A fish rising vertically upward with a uniform velocity of \( 8 \, \text{m s}^{-1} \), observes that a bird is diving vertically downward towards the fish with the velocity of \( 12 \, \text{m s}^{-1} \). If the refractive index of water is \( \dfrac{4}{3} \), then the actual velocity of the diving bird to pick the fish, will be ________ \( \text{m s}^{-1} \).
