Electromagnetic waves carry energy and momentum. Since electromagnetic waves carry momentum, when they hit a surface they transfer this momentum to the surface, causing the momentum to change, which produces force according to Newton’s second law of motion, and thus the light ball in a vacuum chamber was experiencing the force imparted by the laser light which was in equilibrium with the gravitational force on the ball.

This property, is currently being used to develop solar sails which are spacecrafts running on radiation only. Since there is no air resistance in space, a solar sail will experience force form the solar radiation which will propel the spacecraft forward, much like boats with sails are propelled by the force of the winds.