## Core Concept
The critical angle is a fundamental concept in optics, particularly relevant in ophthalmology. It is defined as the angle of incidence above which total internal reflection occurs. This concept is crucial in understanding how light behaves when it passes from one medium to another with a different refractive index, such as from the cornea into the air.

## Why the Correct Answer is Right
The critical angle at the cornea-air interface is approximately 46 degrees. This value is derived from Snell's law, which describes how light bends—or refracts—when traveling from one medium to the next. The critical angle is the angle of incidence in the denser medium (cornea) for which the angle of refraction in the rarer medium (air) is exactly 90 degrees. Beyond this angle, light is totally internally reflected back into the cornea. The refractive indices of cornea and air are approximately 1.376 and 1.00, respectively. Using Snell's law: (n_1 sin(theta_c) = n_2 sin(90^circ)), where (n_1 = 1.376), (n_2 = 1.00), and (sin(90^circ) = 1), we find (sin(theta_c) = frac{1}{1.376}), which yields (theta_c approx 46^circ).

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because the angle provided does not match the calculated critical angle for the cornea-air interface.
- **Option B:** This option suggests a much smaller angle than the actual critical angle, which would not allow for total internal reflection at the cornea-air interface.
- **Option D:** This option suggests a larger angle than the correct critical angle, which does not accurately represent the threshold for total internal reflection at this interface.

## Clinical Pearl / High-Yield Fact
A key clinical application of the critical angle concept is in the understanding of total internal reflection within the eye, particularly in the context of corneal injuries or diseases that might affect the refractive indices of the cornea. For example, in a condition like corneal edema, where the cornea becomes thicker and its clarity decreases, the changes in the refractive index could theoretically affect the critical angle, although this is more relevant in specific diagnostic or surgical contexts.

## Correct Answer Line
**Correct Answer: C. $46^circ$.**