Forming an image

The process of forming an optical image is one that is very well understood and frequently occurs. In the very act of seeing what is on this page the reader is forming a retinal image of its contents which is then conveyed to the brain in the form of electrical impulses for the complex task of interpretation and comprehension. What happens in the visual cortex is poorly understood but the formation of the retinal image via the lens of the eye is straightforward and can be followed by reference to fig. 1.1. The first stage in image formation is to direct towards the object some radiation (light in this case), part of which is scattered so that each point of the object becomes a secondary source of radiation which leaves in all directions. If we look in detail at what happens at a point of the object (fig. 1.1 (a)) we see that the radiation going off in different directions is not only coherent, because it derives from the same point source, but is also all in phase. Next the scattered radiation strikes a lens (fig. 1.1(b)). Because the speed of light in the lens material is different from that in air, rays travelling by different paths to the image point have the same optical path length and so undergo constructive interference there. The amplitude, and hence intensity, of each image point is proportional to that of the corresponding object point and consequently a true image is formed.