Photoelectric Effect: Why Can't Low Freq Light Excite Electrons?

In summary, when light with a frequency greater than the threshold frequency of a metal hits the metal, the kinetic energy of the photons is not enough to cause an electron to be excited.
  • #1
nothing0
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a light with frequency 'v' can't excite electrons from a metal surface if the threshold frequency of the metal is greater than the frequency of the wave( the light,electromagnetic radiation)so why don't the energies of the photons add up together and then excite the electron in the surface of the metal (mean even a very intense light with the same frequency can't do it if it has less frequency intense light means lots of photons can't this photons co-operate?)
 
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  • #2
You got it right. Kinetic energy of emitted electron is function of frequency and not intensity.
For very high intensity of light, it is possible.

You can throw ball up in air only up to certain height. Call your friend to help you and you together can not throw it higher (without using any tool). Friend will be useful to throw heavier ball, but then it is a different attribute.
 
  • #3
nothing0 said:
a light with frequency 'v' can't excite electrons from a metal surface if the threshold frequency of the metal is greater than the frequency of the wave( the light,electromagnetic radiation)so why don't the energies of the photons add up together and then excite the electron in the surface of the metal (mean even a very intense light with the same frequency can't do it if it has less frequency intense light means lots of photons can't this photons co-operate?)

It is possible. Look up "multiphoton photoemission". This is an important technique in studying the energetics of Rydberg-type states. Note that one must use a light source that has a very high photon density. This is because the excited state has a very short lifetime, and the second photon that comes in must interact with that excited electron before it decays back to the conduction band. The probability of this happening (what we call the "cross-section") is quite low and thus, one must use a light source with a very high photon density per unit area to increase the probability of it happening. For ordinary light sources, this probability is practically zero. That's why the ordinary photoelectric effect is commonly observed.

What this means is that the ordinary photoelectric effect is a single-photon photoemission phenomenon.

BTW, note also that there's another way to cause the emission of a photoelectron even using a light with energy lower than the work function. One can apply an external electric field to the surface, thus lowering the effective work function. This is usually known as the Schottky effect.

Zz.
 
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1. What is the Photoelectric Effect?

The Photoelectric Effect refers to the phenomenon in which electrons are emitted from a material when it is exposed to light. This was first observed by Heinrich Hertz in 1887 and was later explained by Albert Einstein in 1905.

2. What is the role of frequency in the Photoelectric Effect?

The frequency of the incident light plays a crucial role in the Photoelectric Effect. Only light with a high enough frequency can excite electrons and cause them to be emitted from the material. This is known as the threshold frequency. If the frequency of the light is too low, no electrons will be emitted regardless of the intensity of the light.

3. Why can't low frequency light excite electrons?

Low frequency light does not have enough energy to overcome the binding energy of the electrons in the material. In order for an electron to be emitted, it must absorb enough energy from the light to break free from the material's surface. Low frequency light does not have enough energy to provide the necessary amount for this to occur.

4. How does the intensity of light affect the Photoelectric Effect?

While the intensity of light does not affect the frequency of light, it does play a role in the number of electrons emitted. The higher the intensity of light, the more electrons will be emitted, as long as the frequency is above the threshold frequency. This is because more photons of light are hitting the material, providing more opportunities for electrons to absorb enough energy to be emitted.

5. What is the significance of the Photoelectric Effect?

The Photoelectric Effect is significant because it provided evidence for the particle nature of light and helped support Albert Einstein's theory of the photon. It also has many practical applications, such as in solar cells and photocells, which convert light energy into electrical energy. It also has important implications in understanding the behavior of electrons and the structure of atoms.

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