Background Information
A critical role of the accessory structures of the eye (the cornea, the lens etc) is to cause refraction of light rays to bring them to a sharp focus on the retina so we can see clearly. This is one of the properties that underlies visual acuity. Visual acuity involves the ability of the eye to distinguish fine detail. Visual acuity is best for "central" vision (vision involving retinal positions at or around the fovea – which is why to see an object clearly, we train our eyes on to it, so that the light rays fall on the fovea). Snellen's Test Types are the most commonly used to assess acuity of distant vision. These charts present letters or symbols (called optotypes ) of decreasing sizes in numbered rows as seen below.
If seen from the distances stated for each line, each symbol subtends a visual angle of five minutes. The relevant details of the symbols (for example the gaps between the horizontal lines in an E) subtend visual angles of one minute.
Myopia (or short-sightedness) is a group of conditions leading to light focusing in front of the retina, resulting in blurred vision of distant objects – ie a decrease in visual acuity. In the following simulation you will investigate the effects of different degrees of myopia on visual acuity.
The simulation involves positioning a person at different distances from a Snellen chart. This in turn will change which rows in the Snellen chart can be seen clearly. You will first be collecting visual acuity data for a person with normal vision, followed by people with varying degrees of myopia.
