Corner Reflectors

Look into the mirrors and see yourself upside-down! Can you get away from your own reflection?

Explanation

If you can see a friend’s eyes in a mirror, they can see you!

Light bounces off a mirror’s reflective surface much like a ball bouncing off a hard surface. The angle at which it approaches is the angle at which it leaves.

To see yourself in a flat mirror, you need to stand directly in front of it. If you move to the side, light that bounces off you will hit the mirror at an angle and be reflected to someone who is standing at the other side of the mirror. So that person will see your reflection, while you see theirs!

The corner mirror is made of three mirrors that are all perpendicular to each other. Light first bounces off one mirror at the same angle at which it approached. It then hits a second mirror that is oriented at 90° to the first mirror. This mirror serves to redirect the light back to its source along a path that is parallel to the original light beam.

The net effect is that light is reflected off these mirrors and returned to its source. This type of mirror is also known as a retro-reflector.

NASA uses a retro-reflector on the Moon to track its distance from Earth in the Lunar Laser Ranging Experiment. Scientists time how long a laser beam takes to travel to the Moon and back and use this to calculate the Moon’s distance (average: 384 467 km).

Bicycle reflectors are composed of many tiny corner reflectors that bounce the light from car headlights back into the driver’s eyes.

Radar reflectors are mounted on sailing boats. These reflect radio signals from other boats, helping sailors detect each other when visibility is low.

A cyclist’s retro-reflector will beam light back at its source, but the cone of return is quite narrow. A driver sitting in a tall truck, bus or van will not see much of the reflected light, as they are seated too far from their headlights. This is why cyclists should use rear lights as well as rear reflectors.

Extras for Experts

Have you heard of the Venus Effect? The phrase comes from paintings of the goddess Venus who is looking into a mirror. People assume that Venus is admiring herself even though her face is pointed directly at the viewer of the painting, meaning that she is actually staring at the viewer instead!

This is often used in movies. The actor appears to be looking at themselves in the mirror, but if we can see the actor’s reflection, they can actually see the camera’s reflection instead!

Things to Try

Look at your image from various vantage points and guess at which point you will be able to see yourself in each mirror.
Next time you see a corner that is arrange like the three perpendicular mirrors, clap into it! The sound will bounce back to you.
Point a laser point or torch into the bicycle reflectors and watch the light go back to its source – place the torch near your eyes to see the reflection, or from a completely different angle to show how the light is no longer reflected at you.
Use a soft ball and bounce it off the mirrors from various angles to demonstrate that the ball comes back to you. This is the path that light is taking!
Dominant eye—stare directly into the 3-point corner of the corner mirror. The eye that appears to be closer to the corner is your dominant eye! Another way to test eye dominance is to keep both eyes open and point at a distant object. Keeping your finger pointed, open and close each eye in turn. Whichever eye still sees the finger pointed at the object is the dominant eye.

Questions to Ask

Have you ever wondered how mirrors work? Next time you see a surveyor’s instrument, look into it to see your eye staring back at you due to a corner reflector inside!

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