REFRACTION 121 



glass till the rays all fall upon one small spot. What do you 

 notice regarding this spot? If the lens were not in the way, how 

 great an area would be covered by the rays that now fall here? 

 Cause the spot to fall on tissue paper or a bit of gunpowder. 

 Account for the intensity of the light and heat at this point. 



140. How Images are formed. When light waves are 

 sent off from an object, each point sends off a separate 

 set of rays which will be collected by a lens and refracted 

 to a separate focus. 



In Fig. 95 let ab be an object and I the lens. Now 

 every ray sent out from a point a and passing through 



PIG. 95 



I will be refracted to a focus a' ; and every ray from b 

 which is refracted, will come to a focus at b'. In the 

 same way rays from all points in ab will be refracted to 

 points between a' and b r . Each focus will appear the 

 same, and will have the same position among the others, as 

 the point in the object from which its rays came. Thus, if 

 a screen be placed so that these foci (plural of focus) may 

 be formed upon it, the group of foci together will form an 

 image just like the object. 



141. The Eye. Just behind the dark opening in 

 the eye is a lens, the crystalline lens. This forms 

 images on a screen called the retina, in the back part 



