Why did Classical theory fail to explain Photo electroc effect ?

When light shines on the surface of a metallic substance, electrons in the metal absorb the energy of the light and they can escape from the metal's surface. This is called the photoelectric effect, and it is used to produce the electric current that runs many solar-powered devices. Using the idea that light is a wave with the energy distributed evenly throughout the wave, classical physicists expected that when using very dim light, it would take some time for enough light energy to build up to eject an electron from a metallic surface. WRONG!! Experiments show that if light of a certain frequency can eject electrons from a metal, it makes no difference how dim the light is. There is never a time delay.

In 1905, Albert Einstein came up with the solution. If Max Planck's idea that energy comes in clumps (quanta) is correct, then light must consist of a stream of clumps of energy. Each clump of light energy is called a photon, said Einstein, and each photon has an energy equal to hf (Planck's constant times the frequency of the light). Therefore the energy of light is not evenly distributed along the wave, but is concentrated in the photons. A dimmer light means fewer photons, but simply turning down the light (without changing its frequency) does not alter the energy of an individual photon. So for a specific frequency light, if a single photon has enough energy to eject an electron from a metallic surface, then electrons will always be ejected immediately after the light is turned on and the photons hit the metal.

i.e. why classical theory fails..................................

pradeep has given a superb answer...... very good...... the thng is CLASSICAL PHYSICS is not that developed incase of ATOMIC STRUCTERS... it cannot explain abt the properties such as frequency, wavelength, magneitc moment etc.. so to deal with an aspect where we make use of a material having DUAL NATURE such as light,,, WAVE MECHANICS n QUANTUM PHYSICS must be taken into account.... which deal with the perfect reasons of  phenomenon such as PHOTO ELECTRIC EFFECT, COMPTON EFFECT, etc...

 If light were strictly a wave, the energy in the light would be represented by the amplitude of the light wave. A more intense light source, even if it was of a lower frequency, would have enough energy to knock electrons away from their orbits, which is necessary to generate a photoelectric current.

But on the contrary it isobserved that light below a certain threshold frequency does not generate any current, no matter how intense the light is. Even though the total light energy hitting the photoelectric cell may be high, it cannot free electrons.

However, if the frequency is increased, even at low intensities, there will be a current. That indicates that the energy from light is delivered in quanta (small units), which is consistent with a particle view of the light. More intense light has more "units" of energy, but if the smallest unit (a photon) has too little oomph (low frequency means low energy per photon), then each collision is too weak to knock the electron. A small number of higher frequency photons will not generate as many collisions, but each collision will set an electron free to jump over to the collector and make a current.