Photons constant light speed

For a typical sodium atom, the initial velocity in the atomic beam is about 1000 m s1 and the velocity change per photon absorbed is 3 crn-s. This means that the sodium atom must absorb and spontaneously emit over 3 x 104 photons to be stopped. It can be shown that the maximum rate of velocity change for an atom of mass m with a photon of frequency u is equal to hu/lmcr where h and c are Planck s constant and the speed of light, and r is the lifetime for spontaneous emission from the excited state. For sodium, this corresponds to a deceleration of about 106 m s"2. This should be sufficient to stop the motion of 1000 m-s 1 sodium atoms in a time of approximately 1 ms over a distance of 0.5 m, a condition that can be realized in the laboratory. 

The critical energy required for a photon to remove an electron is then hvc = q>, where Vc is the critical frequency of the photon and h is Planck s constant. When the frequency of the incident radiation is less than the critical frequency, electrons will not be ejected. Similarly, when the wavelength of the incident radiation is greater than the critical wavelength, Xc, electrons will also not be emitted (recall that V = c/X, where c is the speed of light in vacuum). This relationship between... 

A full explanation of the properties of light requires both the wave theory of electromagnetic radiation and the quantum theory. Most photochemical processes are best understood in terms of the quantum theory, which says that light is made up of discrete particles called quanta or photons. Each quantum carries an amount of energy, S, determined by the wavelength of the light, A. Equation 13.1, in which h is Planck s constant and c is the speed of light in a vacuum,... 

Let us concentrate on the particle aspect only. The main issue is to identify an space (three- or four-dimensional ) where photons propagate with constant speed c. Einstein s second postulate of the special theory of relativity (STR) requires the speed of light in free space to be the same for all inertial observers. This postulate is conventionally interpreted as implying the non-existence of a preferred frame E. As discussed in section II, the exactly opposite view will be adopted here. 

The energy of one light photon (e) is equal to the frequency times Planck s constant or Planck s constant times the speed of light divided by the wavelength ... 

A main distinction between photons and material particles is that in a vacuum photons travel always at constant speed, the speed of light, whereas material particles are able to travel at various speeds relative to the observer, up to a maximum of the speed of light. 

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