CONCAVE AND CONVEX MIRRORS. 95. 



miles a second. The latest computation with electric light has given a rate 

 of 187,200 miles a second ; but the blue rays in the light experimented on 

 probably account for the difference, for blue rays travel quicker by one per 

 cent, than red rays. Romer first found out the velocity of light, which 

 comes to us from the sun ninety millions of miles in eight minutes. 

 Fizeau calculated the velocity by means of a wheel, which was set moving 

 with tremendous speed by making the light pass between the teeth of the 

 * wheel and back again. 



When rays of light meet substances they are deflected, and the pheno- 

 mena under these circumstances are somewhat similar to the phenomena of 

 heat and sound. There are three particular conditions of rays of light : (i) they 

 are absorbed ; (2) they are reflected ; (3) they are refracted. 



Firstly. Let us see what we mean by light being absorbed ; and this is 

 not difficult to understand, for any " black " substance shows us at once that 

 all the sunlight is taken in by the black object, and does not come out again. 

 It does not take in the light and radiate it, as it might heat. The rose is 

 red, because the rays of light pass through it, and certain of them arc reflected 

 from within. So colour may be stated to be the rays thrown out by the 

 objects themselves those they reject or reflect being the " colour " of the 

 object. 



Secondly. Bodies which reflect light very perfectly arc known as 

 mirrors, and they are termed plane, concave, or convex mirrors, according to 

 form. A plane mirror reflects so that the reflected ray di 

 forms the same angle with the perpendicular as the incident 

 ray ri\ in other words, the angle of incidence is always 

 equal to the angle of reflection, and these rays are per- *"'& "s Angle of 

 pendicular to the plane from which they are reflected. The 

 rays diverge, so that they appear to come from a point as far behind the mirror 

 as the luminous point is in front, and the images reflected have the same 

 appearance, but reversed. There is another law, which is that " the angular 

 velocity of a beam reflected from a mirror is twice that of the mirror." The 

 Kaleidoscope, with which we are all familiar, is based upon the fact of the 

 multiplication of images by two mirrors inclining towards each other. 



A concave mirror is seen in the accom- 

 panying diagram, and may be called the segment 

 of a hollow sphere V W. The point C is the 

 geometrical centre, and O C the radius ; F is the 

 focus ; the line passing through it is the optical 

 axis ; O being the optical centre. All perpendicular 

 rays pass through C. All rays falling in a 

 direction parallel with the optical axis are 

 reflected and collected at F. Magnified images 

 will be produced, and if the object be placed 

 between the mirror and the focus, the image Fig< 86 Concave mirror - 



will appear at the back; while if the object be placed between the geo~ 



