Maxwell’s picture of light as electromagnetic waves has given us three predictions about it, A) The intensity of an electromagnetic wave depends on its amplitude but not on its frequency. So, the photoelectric effect can be observed for incident light of any frequency, and the magnitude of the photocurrent should be independent of the frequency of the light. But experimental observations suggest that the photocurrent do depends on the incident light frequency. Monochromatic light with a frequency below a minimum threshold frequency produces no photocurrent, regardless of intensity. This threshold frequency is different for different materials. B) A minimum amount of energy which is required, called the work function, to eject a single electron from a particular surface. If the light falling on the surface is very faint, some time delay may occur before the total energy absorbed by the surface equals the work functions. Hence, for a low intensity of light, we expect a time delay between the incidence of light and when photocurrents appear. The experimental results say other-wise. There is effectively no time delay between the incidence of light and when the cathode emits photoelectrons. This is true no matter how low intense light which is incident. C) Because the energy delivered to the cathode surface depends on the intensity of illumination, we expect the stopping potential to increase with increasing light intensity. Here also the experimental results contradict. From the results it is seen that the stopping potential does not depend on intensity, but does depend on frequency. From the figure 1, which is a photocurrent as a function of applied potential difference shows that the stopping potential is the same for both the intensities. The only effect of increasing the intensity is that there is an increase in the number of electrons emitted per second, i.e., the photocurrent. The experimental results are in clear contradiction to Maxwell’s description of light as an electromagnetic wave. This dilemma was solved by Albert Einstein in 1905 where he proposed a new picture of the nature of light. This new proposal was a postulate that emission of light is not continuous but consists of small packets of energy known as quanta aka photon. These photons can be as small balls. But they are not. Balls have a rest mass and travel much slower than light, while photons travel at the speed of light and have zero rest mass. Also, photons have wave aspect having definite wavelength. This postulate was made much earlier by Max Planck while explaining blackbody radiation. According to Einstein, the energy E of a photon is equal to a constant h times the photon frequency ƒ. From the relationship for electromagnetic waves in vacuum, we have Where h is Planck’s constant. h = 6.6 × 10-34 Js.