Under What Conditions is Light Reflected?

Personal Lab 11

Background

We have found that light can travel through air, water and glass, but not opaque objects such as desk tops, doors, books, etc. Does all of the room light that strikes a glass of water pass through the glass and the water to the other side? What enables us to see the glass of water in a darkened room as we shine a flashlight through the glass? What does someone see when viewing the glass from the side? Why does the light pass through some materials and not others? What would happen if we replaced the glass of water with a mirror?

The art of making glass and mirrors from glass was developed many millennia ago by the most ancient civilizations who experimented with heating sand. What is the property of glass that makes it transparent to light? What is a mirror? How does light interact with a mirror? Do only mirrors reflect light?

Purpose

1) To explore the various ways that light interacts with mirrors of different shapes and with a glass fiber.

2) To generate hypotheses and alternative hypotheses to explain your observations.

flexible mirror	metal spoon
paper clip, large	strand of optical fiber
flashlight	cardboard disk with slit for flashlight
convex lens

Procedure

1. Hold (or tape) flexible mirror so it is flattened against a flat surface such as a tablet of paper, and view the mirror from a distance of one or two feet. Record what you see in the mirror (size, brightness, colors, etc.).

2. Clear off a table top. With the room lights on, place the bent paper clip upright bent paper clip on the table in front of the mirror. Place your eyes nearly level with the table top, move your head to various positions and record when you can and cannot see the bent paperclip in the mirror. Sketch the light paths from the bent paperclip to the mirror and to your eye for several positions of your head. Include the extreme positions, just before you lose sight of the bent paperclip’s reflection in the mirror.

3. Remove the flexible mirror from the tablet of paper, and bend the flexible mirror slightly, so that the bulge is toward you, making a convex surface. Record what you see, and repeat 2 above.

4. Bend the mirror so that it bulges slightly away from you, making a concave surface. Record what you see and repeat 2 above.

5. Hold a metal spoon up to your face at eye level, and observe what you see in both sides of the spoon. Move the spoon to various distances from your eyes, and also try holding the tip of a pen or pencil between your eyes and the spoon. Record details of what you observe in the spoon.

6. Examine the ends of the strand of optical fiber with one of the convex lenses. Put one end of the fiber optic strand near a bright light and observe the other end of the fiber. What happens when you put your finger over the end of the fiber closest to the light? What happens when you move your finger back and forth over the end of the fiber?

7. Extend the strand of fiber from the light source and around a corner and repeat 6. Aim the end of the fiber optic strand farthest from the light at the index card. Darken the room Record what you observe.

Questions

1. Write a general statement which describes the conditions that an image can be seen in a flat mirror (i.e. what are the limits for the relative positions of object, image and your eyes in order to see an image)? Use a sketch to illustrate.

2. Make a table summarizing the conditions in which you created images which were inverted (upside-down), not inverted (right-side up) and multiple.

3. Write a statement for a "law of reflection" from a flat mirror.

4. Write a statement for a "law of reflection" from a concave mirror.

5. Write a statement for a "law of reflection" from a convex mirror.

6. Complete the light path through the optical fiber shown below:

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Last modified 9 Aug 1997
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