Our eyes respond well

Our eyes respond well to most printed material, which is characterized by its dense black characters with well-defined edges which contrast markedly from their light background. Healthy eyes can easily maintain focus on the printed page.
Characters on a computer screen, however, don't have this contrast, or well-defined edges. These characters are brightest at their centers and diminish in intensity toward their edges. Our eyes are unable to maintain focus and remain on plane with these images. They instead drift out to a point called the resting point of accommodation (RPA).
Our eyes constantly move to the RPA, and then strain to regain focus on the screen. This continuous flexing of the eyes' focusing muscles creates fatigue and the burning, tired eyes feeling.