University of Virginia
Physics Department

## Reflection

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.9

Objectives

The student will

• identify and understand the principles of reflection;
• make measurements and calculations of the angle of reflection.

Motivation for Learning

Materials

Procedure

1. Explain to students that they are going to observe and record data, in the form of drawings and narratives, on the activities you are about to demonstrate. Direct them that after the third bounce, as they see patterns start to develop in their data, they may raise their hands and begin to comment on the nature of the bouncing and the data they have collected.
2. Bounce the ball straight down on the floor and catch it as it returns. Pause to allow students to record drawings and narratives.
3. Bounce the ball again, only this time bounce it at a small entry angle so that you have to extend your other hand slightly to catch it as in the illustration. Pause to allow students time to continue drawings and narratives.

 On the second bounce, make it obvious you are creating a slight angle to your dribble. Be careful not to go too wide. In a crowded classroom you don't want a basketball on the loose!

4. Place yourself across from a student who can catch the ball on the up-swing. (Hint: Prearrange this situation as the class starts. Choose a student who will be a responsible demonstration partner and not become so excited with the basketball as to pull others off-task. Instruct the student to hold onto the ball after the initial bounce to him/her.) Bounce the ball to the student. Pause, allowing students to continue data collection.
5. Repeat the bounce to the student. (This will allow for further data collection, or the chance to get original data if the students were a little excited at the prospect of the basketball coming into the "audience.")
6. By now, the students should begin to see patterns in their observations and begin to make inferences. Allow all students who wish to verbalize observations and to begin to make inferences to do so. Help them to clearly articulate their observations and distinguish them from their inferences.
7. You may go further and bounce the ball on the wall using many different entry angles, pausing after each to allow recording of observations.
8. Once students have reached the conclusion that the angle that you bounce the ball in at is pretty close (or equal to) the angle that it bounces back at, then you can proceed with the lab.
9. As a transition into the lab, ask students these thought-provoking questions and engage in discussion with them.
• "Are balls the only things that can bounce off a surface?"
• "Can only solid things bounce off surfaces?"
• "What kinds of non-solid things can bounce off surfaces?"
10. Once students realize and verbalize that light and sound can bounce (or reflect) off of surfaces, introduce them to the lab. Alert them that the introductory activity was a purposeful lead-in to the lab on reflection of light and that they can make hypotheses about light, which are directly related to the physical model they just observed.

Background Information

The reason for using the bouncing basketball is to provide a meaningful, hands-on model to which the students can readily relate and about which they will be enthusiastic.

Students will be able to see in this demonstration that the angle of entry (technically, the angle of incidence) is equal to the exit angle (the angle of reflection).

 Angle of Incidence = Angle of Reflection

They then will be able to apply this principle to a more difficult situation, that of reflected light.

When measuring the angle of incidence and the angle of reflection, use the mirror (or other reflective surface) as the base from which to measure the two angles.

### Student Activity

Materials

• plane mirror
• penlight flashlight
• pair of dirty chalkboard erasers
• 2 sheets of plain white paper
• protractor
• pencils (student must provide for self)

(Caution: Chalk dust will be used to aid in the viewing of the light beams. Students with allergies should be warned and allowances made in the event that the use of the dust interferes with their health.)

Procedure

1. Divide students into groups of four. Throughout the exercise, each student will be assigned a specific job. It is the teacher's discretion to assign or to allow student choice with group jobs. The jobs include:
• Mirror Holder - holds the mirror, maintaining the 90° angle; also reads lab instructions.
• Recorder - writes for the group during the actual performance of the lab.
• Penlight Operator - operates the penlight flashlight, maintains it at safe distances and angles away from eyes of group members; also is responsible for asking the teacher any questions that the group cannot figure out.
• Chalkboard Eraser Operator - gently and lightly taps erasers together to illuminate the beam of light from the penlight; also picks up and returns all lab materials for the group.
2. Neatly arrange materials at your lab stations and know object locations as this lab will be performed in a darkened room.
3. Turn on penlights.
4. Teacher darkens the room.
5. One blank sheet of paper should be laid down on the table first and labeled #1 to indicate it is the first set of data.
6. The Mirror Holder will rest one edge of the mirror on the sheet of paper, holding it perpendicular to the table at all times. (Use the protractor to measure and maintain the angle if necessary.)

7. The Recorder will draw a line on the paper indicating the location of the front edge of the mirror, as indicated in the diagram below.
8. The Penlight Operator will then hold the light a distance of one half the mirror's height above the paper's edge and shine the beam onto the center of the mirror at an angle.

9. The Chalkboard Eraser Operator will gently and lightly tap the erasers over top of the experimental set up, allowing the dust to fall down into and illuminate the beam from the penlight.
10. Looking down over the top of the experimental set up, the Recorder will quickly trace the path the beam takes onto the paper beneath it.

11. Turn the paper over to the clean side and perform another trial by repeating steps 5-10. This time, label the data with a number 2, for the second set of data.
12. Using the second sheet, repeat steps 5-9 two more times to obtain data sets 3 and 4.
13. After all four sets of data have been collected, clean up your lab station and return to your group for analysis of the data.

Data Sheet

1. - 4. Answers will vary based on individual groups.

5. Sample chart:

 Data Set # Angle of Incidence Angle of Reflection 1 2 3 4

6. Have each group to display at least one set of data to provide students with information to answer this question. The mathematical principal: The measure of the angle of incidence = the measure of the angle of reflection.

7. The basketball demonstration was a model for the way light acted in the experiments. The basketball bounced off of the floor (and the wall) just like light bounced off the mirror. In both cases, the reflective surfaces were smooth and as a result the two bouncing objects reflected off the surfaces at the same angle they bounced onto them. The reflection of light is a little more difficult to understand than the bouncing of a basketball. Using the basketball as a model makes the behavior of the light easier to understand.

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

Assessment

While students are measuring the angles on their data sheets, circulate among them and informally assess their measurement skills and accuracy with the protractor. This is a great time to give individual instruction where needed.

Each student should be given a copy of the analysis and conclusion questions. Students should be instructed to discuss the questions with their group members and then write their answers in their own words on their papers.