TUKE 



SKY PHENOMENA 



If the sun is very low (A), or the rain 



falHng far away (B), the bow may 



not all be seen at E. 



While as a rule bows can be seen for only a short time as a storm 

 is receding or approaching, these more distant bows are visible 

 for the whole duration of the storm, giving an opportunity of 

 prolonged study. Another interesting feature of these bows is 

 that sometimes clouds, nearer, and probably lower, often drift in 

 between observer and bow temporarily eclipsing portions of the 

 colored band. 



Why is it that in the case 

 of the distant bows, and also 

 bows formed when the sun is 

 low in the sky, only the lower 

 arcs can be seen ? It will be 

 seen (sketch) that in these 

 cases the light rays after mak- 

 ing their 42 degree turn in 

 the drops do not reach the 

 observer from points straight opposite the sun, so the corre- 

 sponding part of the bow is not visible. 



Violet rays are more refrangible than red rays, yet, as there 

 are two refractions, one on entering the drop, the other on leaving 

 it, and as these are in opposite directions, and as the red ray 



after the first refraction and 

 reflection makes a greater 

 angle with the normal to the 

 surface of the drop than does 

 the violet ray (sketch) the 

 total refraction of the red is 

 greater than that of the violet. 

 In other words the red ray 

 emerges from the drop about 

 two degrees farther from its en- 

 tering direction than the violet 

 ray, that is about two degrees 



lower in the case of drops that 

 The light-ray is refracted at a, reflected . ^ , ^ r.-,-u -ixn, 



at d, and again refracted at c. form the top of the bow. Why, 



then, does the red appear on the top of the bow?' We do not 



see all the colors from the same drop. With two degs. difference 



in direction the red rays that enter: the eye must come from 



