The required equation is
    
    P =  ,  if -6 <= t <= 6,
    P = 0            ,  if 6 <  t  < 18.
where P is the light's power (in watts) falling to the unit area of the island; 
and "k" is the constant value coefficient of the light energy absorption by the island' surface.
The sine function reflects the fact that the energy flux is proportional to the sine of the angle 
between the normal vector to the island surface and the direction to the star.