How can I measure changes in liquid level of the order of 50 μm?

The tank is cylindrical with a radius of about 12.5 mm and a total volume of about 45 ml. The fluid is water-based. The technique needs to be optical or similar because no external body can touch the solution.

I looked at ultrasonic sensors but they don't seem to reach such high precision. Now I'm checking laser sensors as well, but it seems that they don't satisfy the required precision either.