This is the second Investigation in the LabLearner CELL Light and Optics. Students performed experiments in which they measured the angle of incidence and the angle of reflection from a mirrored surface. They investigated the Law of Reflection and used their data to draw conclusions about the relationship between the angles of incidence and reflection.

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A. Begin with a review of reflection by asking the following questions. Instruct students to refer to their Scientist’s Glossary if necessary. To aid the review, to the diagram of a flashlight, a mirror, and incident and reflected light similar to the diagram from the Lab.

• Define reflection. Reflection is the bouncing of light off an object.

• What is an image? An image is a representation of an object formed by light refracted by a lens or reflected by a mirror.

• Define the angle of incidence. The angle of incidence is the angle at which light strikes an object.

• Define the angle of reflection. The angle of reflection is the angle at which light bounces off an object.

• Define the Law of Reflection. The Law of Reflection states that the angle of incidence equals the angle of reflection.

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B. Begin a discussion of students’ observations from the Investigation.

1. Ask students to refer to Problems 4 and 5 in their Student Data Record.

• Ask students: What was the angle of reflection when the angle of incidence was 30°? Student answers may vary slightly. Answers should be very close to 30°.

• Ask students: What was the angle of reflection when the angle of incidence was 60°? Student answers may vary slightly. Answers should be very close to 60°.

• Ask students: What relationship is there between the angle of incidence and the angle of reflection? The angle of reflection is equal to the angle of reflection.

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2. Ask students to refer to Problem 6 in their Student Data Record.

• Ask students: What was the original angle of incidence? Students should answer 0°

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• Ask students: How did the angle of incidence change as the mirror was rotated 20°? The angle of incidence changed from 0° to 20°.

• Ask students: What was the original angle of reflection? 0°

• Ask students: How did the angle of reflection change as the mirror was rotated 20°? The angle of reflection also changed from 0° to 20°.

Ask students to imagine the mirror had been rotated 40°. 

• Ask students: What would the new angle of incidence have been? The new angle of incidence would have been 40°.      
• Ask students: What would the new angle of reflection have been? The new angle of reflection would have been 40°.

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3. Discuss the reasons for possible discrepancies between the angle of incidence and the angle of reflection.

• • Ask students: What factors may have contributed to any discrepancies in your results? Student answers may vary.
  • Ask students, Was the mirror held perpendicular to table? Student answers will vary. 

Point out to students that the angle at which the mirror was held was a vital part of the experiment. A slight variation in how the mirror was held could have caused discrepancies in the results.

• Ask students: Could the results have been affected by where the flashlight was held? Student answers may vary. Students had to approximate center of the beam of light from the flashlight so that it was aligned at the 30° mark and the 60° mark on the protractor sheet. If the flashlight had been held at slightly different angles, the results could have differed slightly.
• Ask students: Could there have been slight errors in the angle of reflection based on how the angle of reflection was marked? Student answers may vary. If the center of the beam of reflected light was marked slightly to one side or the other of the center, the angle could have been observed to be greater or less than the angle of reflection.
• Ask students: Are the slight discrepancies we discussed reasons to think that the Law of Reflection is not true? No. Scientists try to keep errors to a minimum. The errors discussed would cause only slightly different results, so such small variability does not change the validity of the Law of Reflection.

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C. Ask students to locate the passage in Problem 7 in their Student Data Record. Ask a student volunteer to read the passage aloud to the class.

Josh just finished a lab experiment that investigated the reflection of light. He demonstrated that light reflects off of a surface at an angle that is the same as the angle at which it strikes the surface. Josh knows that the Law of Reflection states that the angle of incidence is always equal to the angle of reflection.

Josh decides to investigate reflection further at home. Josh shines a flashlight on his bathroom mirror and easily sees the light reflected off the mirror.

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 Josh remembers that to keep cool in the Summer he needs to wear light-colored clothing because light-colored clothing reflects the light from the Sun. He assumes that he should be able to see light reflect off of light colored clothing. He tries shining a flashlight on a white shirt, expecting to see that the light is reflected but to his surprise, he does not notice any reflection of the light.

1. Ask students the following questions to assess their understanding of reflection in the context of the passage.

• Josh shines his flashlight on what two surfaces? Josh shines his light on a mirror and on a white shirt.
• What happens to the light that is shined on the mirror? The light shined on the mirror is reflected.
• What happens to the light that is shined on the white shirt? Student answers will vary. The passage suggests that Josh cannot observe any reflected light off the white shirt.

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2. Ask students: Why didn’t Josh see any light reflected off of the white shirt? Student answers will vary. 

Instruct students to perform the following activity to answer this question.

• Ask students: Describe the surface of a mirror. The surface of a mirror is shiny, smooth, and flat.
• Refer students to the diagram of light reflected off of a smooth surface in the upper part of the slide.
• Ask students, Describe the surface of a cloth shirt. Student answers may vary. The surface of a cloth shirt rough and not smooth or flat like the surface of a mirror.

Cloth is textured and has small ridges from the threads that are used to weave the shirt. As a matter of fact, the background of this slide is simply an un-close photo of a white cotton shirt!

Refer students to the lower diagram of light reflected off of a rough surface.

• • Ask students, What do you notice about the direction of the light reflected off a rough surface like cloth? The reflected light travels in a many different directions.
  • Inform students that since the light reflected off of a rough surface is reflected in many different directions, it is more difficult to observe than light reflected off a smooth surface. Compared to the mirror, Josh cannot easily observe the light reflected off the white shirt because the surface of the shirt is rougher than the surface of the mirror.
  • Instruct students to answer Problem 7c in their Student Data Record. Problem 7c states: Why was Josh unable to observe the light reflected off of his white shirt? The light reflected off of Josh’s shirt was reflected in many different directions. It was more difficult to see than the light that reflected off of the flat surface of the mirror because it was dispersed in many directions, not in just one direction like the light reflected off of the mirror.

Inform students that the reflection of light occurs off of every surface, not just those that are smooth and flat like a mirror. In fact, if light did not reflect off a surface, we would not be able to see it. The reflection of light will be further studied in subsequent Investigations in the Light and Optics CELL.

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