ACTION OF A CONVEX LENS 25 



different one. About the only variable lenses in the world are those in 

 living vertebrate eyes. 



A lens forms an 'image' of an object, the distance of the image from 

 the lens being fixed as long as the distance of the object from the lens 

 is constant. We can best grasp how the image is formed if we think of 

 it as being made up of a large number of points, each corresponding to 

 a point on the object (Fig. 10). The light reflected from each point on 

 the object — its two end-points, say, as in Figure 10 — travels in straight 

 lines away from that point in all possible directions unless the object 

 happens to have a mirror-like surface. We can be sure of this, for we 

 can walk around an object and see it, from any direction, by means of 



I 





^ / 1 \ ^ vl/ 



Fig. 10 — Formation of an image by a lens. 



Of the rays emanating in all dirertions from each point on the objert, those intercepted by 

 the lens are brought to a focus, thus generating a point in the image. Each image-point lies 

 on the opposite side of the lens axis from the corresponding object point; hence the image 

 is inverted. 



the light coming in that direction from the object to our eyes. All of the 

 rays from an object-point which happen to be intercepted by a lens are 

 brought to a point focus beyond the lens at a particular, fixed distance. 

 If the object-point lies below the axis of the lens, however, the light from 

 it will be focused at an image-point above the axis and vice versa. 

 Hence, when we consider all the image-points formed by the focusing 

 of all the light from each of the object-points, we understand how the 

 image is built up. We also see why it hangs in space at a fixed distance 

 from the lens, is smaller than the object, and is inverted. We can now 

 see the image if we catch it on a screen at the image-distance from the 



