You can't determine the focal length with just the radius of curvature and index of refraction. You need additional information, such as the type of lens (concave or convex).

Here's why:

* Lensmaker's Equation: The lensmaker's equation relates focal length (f), index of refraction (n), radius of curvature of the first surface (R1), and radius of curvature of the second surface (R2):

1/f = (n - 1) * (1/R1 - 1/R2)

* Concave vs. Convex: The sign convention for the radii of curvature depends on whether the lens is concave or convex:

* Convex Lens: R1 and R2 are positive if the center of curvature is on the same side as the incoming light.

* Concave Lens: R1 and R2 are negative if the center of curvature is on the opposite side of the incoming light.

To find the focal length, you need to know:

1. Type of lens (concave or convex): This determines the sign convention for the radii of curvature.

2. The radius of curvature of the other surface: You have provided only one radius of curvature.

Example:

Let's say you have a convex lens with a radius of curvature of 0.70 m for the first surface (R1 = 0.70 m) and an index of refraction of 1.8. To find the focal length, you'd need the radius of curvature of the second surface (R2).

Once you have both radii, you can plug the values into the lensmaker's equation to calculate the focal length.