Group Members: Lucy Dunaway, Robin Tran, David Huston
Big Science Ideas/Concepts: Reflection of Light/Law of Reflection
Possible student misconceptions:
1) A mirror or other smooth surface does not reflect light at a predictable angle.
2) When viewing an object in front of a planar mirror, the student may incorrectly predict the position of the image it forms and the angle of the reflected light.
1) Students should be able to state the law of reflection and draw a ray diagram to illustrate it.
2) Students should be able to apply the law of reflection to predict the position of images formed by objects in front of a planar mirror.
1) Full length mirror hung horizontally
2) Masking tape to mark the center of the mirror, and paper to cover the mirror.
4) 5 planar mirrors for each lab group and mirror supports (clay, slit rubber stoppers, or spring clothespins also work well)
5) Standard size white paper for ray diagram activity ,and a piece of cardboard to place underneath so that pins can be inserted
6) Metric ruler and protractor
7) Butcher block paper for diagramming the position of the 5 mirrors
Intended audience grade level/age:
9th grade physical science or as an introductory activity for the study of optics in a physics class.
Description of Lesson:
1.The teacher will review or ask questions to review the students concerning the following information which has been covered in previous lessons.:
a) Light is a transverse wave that travels in a straight line.
b) When light travels, it can be transmitted , absorbed, and reflected.
c) Light from a rough surface produces a scattered reflection.
d) Light from a smooth surface like a mirror produces an image.
2) The teacher will tell the students, ě Today we will look closer at
the reflections from a flat or planar mirror and the behavior of light
as it reflects from the mirror to form an imageî.
3) The teacher will illicit student response to the follow questions:
a) When you look at an object placed at some position in front of a flat mirror, how do you determine where the image will be seen or whether it will actually be seen at all in the mirror?
b) For example, you may have noticed this sign on the back of a large truck, ěIf you canít see my mirrors, I canít see you?î Is this really true?
4) After hearing the different responses, the teacher will direct the students to the following activity to help them construct the answers to these questions.
Direct student attention to a floor length mirror that is positioned horizontally at the bottom of the board and to a piece of tape used to mark the center of the mirror. The mirror will then be covered, and 5 students will be seated in front of the mirror in positions A,B, C, D, and E . (See diagram 1 on the attached sheet)
Ask the following question: ě If each student focuses at the piece of tape in the middle of the mirror, who will each person be able to see from his specific position in front of the mirror?î
Allow students to return to their groups and have a short discussion. Make a list of their predictions for each position in front of the mirror.
4. Before revealing the images in the mirror, lead the students
to the attached lab activity, Reflection of Light , to help them
confirm or change their predictions.
Allow the students to work as groups to observe the images, make the drawings, measure the angles, and draw some conclusions. Be sure each group places the pins correctly.
Ask the following question: ěHow did the 2 angles that you drew compare in size? For those that might have done it a second time, did you get the same result?î
5. Ask students to refer back to their diagrams, and use these to explain the use of a ray diagram that shows the direction of a moving light ray. Students should be able to define and label the normal ray, the incident ray, the reflected ray, the angle of incidence, and the angle of reflection. After reminding them of their lab results in this activity, write and discuss the law of reflection.
6. Lead students back to the mirror question and their predictions
on the board.
ě After looking at the results of this activity and the law of reflection, how do you feel about your predictionsî Allow groups to change their answers if necessary.
Ask the 5 students to return to their positions in front of the mirror. Leave the mirror covered except for the center position with the tape. (See diagram 2 on the attached sheet) Ask each person whom he is able to see in the mirror.
Person A sees only Person E. Person B sees only person D.
Person E sees only Person A. Person D sees only person B.
Lead students to see that this is what should be expected because of the law of reflection. Person C sees only himself because when a ray reflects at 90 ` , it reflects straight back at 90 `.
Allow each person to shine the flashlight from his face as an incident ray on the center of the mirror. The students will see that the reflected ray in each case confirms the law of reflection and the results mentioned above.
7. Lead the students to work in groups to perform the final lab activity. This can be used as an application and as an assessment. See the attached lab using the 5 mirrors. If the student can perform the task and explain using ray diagrams, he will show an understanding of the objectives in this lesson.
8. Refer students back to the original question from the beginning
of the lesson.
ě If you canít see my mirrors, I canít see you.î Refer back to first 2 lab activities and the results of each. Studentsí discussion here will be used as a means of assessment.
A possible extension:
1) Students can be asked to apply the law of reflection to build a periscope. (This can be found in many activity books. One example is Exploring Physical Science (Prentice Hall).
2) Students can be asked to follow directions to build a kaleidoscope. This is also found in many lab activity books such as the laboratory manual for Exploring Physical Science (Prentice Hall) pp. 265-266.
A relevant internet (or other) resource(and a brief description of how
it could enhance the lesson):
Develop the studentís understanding of reflection and other properties of light by showing and discussing the following films from The Mechanical Universe ( High School Adaptation ): The Wave Nature of Light and Wave -Particle Duality . These films should illicit discussion concerning the history and discoveries leading to the modern theory about light. It also introduces students to the other properties of light and can be used to explain and compare reflection, refraction, and diffraction. The films also provided examples of some practical applications that use these properties.
End of lesson assessment:
Student responses during class discussion will help to assess the objectives of the lesson. Successful completion of the lab activity with the five mirrors and the student diagrams will be used also.