A convex lens of refractive index \( 1.5 \) and focal length \( f = 18 \, \text{cm} \) is immersed in water. The difference in focal lengths of the given lens when it is in water and in air is \( \alpha \times f \). The value of \( \alpha \) is __________. (refractive index of water \( = \frac{4}{3} \))
