Question: What does the theory of photoelectric effect tell us?
About the article "Photoelectric effect", section "Experimental observations of photoelectric emission".

Candidate passages:
Video camera tubes in the early days of television used the photoelectric effect, for example, Philo Farnsworth's "Image dissector" used a screen charged by the photoelectric effect to transform an optical image into a scanned electronic signal. 
Background radiation doses in the immediate vicinities of particles of high atomic number materials, within the human body, have a small enhancement due to the photoelectric effect. 
  Heinrich Hertz discovered the photoelectric effect  The Michelson-Morley experiment was undertaken, which suggested that the speed of light is invariant.
The James-Lange theory of emotion was produced.  
The year 2005 is significant primarily because of the changes that have occurred in the philosophy of physics over the past 100 years.
These changes began in 1905 with the publication of four papers by Einstein that: explained Brownian motion, introduced the special theory of relativity, and described how the photoelectric effect could be explained by the quantization of light, which helped launch quantum mechanics, and developed "E = mc"2.
These papers are commonly called his Annus Mirabilis Papers because they later defined 1905 as a "miracle year" for physics.
Most physicists agree that the first three of those papers deserved Nobel Prizes, but only the paper on the photoelectric effect would win one.
What makes these papers remarkable is that, in each case, Einstein boldly took an idea from theoretical physics to its logical consequences and managed to explain experimental results that had baffled scientists for decades. 
When Einstein published his seminal 1905 paper on the particle theory of light, Millikan was convinced that it had to be wrong, because of the vast body of evidence that had already shown that light was a wave.
He undertook a decade-long experimental program to test Einstein's theory, which required building what he described as "a machine shop "in vacuo"" in order to prepare the very clean metal surface of the photo electrode.
His results confirmed Einstein's predictions in every detail, but Millikan was not convinced of Einstein's interpretation, and as late as 1916 he wrote, "Einstein's photoelectric equation... cannot in my judgment be looked upon at present as resting upon any sort of a satisfactory theoretical foundation," even though "it actually represents very accurately the behavior" of the photoelectric effect.
In his 1950 autobiography, however, he simply declared that his work "scarcely permits of any other interpretation than that which Einstein had originally suggested, namely that of the semi-corpuscular or photon theory of light itself".
Since Millikan's work formed some of the basis for modern particle physics, it is ironic that he was rather conservative in his opinions about 20th century developments in physics, as in the case of the photon theory.
Another example is that his textbook, as late as the 1927 version, unambiguously states the existence of the ether, and mentions Einstein's theory of relativity only in a noncommittal note at the end of the caption under Einstein's portrait, stating as the last in a list of accomplishments that he was "author of the special theory of relativity in 1905 and of the general theory of relativity in 1914, both of which have had great success in explaining otherwise unexplained phenomena and in predicting new ones.
"Millikan is also credited with measuring the value of Planck's constant by using photoelectric emission graphs of various metals.