The resolving power of an optical microscope is determined by its ability to distinguish between two closely spaced objects. The resolving power \( R \) is inversely proportional to the wavelength \(\lambda\) used for observation. Mathematically, it can be given by:
R \propto \frac{1}{\lambda}
Given two wavelengths, \(\lambda_1 = 4000 \mathring{A}\) and \(\lambda_2 = 6000 \mathring{A}\), the ratio of resolving powers \(R_1\) to \(R_2\) is:
\frac{R_1}{R_2} = \frac{\lambda_2}{\lambda_1}
Substituting the given values:
\frac{R_1}{R_2} = \frac{6000}{4000} = \frac{3}{2}
Thus, the ratio of the resolving powers is 3:2, which corresponds to the correct answer from the given options.
Conclusion: The correct answer is 3:2. This choice is justified because resolving power increases with a decrease in the wavelength, thus for smaller wavelength (\(4000 \mathring{A}\)), the resolving power is higher.