Final answer:
When using the Snellen letter eye chart, a child should stand or sit at least 20 feet from the chart to assess their visual acuity properly.
Step-by-step explanation:
The student's question pertains to the proper usage of the Snellen letter eye chart typically found in an optometry office to assess visual acuity. The standard testing distance for the Snellen chart is at least 20 feet.
During the test, various lines of Roman letters in decreasing sizes are presented. The purpose is to determine the smallest line of letters that a person can read from this set distance. For example, if a person is able to read the line of letters that subtends a visual angle of 5 minutes of an arc at 20 feet, they are considered to have normal visual acuity.