Question

Why is photoelectric emission not possible at all frequencies ?


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Answer 1

Photoelectric emission

The process by which free electrons are emitted from the metal surface by the application of light is called photoelectric emission.
It is also defined as, the process by which free electrons are released from the metal when it absorbs light energy. Photoelectric emission is also called as photoemission or photoelectron emission or photoelectric effect.
In this method, light or photons is used to remove the free electrons from the solid metal. Hence, the free electrons emitted from the solid metal are called photoelectrons and current produced due to this process is called photoelectric current.

Metals without light energy

When the light energy is not applied to the metals, the free electrons cannot escape from the metals. However, some of the valence electrons become free from theatoms.
At normal temperature, some valence electrons get enough energy from the heat source. The valence electrons, which gets sufficient energy will breaks the bonding with the parent atom and becomes free.
The free electrons, which breaks the bonding the parent atom, have some kinetic energy. So, they move freely from one point to another point. However, they do not have enough energy to become free from the metal. The strong attractive force of the nuclei prevents the free electrons, which try to escape from the metal.
To overcome the attractive force of the nuclei, the free electrons need sufficient energy from the light. The free electrons, which break the bonding with metal, will moves into the vacuum.

Photons and its effect on metals

Photons are the smallest particles of light. Unlike electrons and protons, photons have no mass. However, photons have energy.
Visible light and all other forms of light such as radio waves, microwaves, infrared light, ultraviolet light, X-rays, and gamma rays are made up of photons. However, the energy of photons is not same for all these lights. For example, gamma rays (photons) have more energy than the infrared light (photons). The energy of photons is depends on its frequency, whereas the intensity of light is depends on the number of photons.
When the light energy is applied to the metal, the free electrons gains energy. In other words, when the light particles (photons) hits free electrons in the metal, they transfer their energy to the free electrons. The free electrons, which gains extra energy from the light, will try to the overcome the attractive force of the nuclei.



If the light energy applied to the metal is further increased to a higher value, the free electrons in the metals gain sufficient energy and overcome the strong attractive force of the nuclei. The free electrons, which overcome the attractive force of the nuclei, will jumps into the vacuum.

Photoelectric emission depends on the frequency of light and does not depends the intensity of light

Frequency of light

The energy of photons is depends on its frequency. Photons with low frequency have low energy whereas photons with high frequency have high energy. High-energy photons or high frequency photons are needed to emit the free electrons from the metals. 
When the high-energy photons are applied to the metals, free electrons are escaped from the metal surface. The minimum energy or the minimum frequency that the photons required to remove the free electrons from the metals is called threshold frequency or threshold energy of photons. This threshold frequency is not same for all metals. It is different for different metals.
If the light energy applied to a metal reaches or exceeds the threshold frequency, it starts emitting the free electrons. On the other hand, if the light energy applied to a metal is below the threshold frequency, it does not emit the free electrons.

Intensity of light

The number of photons strikes the free electrons in the metal depends on the intensity of light. Low intensity light has less number of photons. Hence, if low intensity light is applied to the metal, less number of photons strikes the free electrons in the metal.
On the other hand, high intensity light has more number of photons. Hence, if high intensity light is applied to the metal, more number of photons strikes the free electrons in the metal.

High frequency and low intensity light

If high frequency and low intensity light is applied to the metal, less number of photons hits the free electrons in the metals. However, each individual photon, which hit the free electron, has energy or frequency greater than the threshold frequency or threshold energy. 
Hence, each individual photon provides enough energy to the free electron to escape from the metal. Thus the free electrons emitted from the metal surface is depends on the frequency of

Answer 2

Explanation:

Photoelectric emission depends on the frequency of light and does not depends the intensity of light

Frequency of light

The energy of photons is depends on its frequency. Photons with low frequency have low energy whereas photons with high frequency have high energy. High-energy photons or high frequency photons are needed to emit the free electrons from the metals.

When the high-energy photons are applied to the metals, free electrons are escaped from the metal surface. The minimum energy or the minimum frequency that the photons required to remove the free electrons from the metals is called threshold frequency or threshold energy of photons. This threshold frequency is not same for all metals. It is different for different metals.

If the light energy applied to a metal reaches or exceeds the threshold frequency, it starts emitting the free electrons. On the other hand, if the light energy applied to a metal is below the threshold frequency, it does not emit the free electrons.