State what is meant by a photon.
Electromagnetic radiation of a varying frequency 
and constant intensity 
is used to illuminate a metal surface. At certain frequencies, electrons are emitted from the surface of the metal. The variation with of the maximum kinetic energy 
of the emitted electrons is shown in the graph below.
State the name of this phenomenon.
Describe three conclusions that can be drawn from the graph above. The conclusions may be qualitative or quantitative.
The experiment in (b) is repeated twice, each time making one change. State, with a reason, how the graph obtained would compare with the graph above when:
a different metal is used, but keeping the intensity of the radiation the same
the same metal is used, but with electromagnetic radiation of intensity 
.
that is directly proportional to its frequency 
is Planck’s constant.
.
.
