A periscope lets you see over walls.

But what happens if you rotate one of its mirrors?

Does the image remain the same?

Or does the entire world begin to rotate?

This experiment reveals that a periscope is much more than a viewing tube—it is a device that transforms images in surprising ways.

Building the First Periscope

The first design used:

• One straight PVC pipe
• Two L-shaped PVC joints
• Two plane mirrors

Each elbow was cut with a 45° slot.

The mirrors were trimmed using a glass cutter and inserted into these slots so that each mirror was inclined at 45°.

After assembling the elbows onto the straight pipe, a working periscope was obtained.

Because the elbows could rotate independently, this became a rotatable periscope.

Observation 1 — The Ordinary Periscope

With both mirrors in the usual orientation, the distant scene appeared exactly as expected.

The observer could look over obstacles without changing position.

The light underwent two reflections before reaching the eye.

Observation 2 — Rotating One Mirror

One elbow was rotated by 45°.

The same object was observed again.

The image itself had rotated by approximately 45°.

Nothing else had changed.

Only the orientation of one mirror.

This immediately raises an interesting question:

How can rotating one mirror rotate the entire image?

Observation 3 — Looking Behind You

The elbow was rotated further until the periscope faced backward.

Now an even more surprising effect appeared.

The image had rotated by approximately 180°.

The periscope was no longer simply changing the viewing direction.

It was also changing the orientation of the image itself.

This behaviour can be understood by carefully tracing how successive reflections transform the direction of light.

Why Does This Happen?

Each mirror changes the direction of incoming light according to the law of reflection.

When the orientation of one mirror changes, the relationship between the two reflections also changes.

The final image therefore undergoes a geometric transformation.

The periscope becomes a beautiful demonstration that:

Changing the orientation of optical elements changes not only where we look, but also how we see the scene.

This is an invitation to explore ray tracing and geometrical optics.

Building the Second Periscope

A second design focused on versatility.

Two sheets of chart paper were rolled into cylinders.

One cylinder slid smoothly inside the other.

This created an extendable viewing tube.

Two folded cardboard cuboids were attached to the ends.

Each contained a mirror mounted at 45°.

Circular openings allowed the cylindrical tube to pass through the cuboids.

The result was a periscope that could:

• Extend to different heights.
• Rotate about its axis.
• Be collapsed for easy storage.

Real-World Uses

Periscopes have been used wherever direct line of sight is impossible.

Examples include:

• Submarines observing above the water surface.
• Armoured vehicles.
• Observation posts.
• Inspection of inaccessible spaces.
• Industrial machinery.

Although many modern systems now use cameras and electronic displays, the underlying geometric principle remains the same.

A Deeper Realization

The remarkable feature of a periscope is that it never bends light.

Every ray still obeys the simple law of reflection.

Yet by combining two mirrors in different orientations, we can completely change where we look and even rotate the apparent world.

Complex optical instruments are often built from surprisingly simple components arranged in clever geometries.

Continue the Investigation

The experiment you've just explored is only the beginning. Our hands-on investigation sets are designed to help you recreate, extend, and deepen these ideas through observation and experimentation.

Every investigation has the potential to lead to a new question. If you discover something interesting, improve the experiment, or develop a new variation, share it with the Geometers community. Your work may inspire others and could even be featured here.

Keep experimenting. Keep questioning. Keep discovering.