This is the new updated edition of the first book in the bestselling Uncovering Student Ideas in Science series. Like the first edition of volume 1, this book helps pinpoint what your students know (or think they know) so you can monitor their learning and adjust your teaching accordingly. Loaded with classroom-friendly features you can use immediately, the book includes 25 “probes”—brief, easily administered formative assessments designed to understand your students’ thinking about 60 core science concepts.

Access this probe as a Google form: English

Download this probe as an editable PDF: English 

The purpose of this assessment probe is to elicit students’ ideas about reflection of light. The probe can be used to examine how students use ideas about light to explain how we see objects in a mirror.

P-E-O

mirrors, reflection

The best response is C: She will still only see the same amount of her face, from her eyebrows to her chin. Despite the fact that we look into a mirror everyday and hundreds of times in a year, most people believe that the further away you are from a mirror, the more you will see of yourself. Our familiarity with mirrors clearly does not mean we understand how they work. This probe shows that experience is not always the best teacher. When light falls on a flat mirror it is reflected in a predictable way: the angle at which the light strikes the mirror (angle of incidence) and the angle at which it is reflected (angle of reflection) are the same. As long as the position of your eyes is in the same horizontal plane in relation to the mirror on the wall, it does not matter how far back or close you are to the mirror, you will still see the same image.

When you look into a mirror, you see the portions of your body from which light is reflected off the mirror and into your eyes. In the diagram on page 52, the person can see her head down to the middle of her chest. The light rays reflected to her eye from the bottom edge of the mirror determine the lowest visible portion of her body. The mirror reflects all light rays away at the same angle at which they arrived, so light rays from lower down on her body below the middle of her chest will be reflected to a point above her eyes, and will not be visible. When she steps back from the mirror, the light rays are reflected in the same manner as when she stood close to it. Although the light rays from it are reflected at a more shallow angle, the lowest visible portion of her body remains the same as when she was close to the mirror. The angle of light going in is equal to the angle of light reflected from the mirror, and thus the image stays the same regardless of how close or far back the mirror is.

Elementary Students

In the elementary school grades, students investigate reflection of light with mirrors. Their experiences are observational at this stage. Students are not expected to know how a mirror works. However, this probe is useful in helping students see that even though they may experience something every day and think they know what happens (i.e., believing they will see more of their face when they are further away from a mirror), their predictions do not always match their observations. Science can show us that what we think will happen, based on our everyday experience, is not always what actually happens, thus reinforcing the importance of testing our predictions rather than relying only on our experiences.

Middle School Students

At the middle school level, students learn how mirrors work and connect their expanding knowledge of light reflection to different types of objects that reflect light, such as mirrors. They can investigate how the angle at which light strikes and reflects off of a mirror into the eye determines what is seen in the mirror. However, even with instruction, students have difficulty understanding how an image appears in a mirror and is seen by the eye.

High School Students

At the high school level, students develop sophisticated ideas related to optics. They learn about how light interacts with different types of mirrors, such as concave and convex. However, as in the previous grades, their ideas about mirrors are strongly affected by their everyday experiences looking into mirrors.

You can model this scenario with a small rectangular mirror, about half the size of your face. Describe what is meant by placing the mirror at eye level and backing away from it.

• When children were asked if moving their position would change the position of an image on a mirror, over half thought that it would. Ninety percent of the children thought that moving back from a mirror would allow them to see more of themselves in the mirror (Driver et al. 1994).
• Difficulties in understanding how light travels contribute to students’ misconceptions about how light interacts with mirrors and how light must enter the eye in order to see an object (Driver et al. 1994).
• Mirrors are typically used in curriculum units on light to demonstrate characteristics of light reflection. Several studies have shown that students have difficulty understanding how an image forms on a plane mirror (Shapiro 1994).
• The Annenberg/CPB Private Universe Project (1995) asked high school students a question similar to that asked in this probe. These students too thought that they would see more of themselves as they backed up from the mirror. The video shows the surprise they experienced when they tried it and found they saw the same amount of their body.

American Association for the Advancement of Science (AAAS). 1993. Benchmarks for science literacy. New York: Oxford University Press. American Association for the Advancement of Science (AAAS). 2001. Atlas of science literacy. Vol. 1, “waves,” 64–65. Washington, DC: AAAS.

Driver, R., A. Squires, P. Rushworth, and V. Wood- Robinson. 1994. Making sense of secondary science: Research into children’s ideas. London and New York: RoutledgeFalmer.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.

Matkins, J., and J. McDonnough. 2004. Circus of light. Science and Children (Mar.): 50–54.

National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academy Press.

Robertson, W. 2003. Light, Stop Faking It! Finally Understanding Science So You Can Teach It. Arlington, VA: NSTA Press.

• This probe can be tested by students by placing a small, flat rectangular mirror at eye level flat against a wall and having students back up from the mirror. Students are often quite surprised to find that their prediction that they would see more of themselves as they backed away from the mirror, which was based on their everyday experiences, did not match their result. Seize this opportunity to help students understand why testing a prediction is important in science, even when you think you know what the result will be. Experience is not always the best teacher! Sometimes actual results are unexpected. Be aware that some students are so convinced they will see more of themselves when they back up, that they sometimes actually think they do when they try it!
• With older students, pose the question, how big does a mirror need to be to see your whole body? After drawing reflection diagrams that explain why a mirror only needs to be half your height to see your full body, regardless of how far back or close you are standing, this problem can be linked to the probe scenario.
• Be aware that many students will still not accept the finding that the image in a plane mirror stays the same regardless of distance and may try to test it in the bathroom. Some bathroom mirrors are placed on top of counters. The counter blocks light from the part of their body below the counter from being reflected to the mirror and then to the eye. When you back away from the counter, some of the light from the part of the body that was blocked by the counter is now able to reflect on to the mirror without obstruction and reflect back to your eye, thus allowing you to see more of your body than when you were standing close to the counter. Explain that the difference between this example and the example in the probe of the small, flat mirror on the wall is that there is no obstruction between the person and the small, flat mirror.
• To demonstrate the effect of the bathroom counter described above or other obstruction, place an 18- to 24-inch mirror on a table or counter and have students look into the mirror. Explain that the counter blocks light from the part of the body below the counter from being reflected to the mirror and then to the eye. Move the mirror to a wall. Have them observe how the light reflects without obstruction and reflects back to the eye, thus allowing the students to see more of their bodies than when they were standing close to the counter or table. Discuss the differences and why it seems as if backing up from a mirror in a bathroom makes it seem as if backing up from any mirror allows you to see more of yourself.
• Challenge students to find out why some mirrors, such as car mirrors, appear to reflect a much wider view.
• This probe can be used even if students are not learning about mirrors in the context of lessons about light reflection. The probe makes a case for why students should not rely solely on their everyday experiences when thinking about science ideas. Testing ideas is important because it sometimes reveals that what we strongly think, based on prior experiences, is not always the way the natural world works. Carefully testing our predictions helps us confront and develop new ideas related to understanding the natural world.