According to my optics book when you shine light on an object it exerts a pressure on the object in the direction of propagation. If light is made of alternating electric and magnetic fields, and these fields are in directions transverse to the direction of propagation, then why should the force be parallel to the direction of propagation? Shouldn't the force be in the direction that the fields are pointing? I guess the natural explanation is that photons carry momentum... but I understand that Maxwell was one of the first to suggest a radiation pressure parallel to propagation. How would Maxwell have explained it?