The eye, in common with many optical systems in practical use, is by no means optically perfect; the lapses from perfection are called aberrations. Fortunately, the eyes possess those defects to so small a degree that, for functional purposes, their presence is immaterial. It has been said that despite imperfections the overall performance of the eye is little short of astonishing.
Physiological optical defects in a normal eye include the following :
1. Diffraction of light. Diffraction is a bending of light caused by the edge of an aperture or the rim of a lens. The actual pattern of a diffracted image point produced by a lens with a circular aperture or pupil is a series of concentric bright and dark rings (Fig.1).
At the centre of the pattern is a bright spot known as the Airy disc.
2. Spherical aberrations. Spherical aberrations occur owing to the
Physiological optical defects in a normal eye include the following :
1. Diffraction of light. Diffraction is a bending of light caused by the edge of an aperture or the rim of a lens. The actual pattern of a diffracted image point produced by a lens with a circular aperture or pupil is a series of concentric bright and dark rings (Fig.1).
At the centre of the pattern is a bright spot known as the Airy disc.
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| Fig. 1. The diffraction of light. Light brought to a focus does not come to a point,but gives rise to a blurred disc of light surrounded by several dark and light bands (the 'Airy disc'). |
2. Spherical aberrations. Spherical aberrations occur owing to the
