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The lens maker formula for convex lens is:

\frac{\mu_1}{f}=(\mu_2-\mu_1)(\frac{1}{R_1}-\frac{1}{R_2})\\\\R_2=\infty,\frac{1}{R_2}=0,\ it\ is\ a\ plane\ surface.\\\\\frac{1}{f}=(2-1)\frac{1}{R_1},\ \ \ \ in\ air.\\\\f\ in\ air=R_1\\\\ In\ the\ liquid\\\\\frac{4/3}{f}=(2-\frac{4}{3})\frac{1}{R_1}\\\\f=2*R_1\\\\

So the focal length become two times that in air.

As the lens is immersed in a liquid of higher density than air,  its converging power has reduced.  Hence the focal length increased.

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