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Light and Optics

Investigation 2 – Lab

Back to Investigation 2

BE PREPARED

Supplies and Equipment:

Group Materials:

1 sheet of unlined white paper or a piece of cardboard
1 sheet of unlined white paper
1 pair of scissors
1 flashlight
1 stand-up mirror
masking tape
1 metric ruler

Individual Materials:

1 Student Data Record
1 Protractor Sheet from the Scientist Data Record

Teacher Preparation:

1. Place materials at a central location.

2. Prepare one copy of the two-toned sheet of black and white paper for each student group.

3. Divide the class into five cooperative groups.

Instruction:

1. Ask one student from each group to obtain: one (1) metric ruler, one (1) flashlight, one (1) sheet of white paper or cardboard, one (1) stand-up mirror, and one (1) sheet of white paper. Please note that, on the protractor sheet, the right side of the protractor is labeled “A” and the left side of the protractor is labeled “B.” These labels provide reference points for the experiment.

2. Ask each student group to create an instrument that generates a narrow beam of light. To do this, students should first cut a 5 cm x 5 cm piece of paper or cardboard. Students should place the flashlight on top of the paper or cardboard with the light facing the cardboard. They should trace around the lens of the flashlight so that they see a circle on the cardboard, then cut out the circle.

3. Direct students to cut a slit in the cardboard circle approximately 2.5 cm long and 1 mm wide from the edge of the cardboard to the center of the circle. Then students should tape the cardboard circle to the lens of the flashlight. Use the figure here as a reference.

4. Ask each student group to place the Protractor Sheet on the lab table so that the protractor’s semi-circular portion faces the edge and the straight side faces the inside of the table.

5. Ask one student to hold the mirror on the straight side of the protractor sheet making sure the mirror is aligned along the side and standing upright. Use the illustration below as a reference.

GET FOCUSED

Investigation Two introduces students to the reflection of light off of the smooth surface of a mirror.

INVESTIGATE

Trial 1

In this Trial, students will explore the Law of Reflection. Students will use a flashlight as a light source and a mirror as the reflecting object. Using a Protractor Sheet, students will position the flashlight at two different angles of incidence relative to the mirror and calculate the angle of reflection in each case. In both cases, students will compare the values of both angles to determine if the Law of Reflection is supported by the experiment. By conducting this Trial, students will answer the following questions:

What is the Law of Reflection?

Will changing the position of the light source change the angle of incidence?

How does changing the angle of incidence affect the angle of reflection?

2. Instruct students to record their predictions in the Table in Problem 4 in their Student Data Record.

3. Ask students to locate the Protractor Sheet at the back of their Student Data Record.

4. Ask students to carefully position the flashlight and the mirror on the Protractor Sheet as shown in the diagram in the Problem.

5. Students should follow the procedure in Problems 5a-5e in their Student Data Record.

6. When students are finished with Trial 1, ask students to think about their observations and answer Problems 5h and 5i in their Student Data Record.

Trial 2

In this Trial, the angle of incidence will be changed not by moving the flashlight but by moving the mirror. Students should refer to the diagram in Problem 5 and follow the procedure in Problem 6 in their Student Data Record. Students will first use an angle of incidence of 0º and then an angle of incidence of 20º.

What is the Law of Reflection?

Will changing the position of the reflecting surface change the angle of incidence?

How does changing the angle of incidence affect the angle of reflection?

8. Students should record their data in the tables in Problems 6e and 6h.

9. When students have completed Trial 2, ask them to answer the questions in Problems 6i through 6j.

10. Review the experiment as a class, including the following questions:

• Were you able to observe the direction of reflected light? How? Yes. A piece of paper was held up so that the reflected light was visible and the angle of reflection was marked.

• Do your observations support the Law of Reflection? Why or why not? Student answers will vary. The observations support the Law of Reflection because the angle of reflection was equal to the angle of incidence.