Each letter on an acuity chart subtends what angle to the eye at a given distance?

The correct answer is based on the standard measurement of visual acuity, which is defined by the angle that letters on an acuity chart subtend at the eye. For letters on an acuity chart to be clearly seen as individual letters, they must subtend an angle of five minutes of arc at a distance of 20 feet (or 6 meters). This measurement is foundational in understanding how visual acuity is assessed in clinical settings.

The unit of angular measurement known as a minute (of arc) is important in optometry because it provides a clear and quantifiable way to define how well a person can see. Each letter, when viewed at the standard testing distance, needs to subtend this angle for it to be distinctly recognized. This means that a letter that subtends five minutes of arc at a distance where a person with normal vision can see it would indicate that the test subject has a visual acuity of 20/20.

In summary, the answer reflects the established principle that letters on acuity charts must subtend five minutes of arc for proper visual identification, underpinning the measurement of visual acuity in clinical practice.