112 



HOW WE LIGHT OUR HOMES 



lens, and a thick convex lens. Measure the focal length of 

 each lens as instructed in Experiment 77. 



Place a lens holder at each end of the meter stick. Put 

 the thin convex lens in one lens holder and the thick lens 



FIG. 171 



in the other. Point the end of the meter stick with the thin 

 lens toward some object. Keep your eye close to the thicker 

 lens and move it toward the other lens until you can clearly 

 see the object toward which it is pointed. Record your re- 

 sults. 

 The objective lens (the one nearer the object) had a 



focal length than the eyepiece lens. The image of the 



object was (upright, inverted) . The image formed 



by the objective lens was by the eyepiece lens. 



READINGS WHICH WILL HELP ANSWER 

 THE PROBLEM QUESTIONS 



What is a lens? We learned in a previous lesson 

 that light rays are bent in passing from a medium of 

 one density through a medium of different density, 

 as from water to air. This phenomenon we called re- 

 fraction. Primitive man took this into account when 



FIG. 172. INVENTIONS THAT EMPLOY LENSES 



he speared a fish by aiming below the spot where the 

 fish seemed to be in the water, even though he did 

 not know anything about the science of refraction. It 

 is only in more recent times, however, that man has 

 learned how to make instruments that enable him to 

 employ refraction in useful ways. Cameras, tele- 

 scopes, microscopes, motion picture machines, and 

 projection lanterns are a few of the inventions of this 

 type which enable us to see more and better than we 

 can with ordinary vision. All of these devices are 

 called optical instruments. 



FOCU5 



In all optical instruments we find lenses; hence, in 

 order to know how optical instruments work we must 

 know about lenses and how they affect light passing 

 through them. A lens in an optical instrument is a 

 piece of glass with smooth spherical surfaces. Any 

 other transparent substance with polished spherical 

 surfaces is a lens also. 



Lenses may be divided into two general classes. 

 (Examine Fig. 174 carefully as you read this para- 

 graph.) Convex lenses are thicker at the center than 

 at the edges while concave lenses are thinner at the 

 center than at the 

 edges. Convex lenses 

 are also called converg- 

 ing lenses because 

 they bend to a point 

 the light rays that 

 pass through them. 

 Concave lenses are 

 known as diverging 



-FOCUi 



lenses because they 

 spread the light rays 

 coming to them farth- 

 er apart. 



If you hold a con- 

 vex lens in direct sunlight and move a piece of paper 

 back and forth behind the lens, you will find a point 

 where the rays of sunlight passing through the lens 

 are brightest. In a short while the paper may catch 

 fire. This point at which the rays of light are brought 

 together is called the focus. If the convex lens is re- 

 placed by a concave lens, the light rays passing 



FOCUS 



FIG. 173. FOCAL LENGTH OF 

 CONVEX LENSES 



FIG. 174. THE EFFECT OF CONVEX AND CONCAVE 

 LENSES ON PARALLEL LIGHT RAYS 



through the lens will be spread farther apart instead 

 of being brought to a point. The rule of lenses is that 

 light rays passing through a lens are always bent toward 

 the thickest part. 



What are the parts of the human eye and how does 

 the eye work? It contains a lens which forms images 

 of objects that we look at. But in order to know how 

 the eye works, we must first of all study its main parts 

 and see how they work together. Study Figure 175 

 as you read the following. At the front of the eye is 

 the covering of the eyeball called the cornea (kor'- 

 ne-a). Back of the cornea is the colored part of the eye 

 called the iris (I'ris). In man this is normally some 



