(a) In the alpha-particle scattering experiment, the target atom should be much heavier than the bombarding particle. If a thin sheet of solid hydrogen is used in place of a gold foil, then the target is lighter than the bombarding particle.As a result, the alpha-particles would not bounce back.
OR
(b) It is the frequency and not the intensity of light source that determines the emission as if any incident light of particular (i.e., threshold or more) frequency is incident, then it results in emission while it is independent of its intensity.