B

Step 1: Understand that the refractive index (B1) of a medium is generally inversely related to the wavelength of light. As the wavelength increases, the refractive index typically decreases, which means Option A is incorrect.

Step 2: The critical angle is defined as the angle of incidence above which total internal reflection occurs. Since violet light has a shorter wavelength and a higher refractive index than red light, the critical angle for red light is indeed greater than that for violet light. Thus, Option B is correct.

Step 3: During refraction, the color of light does not intrinsically change; rather, the different wavelengths (colors) of light refract at different angles, but this does not imply a change in color. Therefore, Option C is incorrect.

Step 4: A thin prism typically produces an image that is virtual due to its geometry, especially in standard configurations. Therefore, Option D is also incorrect.

Conclusion: The correct option is B.