By Anupum Pant
Take a glossy black ceramic bowl and fill it with water. Now place it in the middle of a rotating wheel. What do you get?
Thanks to the complex play of centrifugal forces creating a gradient of forces as the function of radius, with earth’s gravity also doing its part, you get a parabolic shaped water surface. If this liquid you use is a liquid metal like mercury, or Gallium (at slightly higher temperatures), you’d have a parabolic mirror in a bowl.
Depending on the speed of your rotation, you can adjust the focal length of the mirror. That’s because the faster you rotate this wheel, the deeper mirror you have and the focal length is smaller. There, you have your own adjustable parabolic mirror.
A great thing about such mirrors is that they can focus parallel rays to a focal point. And this is what telescopes rely on. Large telescopes have large lenses, and it gets incredibly expensive to make these mirrors. So there are telescopes which use a liquid parabolic mirror like the one described above. However, their mirrors are huge.
This idea originated from Issac Newton, but he wasn’t successful in making a smoothly rotating platform. Quoting wikipedia…
The concept was further developed by Ernesto Capocci of the Naples Observatory (1850), but it was not until 1872 that Henry Skey of Dunedin, New Zealand constructed the first working laboratory liquid mirror telescope.
In fact, huge solid mirrors used in telescopes are also made using a similar method. That is to say, the melt is spun like this and is solidified.