Question: Who developed an explanation for the photoelectric effect?

Disambiguated questions and answers:
context: Light, and especially ultra-violet light, discharges negatively electrified bodies with the production of rays of the same nature as cathode rays. Under certain circumstances it can directly ionize gases. The first of these phenomena was discovered by Heinrich Hertz and Wilhelm Hallwachs in 1887. The second was announced first by Philipp Lenard in 1900.
question: Who discovered and developed an explanation for the photoelectric effect in 1887?
short_answers: Heinrich Hertz

Hertz
wikipage: Photoelectric effect

context: No context provided
question: Who developed an explanation for the photoelectric effect in 1888-1891?
short_answers: Aleksandr Stoletov

Stoletov

context: In 1905, Einstein proposed an explanation of the photoelectric effect using a concept first put forward by Max Planck that light waves consist of tiny bundles or packets of energy known as photons or quanta. 
question: Who developed an explanation for the photoelectric effect in 1900?
short_answers: Max Planck

Planck
wikipage: Photoelectric effect

context: In 1900, while studying black-body radiation, the German physicist Max Planck suggested that the energy carried by electromagnetic waves could only be released in "packets" of energy. In 1905, Albert Einstein published a paper advancing the hypothesis that light energy is carried in discrete quantized packets to explain experimental data from the photoelectric effect. This was a key step in the development of quantum mechanics. In 1914, Millikan's experiment supported Einstein's model of the photoelectric effect. Einstein was awarded the Nobel Prize in 1921 for "his discovery of the law of the photoelectric effect", and Robert Millikan was awarded the Nobel Prize in 1923 for "his work on the elementary charge of electricity and on the photoelectric effect".
question: Who developed an explanation for the photoelectric effect in 1905?
short_answers: Albert Einstein

Einstein
wikipage: Photoelectric effect

Long answers:
knowledge: content: Stoletov invented a new experimental setup which was more suitable for a quantitative analysis of the photoeffect. 
wikipage: Photoelectric effect

content: In 1900, while studying black-body radiation, the German physicist Max Planck suggested in his "On the Law of Distribution of Energy in the Normal Spectrum"[43] paper that the energy carried by electromagnetic waves could only be released in packets of energy.
wikipage: Photoelectric effect
long_answer: Before Albert Einstein's discovery of the law of the photoelectric effect in 1905, a number of scientists had discovered and developed an explanation for the photoelectric effect. In 1887, Heinrich Hertz and Wilhelm Hallwachs discovered that light, and especially ultra-violet light, discharges negatively electrified bodies with the production of rays of the same nature as cathode rays. Between 1888 and 1891, Aleksandr Stoletov invented a new experimental setup which was more suitable for a quantitative analysis of the photo effect. In 1900, while studying black-body radiation, the German physicist Max Planck suggested that the energy carried by electromagnetic waves could only be released in packets of energy.