Hey!

you have not mentioned the height of tube, so considering it 'H'.

Now, let the tube is filled to 'h' height, then the part of tube which is empty/no water = (H-h)

So, apparent depth of the filled portion must be (H-h) cm. And as we know,

refractive index of water = apparent depth/real depth

1.33 = (H-h)/h

If we know the height of the tube and replace it with 'H', then we will be able to find 'h'.

Hope I answered your query,thankyou!