I tend to differ both with elessar and nikhil. Because as the height of water column has been given hence we cant use the formulae as they assume 0 width for lenses.
The solution can be obtained by first principles.
If you look at the figure then you will see for condition of image being formed on the object the rays hitting the concave mirror should be normal to it. This means that the refraction on the water surface should be such that the rays appear to come from centre of curvature of the concave mirror. This i have shown in the figure. Thus we can say that for small apperture -
sin i 
tan i = MN/OM sin r tan r = MN/IM
Hence
Refractive
= sin i/ sin r = (MN/OM)/(MN/IM) = IM/OM = (R-h)/OM
Hence OM = (R-h)/
Hope this helps !
Moderation Team
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