

2. Hold your right hand in front of the mirror. What does the mirror show you? Is your righthand image positioned the same or differently from the actual position of your right hand? Does your righthand thumb in the mirror image show the actual direction in which it is pointing?

3. Place an object five inches from a mirror. How far behind the mirror does the image appear to be?

4. Move an object that is in front of a mirror away from the mirror. What happens to the mirror image?

5. A mirror image of an object is said to be superimpos- able if the image and the object fit exactly one on top of the

SCIENCE ACTIVITIES

Figure 3. Objects and their mirror images.

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Table 2. Observations from the Reflection of Col- ored Paper

What Color was Absorbed by

Color of Paper Reflected Color the Paper?

I

other. A mirror image that is not the same as the object is said to be nonsuperimposable. Is your right hand and its mirror image superimposable? Look at the objects shown is Figure 3. Draw their mirror images. Are these images super- imposable or nonsuperimposable?

Conclusion This exercise provides insight into the study of light as it

is reflected by mirrors and selectively reflected by colored objects. Doing the activities in Parts A-C helps students understand the measurement of angles, the relationship of

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