16, On the Pressure of Light. 
By M. N. Sana, M.Sc., Lecturer on Mathematical Physics, and 
. CHAKRAVARTI, B.Sc., University College of Science, 
Calcutta. 
The object of the present paper is to describe a simple 
apparatus by means of which the pressure of light can be easily 
demonstrated, and qualitatively measured with the entire elim- 
ination of all sorts of disturbing effects. The materials re- 
quired are not difficult to procure, and are readily available in 
all well-equipped laboratories. 
e wish first to give a short history of the subject and a 
short sketch of the theory.! As early as the seventeenth cen- 
tury Kepler supposed that light exerted a pressure on surfaces 
on which it is incident. The hypothesis was called into being 
for explaining the tails of comets. 
With the rise of Newton’s corpuscular theory of light, the 
ure no longer remained a guess, but could be deduced 
ela 
negative. Later on, the failure of these experiments were used 
light ments against the validity of the corpuscular theory of 
on 
‘mount of pressure is extremely small. It can be shown that 
if light consists of unidirectional rays, the pressure amounts to 
3 ‘Amount of radiant energy falling on unit surface per unit 
of time, measured in absolute units), where ¢ velocity of light, 
‘i d the Surface is a perfectly absorbing one, e.g. 4 surface 
cated with lamp-black. 
If the surface on which the light is incident be perfectly 
haecting, the pressure is just double. But if, on the other 
» the surface be transparent (e.g. glass), there will be no 
Te at all, or more accurately a very small amount of 
a. <n inemeepeinnining statins TL — 
1 * 
433. cere historical part, see Lebedew, Ann-d. Phys., Bd. 6, page 
2 ols and Hull, Phys. Rev., 1903. 
Maxwell, Electricity and Magnetism, Vol. II, page 792. 
