Physics: Optics If You Can t See Me, Can I See You? SUMMARY Students use their knowledge of optics and the law of reflection to assess the message often seen on the backs of buses and trucks: If you can t see my mirrors, I can t see you. On diagrams showing the positions of vehicles, mirrors, and drivers, students draw ray diagrams to determine whether this statement is true. (Note: The activity can be used as the framing question for a unit on the optics of mirrors, as an in-class or homework assignment during the unit, or as part of an assessment at the end of a unit.) CURRICULAR PLACEMENT This activity can be used when studying optics in physics or physical science courses. OBJECTIVES Apply the law of reflection to a real-world issue of driving safety. MATERIALS NEEDED Copies of Reproducible Master 1: Mirror, Mirror Multiple copies per student of Reproducible Master 2: On the Road Protractors (one for each pair) Rulers (one for each pair) Copies of Reproducible Master 3: Driving Behind the Bus Scissors (one for each pair) Sheet of paper (one for each pair) CLASSROOM ACTIVITY 1) Distribute Reproducible Master 1: Mirror, Mirror. Ask students, Have you ever seen the statement If you can t see my mirrors, I can t see you, on the back of a bus or a truck? Tell them that from here on, you ll refer to this as the mirror warning. Ask students, Do you think this is true? Why or why not? 2) Explain that students will first look at possible ways to position a bus mirror and determine the best one. Have students work in pairs, and give each pair three copies of Reproducible Master 2: On the Road, a ruler, and a protractor. Have pairs work together to use their protractor and ruler to sketch the field of view for a few different mirror positions. They should decide which position gives the bus driver the best view of the cars next to and behind the bus. Developed by Education Development Center, Inc. Page 1 of 6
3) Have students share their results with the class and explain how they chose the best position for the mirror. If possible, have students show on the board how he or she used the law of reflection to determine the limits of what the driver could see. Sample answer: 4) Distribute Reproducible Master 3: Driving Behind the Bus and have students work in pairs to complete it. Students will go through three steps to complete the RM: Step 1: Students begin testing the mirror warning. They may want to model the situation by cutting out a rectangle to represent an average car (in proportion to the bus on RM 2) and trying it in different positions on the RM to determine the best mirror position. Students will also try to find any situations in which the mirror warning is false. Teacher Tip There should not be any situations in which the mirror warning is false, with one minor exception: It s possible that the bus driver may be able to see a small corner of the car even if the car driver can t see the bus s mirror. Developed by Education Development Center, Inc. Page 2 of 6
Step 2: Students investigate the inverse of the mirror warning: If the car driver can see the bus mirror, does that mean that a bus driver looking in the mirror can always see the car? Again, the students can test this by using their paper model. Teacher Tip The inverse of the mirror warning is not always true. There are many positions in which the car driver can see the bus mirror but the bus driver cannot see the car in the mirror. Step 3: Since students can t test every possible mirror angle and car position to prove or disprove the mirror warning, ask them to use what they know about the law of reflection to describe more generally, in words supplemented by a diagram, why the warning is or isn t always true, and why the opposite of the mirror warning is not always true. 5) As a class, discuss what students found. Ask, How can you explain why the mirror warning is always true? What does this have to do with driving safety? EXTENSION Remind students that If you can t see my mirrors, I can t see you is a statement written on many buses and trucks to remind other drivers of safety considerations. Given what they know about safe driving, what other safety statement would they want to see written on buses, trucks, or cars? Have students work with a partner to do the following: Choose a statement that reflects a safety concern they believe is especially important. Choose the statement s exact words and any design elements they d like to include, such as color, symbols, or images, or a particular size or type of font. Decide which vehicles should carry their statement and where on the vehicle it should go. Present their idea to the class and explain why they think this is an important safety message. Developed by Education Development Center, Inc. Page 3 of 6
Reproducible Master 1 Mirror, Mirror As a driver, you pay close attention to what you see in your own rear- and side-view mirrors. But this message, often seen on the backs of buses and trucks, says that you also need to pay attention to other vehicles mirrors. Positioning the side mirrors on a bus is a complicated process. It s so important to safety that the placement of mirrors on school buses is regulated by federal law! Your challenge: Use your knowledge of optics, particularly the law of reflection, to determine if this warning is true. Developed by Education Development Center, Inc. Page 4 of 6
Reproducible Master 2 On the Road Imagine that you are driving this bus. How would you position your mirror? To simplify this question, consider only one mirror (buses usually have multiple mirrors on each side) and look at the problem in only two dimensions (real mirrors need to be adjusted left-to-right and upand-down). Draw a line segment to show a possible position for the side-view mirror. Then draw a ray diagram and shade the area that would be visible to you (the bus driver) when you look in that mirror. Use multiple copies of this RM to try different mirror positions, then choose the position you think is best. Developed by Education Development Center, Inc. Page 5 of 6
Reproducible Master 3 Driving Behind the Bus Now that you ve considered what a bus mirror enables a driver to see, it s time to reexamine the mirror warning: If you can t see my mirrors, I can t see you. Step 1: Test the mirror warning. You can model it by cutting out or drawing a car-sized rectangle (in proportion to a drawing of the bus) and placing it in different positions on your completed RM 2. Can you find any situations in which the mirror warning is false? Car Step 2: Investigate the inverse of the mirror warning: If you can see my mirrors, I can see you. Is this always true? Again, you can test this using the paper model. Step 3: It s not possible to test every mirror angle and car position to prove the mirror warning. Use what you know about the law of reflection to describe more generally, in words supplemented by a diagram, why the mirror warning is or is not always true. Developed by Education Development Center, Inc. Page 6 of 6