92 PRESSURE OF LIGHT 



NOTE 6, p. 73. 



THE AMOUNT OF MATTER WHICH CAN BE PUSHED OUT 

 BY THE PRESSURE OF SUNLIGHT 



Consider a square centimetre area perpendicular to 

 the rays from the sun. The momentum streaming 

 through it per second is 



F S P 

 E = V r* 



where S is the solar constant in ergs per sec. per sq. cm. 

 at the earth's distance b from the sun and r is the 

 distance of the square centimetre from the sun. 



Let the square centimetre be the origin of matter which 

 experiences light-pressure equal to n times the gravitation 

 pull. Let the matter be supposed to be all within a short 

 distance behind the square centimetre. The maximum 

 amount which can be repelled is that which absorbs all 

 the sunlight and all its momentum. Let it be m. 



The gravitation pull on m is 



= 



r* 



where G is the gravitation constant and M is the mass of 

 the sun. 

 If E = nP 



whence 



