At the end of nineteenth century the physicists thought that all the problems related to light can be solved on the bases of waves theory.But in the beginning of 20th century many new phenomena were discovered like. photoelectric effect, Compton,s effect,black body radiation and others which couldn't be explained on the bases of theory of light.
In an effect to explain the observed radiation spectrum, he put forward his quantum theory of light. According to this theory, light is emitted in the form of photons or bundles each having energy hf, where f is the frequency of the photon and is the plank's constant. Its value is h = 6.63*10^-34 j.sec.In 1905, Albert Einstein explained the photoelectric effect on the bases of quantum theory of light.
DUAL NATURE OF LIGHT (PRESENT VIEW ABOUT NATURE OF LIGHT):
Now a days we believe that light has a duel nature. Photoelectric effect and Compton's effect can only be explained, if light is considered to travel in the form of photons (particle nature). Similarly, phenomena of interference and diffraction have been explained on the classical wave theory of light (wave nature). There is no single experiment in which both natures are shown simultaneously. It means that both the electromagnetic wave theory of light are equally correct.Hence light has a dual nature.That is under certain conditions some times it behaves like waves and sometimes it behaves like a particle.