Parabolic mirrors

Do the walls have ears?

Two people are needed for this task. One person speaks into the ring at the centre of one reflector and the other listens at the ring of the other reflector.

The person with his/her ear next to the ring in the other reflector can hear the voice of the speaker – even a whisper – over the noise of the room.

The reflectors are parabolic in shape. The sound waves exiting the focal point at the centre of the ring are projected off of the concave surface in a parallel fashion towards the other reflector. When they reach the other reflector, they are all projected towards that reflector’s focal point. Without the reflectors, the sound of the speaker’s voice would simply spread to the surroundings, and only a small portion of the sound waves, insufficient for the human ear, would reach the listener.

The same principle holds true for light and heat. With the help of two parabolic mirrors, we can light a match. We simply station the mirrors at a distance of tens of metres from one another, hold a match at the focal point of one of the mirrors, and place a small, hot halogen light bulb at the focal point of the other.

A satellite dish on a rooftop operates on the same principle. The micro head located at the focal point effectively catches the TV signal sent by a communications satellite and reflected off from the dish. Satellite dishes always face the same direction, because the communications satellites remain stationary, with respect to the Earth, on their orbits at a height of 36,000 km above the equator. So, you can have ears on your roof, not just on your walls!

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Cat Matikainen