Light: Shadows and Reflections · Lesson 5 of 6
Periscopes and Kaleidoscopes
“Arrange mirrors to see around obstacles and make changing patterns.”
• Trace light reflected by the two mirrors of a periscope. • Explain how a periscope lets a viewer see an otherwise hidden object. • Describe how three mirrors create multiple changing views in a kaleidoscope. • Plan safe constructions and investigations with two or more mirrors. • Use reflection to reason about what a small mirror can show.
Using Two Reflections to See Around an Obstacle
Suppose a wall is too high to see over directly. A periscope provides another route for light from an object beyond the wall to reach your eye. Put two flat mirrors inside a Z-shaped tube so that light entering its upper opening strikes one mirror, travels down the tube, strikes the other, and travels toward the lower viewing opening. The object need not be in a straight line with your eye.
A periscope does not make light curve continuously around a corner. On each stretch between the object, a mirror, the next mirror, and the eye, the light travels along a straight path. It changes direction at each mirror. This combines the two earlier ideas: straight-line travel and reflection.
Problem
You stand behind a taller friend and use a simple periscope. Explain how light from the scene ahead reaches you.
- 1.The upper opening is raised above the obstruction, so light from ahead can enter it.
- 2.The first mirror redirects that light down the tube. The second redirects it toward the viewing opening.
- 3.The light reaches your eye even though your direct line of sight is blocked.
Problem
You have a tube that bends like a simple periscope and two small plane mirrors. Where should they go?
- 1.Find the two places where a straight incoming light path must change direction.
- 2.Place a mirror at each bend, facing so the first sends light along the middle section and the second sends it to the eye.
- 3.Check the path with a safe torch beam or by looking at a well-lit object; adjust mirror tilts until the view appears.
Submarines can use periscopes to view outside while the observer remains below the surface; soldiers can view over a protective barrier. A classroom model can be made from a cardboard Z-shaped box. Secure mirror edges and avoid looking at the Sun or reflecting strong light into someone’s eyes.
Three Mirrors and Changing Patterns
A kaleidoscope makes patterns using several reflections. Arrange three equal-width mirror strips into a triangular tube, with their reflecting faces inward. Near one end place small coloured beads or pieces behind a transparent sheet and a tracing-paper cover. Look from the open end toward a lighted area. You see repeated views of the coloured pieces because one mirror can reflect an image already seen in another.
Turn the tube slightly. The coloured pieces move relative to the mirrors, and their repeated reflections form a new pattern. Pointing an open-ended version toward a tree or another bright object can also produce patterns. Artists and designers can use these changing arrangements as ideas. The device is an application of reflection, not a new light source.
Problem
A kaleidoscope has three inward-facing mirrors and a few coloured beads, yet the viewer sees many shapes. Why?
- 1.The beads provide the original coloured objects.
- 2.Each mirror reflects them and can also reflect images produced by another mirror.
- 3.These multiple reflections create repeated parts of the pattern. Turning the tube changes the arrangement.
Investigate More than One Mirror
Place two plane mirrors at an angle and observe an object between them. Change the angle, then try holding the mirrors parallel. Record how many images you can distinguish and whether the repeated views change. This investigation extends the repeated-reflection idea without needing a numerical rule for the number of images.
A small mirror can sometimes show an image of a much larger tree. The mirror does not have to be as large as the tree: it only needs to redirect the light from the parts of the tree that enter the viewer’s eye through the available mirror area. What can be seen depends on the positions of the tree, mirror, and observer. Predict first, then test with a safely placed mirror.
A small plane mirror can give a view of a distant large object. The visible portion depends on the viewing position and light paths, so a small mirror does not guarantee that every observer sees the entire object.
| Device | Mirror arrangement | What the observer gains |
|---|---|---|
| Periscope | Two mirrors placed to redirect the path twice. | A view around or over an obstruction. |
| Kaleidoscope | Three mirror strips facing inward in a triangular tube. | Repeated, changing patterns. |
| Two-mirror investigation | Two mirrors at an angle or facing each other. | A chance to observe multiple reflections. |
Quiz
How many reflecting changes of direction does light normally make in the simple two-mirror periscope?
Why can a periscope show a scene blocked from direct view?
How are the three strips arranged inside the described kaleidoscope?
Why do patterns change when a kaleidoscope is turned?
Can a small plane mirror ever show a large distant tree?
On the route through a periscope, where does the light change direction?
Practice Problems
- Draw and label the object, both mirrors, and the viewer’s eye in a simple periscope; trace the light route with arrows.
- Explain why a periscope does not contradict straight-line travel of light.
- Describe how to test and adjust the mirror positions in a Z-shaped cardboard tube.
- Explain why a few beads can appear as many shapes in a kaleidoscope.
- Predict what might happen to the images when two mirrors are moved from an angled arrangement to a nearly parallel one; state what you would record.
- A small mirror shows part of a large tree. Explain why the mirror need not be tree-sized and what might limit the view.
- Compare how a periscope and a kaleidoscope use multiple reflections for different purposes.
Key Takeaways
• A simple periscope uses two plane mirrors to redirect light around an obstacle. • Light follows straight stretches between mirrors and changes direction at their surfaces. • Three inward-facing mirrors produce repeated views in a kaleidoscope; turning it changes the pattern. • A small mirror may show a large distant object, depending on the positions of object, mirror, and eye.