When a wave passes through different media, its frequency remains the same, but its velocity changes.

The wavelength is the ratio between velocity and frequency, so it changes proportionally.

If you multiply 36 kHz by 8326 m, you get a value only slightly less than the speed of light in vacuum, which is true for the propagation of electromagnetic waves in most gases.

On the other hand, with 170 m, you will get a speed of VLF radio waves in sea water that is much lower than in vacuum.

The speed of electromagnetic waves in most media depends strongly on frequency.

At frequencies corresponding with visible light, only in few materials the speed is lower than half of the speed in vacuum (i.e. the refractive index is greater than 2).

On the other hand, for low frequency radio waves, speeds that are 10 times slower or even 100 times slower than in vacuum are not unusual.

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mcswell 5 hours ago | parent [-]

I tried to do a little (web) research on this. It is of course the reason a prism separates white light into its components. I didn't find out much about sea water, though.

And then there's "slow glass", in which the passage of light through half an inch of glass takes years; the subject of the short story "Light of Other Days" :).

	▲	ninalanyon 4 hours ago \| parent [-]
		One of my favourite stories, heartbreaking though it is. Arthur C. Clarke and Stephen Baxter wrote a novel of with the same title, but the two stories have nothing in common. It's worth a read in these surveillance heavy times.