154 HOW WE USE LIGHT 



act upon light like a lens. When light from an object passes 

 through the eye, it forms an unreal or virtual image on the cornea. 

 The position where an image is formed depends upon the distance 

 the object is from the eye. However, the eyeball can change its 

 shape and move the back surface of the eye nearer or farther away 

 from the pupil. This is taken care of automatically so that in the 

 normal eye there will always be a clear image formed. This 

 adjustment is called accommodation. Father says most people 

 wear glasses to remedy eye defects but some wear them for " looks." 



PROBLEM V. WHAT IS COLOR? 



What Is White Light ? If you pass sunlight through a 

 triangular piece of glass called a prism, the white light 

 is separated into the following colors : red, orange, 

 yellow, green, blue, and violet. The waves of all these 

 colors differ in length. The longest wave affecting sight 

 gives us the color red, the shortest, violet. Color, then, 

 is a property of light which depends upon its wave length. 

 When light is refracted, the blue rays are bent more than 

 the red rays, or the shorter the wave length, the more it is 

 bent. The prism bends the beam of light twice in the 

 same direction, once as it enters the glass and again as 

 it leaves the glass and enters the air. Both times it 

 separates the colors and so produces a band of colors on 

 a screen. This band of colors is called the "solar spec- 

 trum." The rainbow is caused by light being broken up 



into these colors by 

 bending or refrac- 

 tion in falling rain- 

 drops. When all 

 the different wave 

 lengths from the sun 



Why does a prism separate sunlight into colors ? . -, , ,-, 



are mixed together, 



white light is produced. White substances are those which 

 reflect all wave lengths, hence all colors, while black sub- 

 stances are those which absorb all wave lengths of light. 



