Optics - 1

A diver is looking up through the water sees the outside world contained in a circular horizon. The refractive index of water is $\dfrac43$ and the diver's eyes are $15~cm$ below the surface of water. Find the radius of the circular horizon as seen by the diver.

If the radius is given by $\dfrac{a \times b^2}{\sqrt{c}}$ where $a,b,c$ are co-prime positive integers, enter the value of $a + b + c$ .

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