**Core Concept**
Retinoscopy is a clinical technique used in ophthalmology to determine the refractive error of the eye. This involves measuring the reflection of light from the retina in response to a light source, providing information about the eye's refractive power. It is an essential tool in the diagnosis and management of refractive errors.

**Why the Correct Answer is Right**
Retinoscopy is based on the principle that the reflection of light from the retina is influenced by the refractive state of the eye. When a light source is shone into the eye, the reflection from the retina will be deviated by the amount of refractive error present. By observing the movement of the reflection, the examiner can determine the presence and type of refractive error, such as myopia, hyperopia, or astigmatism. This information is critical for prescribing corrective lenses or refractive surgery.

**Why Each Wrong Option is Incorrect**
**Option A:** Examination of Retina - While retinoscopy involves observing the reflection from the retina, it is not primarily used for examining the retina itself. Other techniques, such as fundoscopy, are more suitable for this purpose.
**Option B:** Assessing surface of cornea - Retinoscopy does not directly assess the surface of the cornea. Instead, it measures the refractive error of the entire eye, which may be influenced by the cornea's shape and curvature.
**Option D:** Examination of vitreous - Retinoscopy is not used to examine the vitreous humor, which is the clear gel-like substance inside the eye.

**Clinical Pearl / High-Yield Fact**
Retinoscopy is a non-invasive and objective method for assessing refractive errors, making it an essential tool in ophthalmology. It is particularly useful in pediatric patients, who may not be able to cooperate with other refractive error assessment methods.

**✓ Correct Answer: C. Refractive errors. Refractive errors.**