Up periscope!

A lesson on the reflection of light takes on new dimensions when kids build their own periscopes

Take a look around. Almost everything you see is the result of light reflecting off objects, entering your eyes and instantly being translated by your brain. Light travels through space in straight lines at 186,000 miles (300,000 km) per second and its effects appear to be instantaneous. As children understand the concept of light traveling in straight lines, they'll need to learn that one of three actions occurs when light hits an object: light either bounces off the object, goes through the object or is absorbed by the object. In this lesson, we'll focus on reflection of light.

Exploring the angles

When light strikes the surface of an object, most of it is reflected. The angle at which light strikes the object is called the angle of incidence. The angle at which light reflects off of the object is called the angle of reflection. The angle of incidence always equals the angle of reflection.

Light that strikes a smooth, polished surface such as a mirror is not scattered by surface roughness. As a result, all the light "beams" reflect off the mirror in the same direction, as shown below. On the other hand, when light hits a rough surface, such as a sheet of paper, the "beams" of light reflect and scatter, as shown below, and make the paper seem dull. If you have access to a microscope, students can take a closer look at how rough paper really is.

Build a periscope

Have you ever wished to be taller so you could see what was on a high shelf? Let's build a periscope! You'll be able to see not only high places but around corners as well.

Periscopes are optical instruments that use a system of mirrors to reflect images through a tube. Light from a distant object hits the top mirror and is instantaneously reflected 90[degrees] down the tube. At the bottom of the tube, the light hits another mirror and is reflected into the viewer's eyes. Here are plans for building a periscope that not only allows you to see higher but also rotates at 90[degrees] increments:

Materials:

two mirrors (5 cm x 5 cm)

utility knife

masking tape

one 8'' x 20'' (20 cm x 50 cm) sheet of black foam board or cardboard

ruler

scissors

silicone caulking

Procedures:

1. Divide the foam board into four equal parts. The width should be the same size as that of the mirrors you're using.

2. Using a utility knife, cut only halfway through the foam board on the three division lines. NOTE: An adult should cut the foam board.

3. On the first and third columns, cut slits at 45[degrees] angles, as shown at right. The slits must be long enough and wide enough for the mirror to slide through.

4. On the second and fourth column, cut two square holes approximately 1.5'' x 1.75'' (3.75 cm x 4.375 cm).

5. Carefully bend the foam board where the long cuts were made to form a 2'' x 2'' x 20'' (5 cm x 5 cm x 50 cm) rectangular prism. Tape the two edges together with masking tape.

6. Slide each mirror through the 45[degrees] slots. (Be sure the mirrors face each other inside the rectangular prism.

7. If you have any foam board remaining, cut two squares to cover the ends of the rectangular prism and glue them in place, as shown at left.

Make one end rotate

Materials:

thin cardboard 2'' x 8'' (5 cm x 12 cm)

white glue

utility knife

Procedures:

1. Measure and mark 4'' (10 cm) down on all sides and edges of the rectangular prism.

2. Carefully cut through the foam board and disconnect this four-inch section.

3. Bend the 3'' x 8'' (7.5cm x 20cm) cardboard strip every 2'' (5 cm) to form a square.

4. Glue half the width of the cardboard strip inside the edge of the longer periscope section, as shown at right. Allow to dry.

5. Place the two sections of the periscope back together. Pull the short section off, rotate it 180[degrees] and reassemble the periscope.

6. Look in your periscope. What is your angle of view?

7. Continue rotating the top section. Notice how the angle of your view changes dramatically!

Take a look around

Hold your periscope at eye level and look through the lower hole. What do you see? Then hold the periscope at eye level and look through the upper hole. What do you see?

Can you see your friends through the periscope without them seeing you? How did you do this?

Why do you think longer periscopes cause the images to appear farther away than when you look through a shorter periscope?

Extensions:

1. How could you find out how smooth your periscope's mirrors are?

2. Make a periscope that will allow you to examine the back of your body. How many mirrors do you need to install on the inside of your periscope to create the correct angle for viewing your back?

3. Make a periscope that extends in length and rotates.

4. Research the way in which periscopes on submarines work. (Hint: Lenses are added to submarine periscopes. Why do you think that is the case?)

Websites:

* Making a Periscope: www.opticsforkids.org/resources/5_Making_Periscope.pdf

* Periscopes: www.xited.co.uk/dandt/periscope/make.htm

* Up Periscope: www.exploratorium.edu/science_explorer/periscope.html

* Make a Periscope: www.orc.soton.ac.uk/OSA/lightwave/experiments/periscope/periscope.html

Books:

* Cut and Make Periscope by A.G. Smith (Dover Publishing Co., 1991, ISBN: 0-486-26426-2).

* Science Smart by Gwen Diehn, Terry Krautwurst, Alan Anderson, Joe Rhatigan and Heather Smith (Main Street, 2003, ISBN: 1-402-70515-8).

John Cowens teaches science at Fleming Middle School in Grants Pass, OR and is a Teaching Editor of Teaching K-8. E-mail: snewoc@yahoo.com

Copyright Early Years, Inc. Oct 2003
Provided by ProQuest Information and Learning Company. All rights Reserved