The eyes lead the body.
(Dr J. Sheedy, School of Optometry, University of California at Berekely)
Light is electromagnetic radiation that is visible. The electromagnetic spectrum is extremely wide but the visible part is extremely narrow (Figure 10.1).
The eye is a fluid-filled membranous sphere that converts electromagnetic radiation into nerve impulses that it transmits to the brain along the optic nerve (Figure 10.2). Light enters through a transparent outer covering called the cornea. The cornea plays a major role in refracting the light. Further refraction occurs as the light passes through the lens. The pupil works like the aperture of a camera to vary the amount of light entering the eye. In bright light, the iris contracts, the pupillary diameter decreases and only the central part of the lens forms an image on the retina. In poor light, the iris expands and a larger area of the lens is used. Because the peripheral regions of the lens focus the light slightly in front of the image formed by the central part (a characteristic of all simple lenses termed 'spherical aberration') slight blurring
Figure 10.1 The electromagnetic spectrum. Visible light falls in the range 380-760 nanometers (nm).