# Particle theory of light

1. Jul 2, 2007

### Minnie

Why does the concept of a threshold frequency for metallic surfaces support the particle theory of light? I understand why it conflicts with the wave theory, but why does it support the particle theory??
Any help would be gr8!
Minnie, xoxo.

2. Jul 2, 2007

### ranger

How can you relate frequency to a photon?

3. Jul 2, 2007

### Minnie

The photoelectric effect is the emission of photoelectrons from a clean metal surface due to incident light whose frequency is greater than a threshold frequency. This phenomenon cannot be explained using a wave apporach to light, and its explanation led to the development of the photon or particle-like model for light.
In considering the photon model, a beam of light consists of a stream of photons, each carrying an energy, E(photon)=hf (h=planck's constant; f=frequency). The total energy in the beam will be Nhf, where N is the number of photons in the beam.
Not entirely sure what you were asking, since light occurs at many different frequencies and wavelengths, according the the equation c=f x wavelength and with changing frequency, there is a change in colour of light, or type of radiation if we move outside the visible spectrum.
Hope this helps!!
Minnie, xoxo.

4. Jul 2, 2007

### cristo

Staff Emeritus
I think ranger's question was meant to provoke an answer to your original question "why does the photoelectric effect support the particle theory of light?"

The photoelectric effect supports the particle theory of light because it shows that the energy required to release electrons from a metal is totally dependent upon the frequency of the light, and not the intensity. Therefore, certain frequencies of light, no matter how intense, cannot cause an electron to be emitted from the surface of a metal.

5. Jul 10, 2007

### lightarrow

Who said it? Mandl & Wolf - Optical Coherence and Quantum Optics - shows the opposite.

Last edited: Jul 10, 2007
6. Jul 11, 2007

### Parlyne

I couldn't have said it better. A much better statement of the consequence of the photoelectric effect experiment would be to say that the photoelectric effect demonstrates that the world has quantum mechanical properties; but, does not determine whether the most relevant properties are those of light or of matter.

7. Jul 11, 2007

### bel

Well, the particle theory was proposed first, because when light greater than or equal to the threshold frequency was shone on the correponding (to the threshold frequency) surface, even for a weak frequency, which supports the quantisation of electromagnetic radiation, i.e., light, and that for a fixed frequency and retarding potential, the photocurrent was directly proportional to the light intensity (amplitude). The quantisation of EM radiation was supported because throwing lots of little packets of energy each without enough energy to free an electron did not free any electrons but throwing even very little amount of little packets of lots of energy freed some electrons. Secondly, because of this quantisation and the old corpuscular theory, the photon was proposed. The photon was shown to be an actual entity by the Compton effect.