Mirror, Mirror on the Wall...
We see ourselves in a mirror because mirrors reflect light. Light interacts with materials in different ways; for instance, shiny surfaces like mirrors are able to change the direction of light. In this lesson plan, students will demonstrate how light bounces off a reflective surface by redirecting a light beam from a flashlight to different locations within the classroom. Students will then be challenged to use their knowledge about mirrors to find a way to guide a light beam to a specific target using multiple mirrors.
- Understand that some materials redirect light.
- Describe how a mirror works.
- Investigate how you can redirect a light beam using reflective materials.
NGSS AlignmentThis lesson helps students prepare for these Next Generation Science Standards Performance Expectations:
- 1-PS4-3. Plan and conduct investigations to determine the effect of placing objects made with different materials in the path of a beam of light.
|Science & Engineering Practices||Disciplinary Core Ideas||Crosscutting Concepts|
|Science & Engineering Practices||Planning and Carrying out Investigations.
Plan and conduct investigations collaboratively to produce evidence to answer a question.
||Disciplinary Core Ideas||PS4.B: Electromagnetic Radiation.
Some materials allow light to pass through them, others allow only some light through and others block all the light and create a dark shadow on any surface beyond them, where the light cannot reach. Mirrors can be used to redirect a light beam.
||Crosscutting Concepts||Cause and Effect.
Simple tests can be designed to gather evidence to support or refute student ideas about causes.
Per student group of 2:
- Three small square mirrors (4"x4"). Note: The glass edges of the mirrors might be sharp. Cover them with tape to prevent students from cutting themselves. Alternatively, you can use other highly reflective materials, such as old CDs or squares of heavy-duty flat aluminum foil.
- 6 clothespins or binder clips
- Flashlight, ideally with a narrow beam
- Marker or pen
- Printed target
- White cardstock paper
Background Information for TeachersThis section contains a quick review for teachers of the science and concepts covered in this lesson.
Light mostly travels through space in straight lines. This is why the concept of light rays is commonly used in physics. A light ray is represented as a straight line or arrow pointing away from the light source that indicates the direction of light (Figure 1, left). A bundle of parallel light rays is called a beam of light (Figure 1, right). Picturing light as lines makes it possible to illustrate what happens if light hits an object that is in its way.
Figure 1. A straight line or arrows traveling from a light source depicts a light ray (left) or a light beam (right).
Materials can interact with light in different ways. Once light hits a material, the light can either be absorbed, transmitted, or reflected. Often, a combination of absorption, transmission, and reflection happens. How we see a material depends on how the light interacts with the object. We can only see materials that reflect or bounce back at least some light, or materials that emit light themselves. The reflection of light is similar to the bouncing of a ball in that the angle at which the light hits the reflecting surface is identical to the angle at which it bounces back (Figure 2). This circumstance is called the law of reflection. Additionally, the reflected ray, the incident ray, and the normal line all lie in the same plane, the plane of incidence (Figure 2).
A grey trapezoid represents a plane mirror. A line perpendicular to the mirror depicts the normal line. Two arrows pointing at an angle towards the mirror and away from the mirror represent the incident ray and the reflected ray. The angle of incidence and angle of reflection are equal. A rectangle illustrates the plane of incidence.
Figure 2. The law of reflection states that when a light ray hits a reflective surface, its angle of incidence is equal to its angle of reflection.
A mirror is a perfect example of reflection on a smooth material. Because it is so smooth, all light reflects at the same angle (Figure 3, middle) giving the material a shiny appearance. Materials that do not have a smooth surface reflect light in a diffuse way (Figure 3, right). Bumps on their surface cause light to reflect in many different directions. These materials appear matte.
Figure 3. Illustrations of how light reflects on shiny (middle) and matte (right) surfaces.
Using the law of reflection, you can predict where a light ray will go after hitting a smooth, reflective surface such as a mirror. In fact, you can draw a diagram of the light's path through a mirror maze if the incident angle for the first mirror is known. An example of such a diagram is shown in Figure 4.
Figure 4. A diagram showing the path of light through a mirror maze.
The fact that light reflects off a smooth surface is the reason why we can see ourselves in a mirror. We see objects when they reflect light that then enters our eyes. If we stand in front of a mirror, some of the light that bounces off of us hits the mirror. The mirror reflects this light back into our eyes, and we see an image of ourselves! Mirrors are extremely smooth. This makes light bounce back in a regular pattern and leads to a clear image of objects seen in a mirror. Aluminum foil's surface is not as smooth, making light reflect in a slightly diffuse way. This explains the blurry image of objects seen in aluminum foil.
In this lesson plan, students will first investigate how mirrors can change the direction of light. Then they will use their gained knowledge to redirect a light beam onto a specific target using reflective surfaces.