What is the power of the lens used to view retina with this instrument?
**Core Concept**
The question is related to the anatomy and optics of ophthalmoscopy, specifically the lens power required to visualize the retina with an ophthalmoscope. The correct answer is related to the type of lens used in an ophthalmoscope to obtain a clear view of the retina.
**Why the Correct Answer is Right**
The correct answer is 15 diopters (-15 diopters). This is because an ophthalmoscope is typically used to view the retina, which is located at a distance of about 1.5 cm from the cornea. A lens with a power of -15 diopters would converge light rays from the retina to form a clear image at the observer's eye. This is in contrast to a direct ophthalmoscope, which uses a lens with a power of +78 diopters to view the retina directly.
**Why Each Wrong Option is Incorrect**
**Option A:** 10 diopters is too weak to provide a clear view of the retina, resulting in a blurry image.
**Option B:** 20 diopters is too strong and would result in an over-converged image, making it difficult to view the retina.
**Option C:** 5 diopters is also too weak and would not provide a clear view of the retina.
**Clinical Pearl / High-Yield Fact**
When using an ophthalmoscope, it's essential to adjust the lens power to obtain a clear view of the retina. A power of -15 diopters is typically used to view the retina in a healthy individual. However, the power may need to be adjusted in patients with high myopia or other conditions that alter the shape of the eye.
**Correct Answer: D. -15 diopters**