What is the angle subtended by the topmost letter in the Snellen’s chart at the Nodal point of the eye when the person seeing it at 6m from the chart?
**Core Concept**
The Snellen's chart is a visual acuity test used to measure a person's ability to distinguish between different sizes of letters at a standard distance. The chart is calibrated so that a person with normal vision can read the top line at 6 meters. The angle subtended by the topmost letter at the nodal point of the eye is a critical factor in determining visual acuity.
**Why the Correct Answer is Right**
The Snellen's chart is designed so that the top line subtends an angle of 5 minutes of arc at the nodal point of the eye. This is based on the idea that a person with normal vision can distinguish between two lines that are separated by an angle of 5 minutes of arc. The nodal point is the point in the eye where light rays converge after passing through the cornea and lens. At this point, the angle subtended by the topmost letter is measured in minutes of arc, which is a unit of angular measurement.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the angle subtended by the topmost letter at the nodal point is not 1 minute of arc. While the Snellen's chart does involve angular measurements, the angle subtended by the topmost letter is actually 5 minutes of arc.
**Option B:** This option is incorrect because the angle subtended by the topmost letter at the nodal point is not 10 minutes of arc. The Snellen's chart is designed to measure visual acuity at a specific distance, and the angle subtended by the topmost letter is a critical factor in determining this measurement.
**Option D:** This option is incorrect because the angle subtended by the topmost letter at the nodal point is not 20 minutes of arc. While the Snellen's chart does involve angular measurements, the angle subtended by the topmost letter is actually 5 minutes of arc.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Snellen's chart is a standardized visual acuity test that measures a person's ability to distinguish between different sizes of letters at a standard distance. The angle subtended by the topmost letter at the nodal point is a critical factor in determining visual acuity, and a person with normal vision can read the top line at 6 meters.
**Correct Answer:** C. 5 minutes of arc.