When you take a container with water and begin to heat it, after a while, you will notice that bubbles begin to form. If you wait a bit longer, you will easily (it's so clear) start noticing smudgy heat waves in the water (which I believe is due to change in refractive index of the water). These waves tend to occur at discrete intervals and it takes quite a time for the first wave to be noticed.

What exactly are these waves and how are they created?

One possible explanation would be that waves of heat is moving through the liquid which causes temporary change in refractive index at its location but this explanation is quite weird in the sense that thermal energy is moving in packets instead of being transferred continuously.

  • $\begingroup$ The same kind of thing happens on top of a hot car. Remember that air is a fluid, just like water. $\endgroup$ Jan 9, 2018 at 17:25
  • $\begingroup$ @JakobLovern This one is very weird that it appears to move inside the water. $\endgroup$
    – Yashas
    Jan 9, 2018 at 17:30
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    $\begingroup$ Convection currents cause the movement of the water and as the temperature of the water is not uniform the refractive index of water is not the same. $\endgroup$
    – Farcher
    Jan 9, 2018 at 17:31
  • $\begingroup$ @Farcher Aren't those currents sort of continuous? The waves I'm talking about appear to happen at discrete intervals. Shooooosssh.... one wave.......soooooooosh... another wave..... $\endgroup$
    – Yashas
    Jan 9, 2018 at 17:33
  • $\begingroup$ @Yashas - The boiling process can, of course, cause quasi-repetitive movements of the water. $\endgroup$
    – freecharly
    Jan 9, 2018 at 17:38

1 Answer 1


You are correct, the "smudgy heat waves" are convectively moving areas of water with different temperature and thus different refractive index. This phenomenon has e.g. been used in Schlieren Photography.

  • $\begingroup$ Why do the waves appear in discrete intervals? Why does it take so much time (close to the boiling point) for these waves to start appearing? $\endgroup$
    – Yashas
    Jan 9, 2018 at 17:36
  • $\begingroup$ @Yashas - Water bubbles form during the heating of a container from below. This takes some time even at boiling temperature. Also, it takes a while that a big enough bubble is formed that will detach from the heated bottom and rise. After the detachment of the bubble a new bubble can form driven by the heat conduction ftom the bottom. This can happen at discrete time intervals and can give rise to the bubbling sound of boiling water. The movement of the bubbles also drives water currents in addition to the convection of the heated water itself. $\endgroup$
    – freecharly
    Jan 9, 2018 at 17:51

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