Answer:
A spectroscope
Explanation:
A spectroscope is used by scientists to separate a source of light ( usually white light ) into its various compositions. i.e it is used to determine the chemical makeup of a visible source of light. hence to determine whether the red light produces photo electrons, which is a chemical makeup of visible light a spectroscope should be added to the circuit.
photo electrons are electrons emitted/ejected from a surface usually metallic surface in the form of a photoelectric effect when a source of light is shines on the surface
