Whispering Dish

Make a feeble sound such as shaking a bunch of keys or coins in front of the ring, the focus of the Dish. The listener, standing in front of the focus of the other dish hears the sound very clearly.

The effect is due to the parabolic shape of the dishes and the positions of the speaker and the listener. They have to be stationed at the foci of the parabolic dishes. The waves reflected by the speaker’s dish are brought to a focus by the other dish.

Whisper a message standing at a particular position in front of one of the discs. The listener, standing in front of the other disc very clearly hears the whisper while no one standing in the path of the two discs can hear the sound.

The effect is produced by two factors – the geometry of the discs and the position where the speaker and the listener stand. The Discs are parabolic in shape. The jump of a frog along jumper, the motion of a discuss, a shot put and a javelin are parabolic. Several comets are known to move in parabolic orbits. Just as we describe a circle with a fixed point, namely its centre, a fixed point known as focus also describes a parabola. The focus lies in front lies in front of the parabolic curve. In this exhibit the position of the speaker as well as the listener is the focus of the parabolic discs. One of the properties of the focus of the parabolic surface as applied to sound is that the waves emanating at the focus and incident on the surface are reflected and rendered parallel. The reflected waves strike the parabolic surface at the other end and converge at the focus. Since sound waves converge, their intensity is maximum there. This is similar to burning a piece of paper in sunlight with the help of a converging (convex) lens, where the lens collects the parallel beam of sunlight and converges them to a point. At that point, the intensity of light is high.

Parabolic surface, because of this property to bring waves to a focus, thereby increasing the intensity of the waves, find a number of applications. The dish antenna we see with the cable operator is one such. The radio waves transmitted by the various channels are collected by the dish antenna. The receiver is placed at the focus of the antenna. The radio signals received are amplified and transmitted to our houses through cables.

Parabolic surfaces help the astronomers immensely. It is common knowledge that telescopes are used to study the celestial objects like the planets, stars and galaxies. The primary aim of such telescopes is not magnifying the object but to gather more light for accurate analysis. It is the study of light from those objects that ultimately leads to have a glimpse of the dynamic processes that punctuate their very existence. Many large telescopes, including the one at Vainu Bappu Observatory in Tamilnadu, have parabolic shapes to enhance their light gathering capacity.

Then, there are celestial objects that emit less energetic radio waves that our eyes are insensitive to. The parabolic antenna, like the ones used by the cable operators, sense the radio signals from objects located even beyond the realms of our galaxy, the Milky Way. An analysis of the radio waves provides additional information about the object hitherto not known through studying visible light alone. One advantage with radio telescope is that, one does not need highly polished surface that an optical telescope demands. Since the radio waves have longer wavelengths, even a parabolic mesh is akin to a mirror.