90 PRESSURE OF LIGHT 



or since V/V = p, the refractive index, 

 M' = /xM 



Draw B^ = pA^. Then B 1 C ?> represents M'. 



Draw C 1 D 1 parallel to B 1 A 1 , meeting the normal at B x 

 in D r 

 We have 



B 1 C 1 _ sin BjDjC^ sin i _ 

 D^ = sin D^Cj = sin r ~ M 



Then B^ = pU^ 



or, D^ = A x B r 



The momentum D^ = A 1 B 1 is converted into 

 momentum B^ by the addition of momentum BjDj. 



Then B l D l represents the force acting on the beam of 

 light, and D 1 B 1 represents the equal and opposite force 

 acting on the refracting medium, a force outwards along 

 the normal. 



We see also that in the prism experiment, fig. 27, the 

 torque is on this theory produced by two pulls outwards 

 on the glass at D and E. 



NOTE 5, p. 71. 



THE PRESSURE OF SUNLIGHT AGAINST AN ABSORBING 

 SPHERE COMPARED WITH THE GRAVITATION PULL 



The total pressure of light against an absorbing sphere 

 radius a is 7r# 2 S/V, where S is the stream of solar energy 

 per second, and V is the velocity of light, equal to 

 3 x io 10 cm./sec. 



The value of S at the earth's distance b, is 2-5 calories 

 per minute, or 175 x io 6 ergs per second. Therefore 

 the value of the total pressure at any distance r is, 



^2 175 x io 6 g 

 7 x io 1( V 2 



