The Particle Nature of Light

A FIGURE 7.8 The Photoelectric Effect (a) When sufficiently energetic light shines on a metal surface, the surface emits electrons, (b) The emitted electrons can be measured as an electrical current. 

The lag time would be the minimum amount of time required for the dim light to transfer sufficient energy to the electron to dislodge it. 

In 1905, Albert Einstein proposed a bold explanation for the photoelectric effect light energy must come in packets. According to Einstein, the amount of energy (E) in a hght packet depends on its frequency (y) according to the following equahon  

A FIGURE 7.9 The Photoelectric Effect A plot of the electron ejection rate versus frequency of light for the photoelectric effect. Electrons are only ejected when the energy of a photon exceeds the energy with which an electron is held to the metal. The frequency at which this occurs is called the threshold frequency. 

The enei of a photon is directly proportional to its frequency and inverseiy proportional to its waveiengfii. 

Three phenomena involving matter and light confounded physicists at the turn of the 20 century (1) blackbody radiation, (2) the photoelectric effect, and 

Because the atom can change its energy only by integer multiples of hv, the smallest change occurs when an atom in a given energy state changes to an adjacent state, that is, when A = 1  

Presence of a threshold frequency. Light shining on the metal must have a minimum/ret/Mcncy, or no current flows. (Different metals have different minimum 

Absence of a time lag. Current flows the moment light of this minimum frequency shines on the metal, regardless of the light s intensity. The wave theory, however, predicts that in dim light there would be a time lag before the current flowed, because the electrons had to absorb enough energy to break free. 

Let s see how Einstein s photon theory explains the photoelectric effect  

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Still, the interference patterns from the double slit experiment clearly indicated that light was a wave. How could Einstein reconcile his conclusions about the particle nature of light with the results of the double slit experiment Was it possible that light could act as either particle or wave ... 

Does the particle nature of light cause its wave aspects Or vice versa All these questions may only be asked from the point of view of classical physics, they only have meaning from the classical view. Once quantum mechanical physics enters the scene, no one even attempts to answer the questions on the classical level, if my guess that brain and mind are parallel aspects of a more fundamental reality is nebulous, perhaps it will take on some relevance when a "quantum mechanics of philosophy" will be available, whether a process of mind studying mind will accomplish such a feat is still an open question. 

The particle nature of light was postulated in 1905 by Einstein to explain the photoelectric effect. When light is incident on a metal surface in an evacuated tube, electrons may be ejected from the metal. This is the operational basis of photomultipliers and image intensifies, which transform light to an amplified electric signal (see Section 3.1). 

The photoelectric effect exemplifies the particle nature of light. 

The Wave Nature of Light 217 The Particle Nature of Light 220 Atomic Spectra 223 Line Spectra and the Rydberg Equation 223 The Bohr Model of the Hydrogen Atom 224 7.4... 

The particle nature of light is characterized by the specific quantity of energy carried in each photon. 

I have expanded and clarified the description of the photoelectric effect and the particle nature of light in Section 7.2, including a new figure (Figure 7.9) that depicts a graph of the rate of electron ejection from a metal versus the frequency of light used. 

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