Solar sails use the momentum of particles of light to propel itself forward by bouncing the said light particles back. A large amount of people are led to believe that momentum is quantified by the equation, (p=m*v), if this was the case then photons would have no momentum as they do not have mass. This equation, however; is an oversimplification of the actual universal momentum equation that is described by special relativity (E2=(p*c)2+(m*c2)2) where c is the speed of light constant and E is energy. Photons have no mass but contain energy so the equation can be reduced to (p=E/c). A more detailed breakdown of the equation would result in (p=h/λ) where h is the planks constant (6.63*10-34 J*s) and λ is the photons wavelength in meters.
As the photons change its momentum by reflecting off the sail, equal and opposite momentum is imposed on the sail and pushes it forward. The amount of transferred momentum of one photon is extremely minuscule but there is a vast amount of them pelting the sail every second. The speed of the sail is ever increasing as there is no force acting on it in the negative direction (ideally) so it will continue to constantly gain velocity throughout its life cycle.