Elementary concepts of image formation by a thin lens

2 Elementary concepts of image formation by a thin lens 

The lateral magnification is defined as the ratio of the image height to the object height hence, 

The ray model of light is of limited usefulness. If we are to understand the fundamental processes involved in the formation of an image by a lens, we must consider the wave nature of light. The simplest form of the wave theory of light is based on a geometrical construction known as Huygens principle, which is usually stated as follows  

All points on a wavefront can be considered as point sources of secondary spherical wavelets after a time t, the new position of the wavefront is given by the surface that is tangent to these secondary wavelets. 

Even though Huygens principle makes no assumptions about the nature of the wave, the principle is extremely useful, especially when stated in a precise mathematical form (see Section 1.3). 

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