Answer:
Due presence of empty as well as heavy particles.
Explanation:
The main reason for this is the presence of empty spaces as well as heavy particles in the nucleus of Helium atom. Most of the beam pass through the helium atom due to the presence of empty spaces in the atom whereas some beams deflect from the helium atom because both repel each other due to same charge i.e. positive charge. Few beams of light bounce back with an angle of 90 degree due hitting with the nucleus where heavy particles such as neutron and proton are present.