Basking in reflected light

Meera Chandrasekhar and Dorina Kosztin

Light and how it behaves, has fascinated and puzzled humans since time immemorial. Much of what humans observe about light comes from how light interacts with the various materials within our view.

Young students’ conceptions about light can confound adults who have learned about the physics involved in how light behaves. Yet it is important for the teacher to know where students will trip up, so we can set up our teaching and discussions so that misconceptions are revealed, confronted, and dispelled.

In this article we will present a series of activities for use in the classroom. We begin with an activity to introduce students to how different kinds of materials and their surfaces modify the behaviour of the light that falls on them. This activity is followed by a reading page and an exploratory activity in which students examine the reflection of light. A quantitative activity, in which students discover the laws of reflection, is followed by an activity in which students apply what they have learned about light in building a periscope. These activities were used as part of an extra-curricular program called Exploring Physics, a program for students in grades 4-6 in Columbia, Missouri, USA.

All the activities that we will present are best performed in groups of two or three students. Students are given the materials, and are expected to make observations and discuss them among the group. They write their observations and interpretations of observed phenomena in their individual notebooks. It is all right if their initial statements are not completely accurate. Responses may be based on students’ misconceptions, and should be used by the teacher to guide later discussions to dispel misconceptions. As the activity progresses, the teacher asks student groups to share their observations so that the entire class can be involved in the discussion. The teacher should ensure that all groups provide inputs. One method of doing so is to call on each group to provide inputs on one aspect. If you have small dry-erase boards (whiteboards) or small chalkboards in the classroom, student groups can write or draw their inputs on their board, which they hold up while reporting back. The teacher guides the discussion with questioning so that students can reason through their deductions and justify their answers, or correct the answer if necessary.

Activity 1: How does light behave?

Purpose: How does light behave when it falls on different kinds of surfaces?

Materials:

• Light sources such as a ray box, pen laser, or flashlight. For young students, a small flashlight is the safest. Use a pen laser only if you are sure that your students will not point the laser at each other.
• A small mirror (approximately 5 x 5 cm2)
• Opaque materials with matte and rough surfaces, (e.g., cardboard, construction paper, plastic, wax paper) preferably a similar size as the mirror.
• Opaque materials with shiny surfaces, (e.g., aluminum foil)

Directions for students:
Point the source of light at the different materials and make observations about the behaviour of light after it has hit the surface.

Describe what the light does when it falls on different materials and different kinds of surfaces. Make a drawing of your observation, and enter your observations in a table. Use four columns in the table, with headings for the material, the kind of surface, your observations and a drawing of what you observed.

Compare and contrast the similarities and differences of what the light does when it falls on the various surfaces.

Activity 2: Reading Page: Light on Surfaces
When light falls on a surface, one of four things can happen – it can be reflected, refracted, scattered, or absorbed.

Reflection occurs when light bounces back from a surface. A skinny beam of light (such as a laser) still bounces back as a skinny beam. Reflection occurs from smooth surfaces. We usually think that reflection occurs from shiny surfaces, such as a mirror. A smooth, transparent glass surface, such as a window pane, can also reflect light, but less efficiently than a shiny surface will. The surface need not be flat, since curved surfaces reflect light too. Reflection of light is governed by a set of rules, which we will learn about in the next activity.

Refraction occurs when light travels from one material to another. The speed of light in different materials is different – light is fastest in vacuum, and slower in other materials. This change in speed causes light rays to bend when travelling from one medium to another.

Scattering is sometimes called diffuse reflection. Scattering usually occurs when the surface is not smooth – and by that, we mean smooth on the level of the wavelength of light (about 0.0000005 m). A sheet of paper is smooth, but not on that level. When light bounces off paper, each incoming ray might bounce off in a different direction. Scattering is why a whole room appears lit by a couple of light bulbs, and why the sky looks bright during daytime (and also why it looks blue).

Absorption occurs when a material absorbs some of the light falling on it. When the molecules of the material are able to absorb some of the light, the amount of light the material reflects, transmits, and scatters is less than the amount of light that falls on it. For example, when you look through sunglasses, the scenery looks dimmer than without sunglasses. That’s because the sunglasses absorbed some of the light that travelled through it. Some materials may absorb certain colours but not others. For example, a yellow-coloured sheet of cellophane absorbs a lot of blue and green light, so the light coming through it appears yellow.

Notice that absorption deals with the amount of light, rather than the direction of the light rays, as reflection, refraction, and scattering do.

Activity 3: The Reflection Maze

Purpose: How does light travel after it reflects?

Materials:
• Laser
• 10-15 wall blocks (1 cm x 8 cm x 8 cm wooden blocks that can stand on edge)
• Mirrors with bases
• Toy for target
• Flat surface – e.g., desk or whiteboard

Directions for students:
Build a maze of any shape you choose using the wall blocks. The corners don’t have to be right angles. Place the toy target at the maze exit.

Predict first: (don’t use the laser yet!) Place the mirrors and angle them (use your best guess) so when the laser is placed at the entrance to the maze, the laser beam can bounce around the corners and eventually reach the target. Draw the mirrors in your picture.

What criteria did you use to arrange the mirrors? Test your maze with the laser. What “tweaking” did it take for the laser to hit the target?

What is the least number of mirrors you could use to complete your maze? What did you find out about reflected light in this activity?

Activity 4: Straight Line Reflections

Purpose: What are the “rules” for reflection?

Materials for demonstration activity:

• A large mirror (preferably unbreakable)
• Wood blocks or books to support the mirror in a vertical position
• Six dry-erase or markers with fl at bottoms (so they can stand on end). Alternately, use a set of tall objects (about 1 cm dia. x 10 cm tall)
• A large protractor
• A white-board or large sheet of paper

Alternate materials for student version of activity: A small mirror, mirror stand, sheet of paper placed on cardboard, tape, 6 push-pins, and protractor.

Directions:
Support the mirror in a vertical position against the whiteboard using wood blocks or books.

Place three markers so that they form a straight line as a model of a ray of light directed at the mirror. This straight line models the incoming (or incident) ray of light. Draw the incident ray on your white-board. Note that students find the concept of a ray of light abstract, because it is not how they see reflections in a mirror. Setting up the markers in a line makes the concept of a ray of light more concrete.

Look along this incident ray by placing your eye level with the markers and sighting down the line. Use only one eye (close the other one!) Describe what you see.

To find the reflected ray of light, you need to just look in the mirror! When you looked along the incident ray, the three markers line up behind each other. Look directly in the mirror and find the angle where the images of the three markers line up.

This line is the direction of the reflected ray. You may need to move your head around; remember to close one eye.

Place three more markers along the path of this reflected ray, and draw a line on the whiteboard as the model of the reflected ray.

Draw a line along the mirror and remove the mirror. At the point where the incident and reflected rays touch the mirror, draw a line (P) on the whiteboard that is perpendicular to the mirror.

Use the protractor to measure the angles between the “incident” ray and the perpendicular, and the “reflected” ray and the perpendicular. What do you find?

Activity 5: Designing a Periscope

Purpose: How does a periscope work?

Materials:
• Small blackboard or whiteboard
• 10-15 wall blocks
• Two mirrors on mounts that hold them vertical
• Object, chalk/marker, protractor, ruler

Directions: In this activity you will set up mirrors so you can figure out how a periscope works. Follow the instructions below.
Draw a long horizontal line on the board using a ruler.

Place wall blocks.

Place the object at the spot marked with the star.

Look along the arrow as shown in the picture. Can you see the object? If not, why can’t you see it? What do you think you must do to see it?

To see around a corner the rays of light have to bend around the corner! How can we get the light around the corner using mirrors?

Try it: Place two mirrors facing each other on the horizontal line at the spots marked X.

Look along the arrow and adjust the mirrors so that you can see the object clearly. Describe what you need to do.

Now remove the blocks, but leave the mirrors in place. Measure the angle between each mirror and the horizontal line.

What was the angle for Mirror 1? For Mirror 2?

What do you observe about the two angles? Explain.

How did the mirrors allow you to see around the corner? Explain.

With a periscope, you can look over a wall too. You will have to turn the periscope so that the mirrors are above one another rather than side by side.

Conclusion
These activities will allow students to have concrete experiences that allow them to see why scientists use the concept of a ray of light. It might seem artificial to choose just one ray and examine its behaviour, until students realize that all rays of light that bounce off a reflecting surface must obey the same law of reflection. It’s just that the different rays might hit the mirror at different points, and the perpendicular against which we measure the incident and reflected rays might just fall at different points along the surface of the mirror.

This work was supported by the National Science Foundation, NSF-HRD-96-19140 and NSF-DUE 0928924. Several other activities can be accessed through the website of one of the authors, http://epic.physics.missouri.edu/PhysActivities.html

Meera Chandrasekhar is a Professor of Physics at the University of Missouri in Columbia, Missouri. Dorina Kosztin is a Teaching Professor at the University of Missouri in Columbia, Missouri. They can be reached at meerac@missouri.edu and kosztind@missouri.edu.