Final answer:
Without specific formulae or values, it is not possible to give an exact focal length for the concave lens immersed in a medium with a higher refractive index (1.75) than its own (1.5). However, the lens will act as a convergent lens under these conditions.
Step-by-step explanation:
When a concave lens of glass with a refractive index of 1.5 is placed in a medium with a higher refractive index of 1.75, the lens will no longer cause light rays to diverge as it did in air. The lens' capacity to diverge light depends on the ratio of its refractive index to that of the surrounding medium. Since the refractive index of the lens is now less than the refractive index of the medium, the lens will no longer function as a divergent lens.
The new focal length of the lens can be calculated using the Lens Maker's Equation, which tells us how the focal length of a lens depends on the radii of curvature of its surfaces (in this case, the same R for both sides), the refractive index of the lens material (1.50), and the refractive index of the surrounding medium (1.75). However, we do not have enough information to provide an exact focal length without the specific values or formula. But, generally, a lens will act as a convergent lens in a medium with a higher refractive index than its own and as a divergent lens when in a medium with a lower refractive index.