Energy of a photon

Electromagnetic radiation of which visible light is but one example has the properties of both particles and waves The particles are called photons, and each possesses an amount of energy referred to as a quantum In 1900 the German physicist Max Planck proposed that the energy of a photon (E) is directly proportional to its frequency (v)... 

The energy of a photon provides an additional characteristic property of electromagnetic radiation. 

Figure 12.22 Schematic diagram showing the flow of excitation energy in the bacterial photosynthetic apparatus. The energy of a photon absorbed by LH2 spreads rapidly through the periplasmic ring of bacterio-chlorophyll molecules (green). Where two complexes touch in the membrane, the energy can be transmitted to an adjacent LH2 ring. From there it passes by the same mechanism to LHl and is finally transmitted to the special chlorophyll pair in the reaction center. (Adapted from W. Kiihlbrandf, Structure 3 521-525, 1995.)...

The frequency of the electromagnetic radiation that can be absorbed by the nuclear system is easily calculated by equating the energy of a photon and the energy level separation ... 

EXAMPLE 1.4 Sample exercise Calculating the energy of a photon... 

Self-Test 1.4A What is the energy of a photon of yellow light of frequency 5.2 X 1014 Hz ... 

This conversion problem requires two steps. Equations and relate the energy of a photon to its frequency and wavelength. 

The wave picture was retained in that a photon was considered to have a frequency cuid that the energy of a photon was proportional to its frequency). 

The energy of a photon is now recognized as being proportional to the frequency of the photon. The constant of proportionality relating the photon s frequency and energy is known as Planck s constant. It has a value of 6.626 x 10-34 J s, and is denoted by the letter h. In this activity, you will measure the value of Planck s constant by observing the photoelectric effect. 

Planck s Law The relation between energy of a photon and its frequency, E = hv. 

Planck s constant (h) A universal constant of nature that relates the energy of a photon of radiation to the frequency of the emitting oscillator. Its numerical value is about 6.626 x ICh27 ergs/s. 

Quantum theory considers radiation as a stream of energy packets - photons or quanta - travelling through space at a constant velocity (c when in a vacuum). The energy of a photon is related to the frequency of the radiation, as defined in wave theory, by the expression... 

Radiation is not emitted continuously, but is emitted in small bundles called photons. The energy of a photon is dependent on the wavelength of the radiation. When a photon of radiation is absorbed into a metal such as the cathode (negative plate) of the UV tube, the energy of the photon is imparted to an electron within the metal, causing it to leave the surface of the metal and be drawn toward the anode (positive plate). The energy that the electron must have to leave the metal is called the work function of the metal. The sensitivity range of the radiation detector is dependent upon the work function of the metal used in the cathode. 

At a quantum-mechanical level, there is a simple relationship that ties together the twin modes by which we visualize photons we say that the energy of a photon particle is E and the frequency of a light wave is v. The Planck-Einstein equation, Equation (9.3), says... 

Finally, in the early 20th century Albert Einstein explained the photoelectric effect based on quantized packets of electromagnetic radiation called photons. These quickly led to the familiar relationships of the energy of a photon,... 

The energy of a photon is proportional to its frequency and inversely proportional to its wavelength. 

Since the energy of a photon is hv = 3.3 x 10 J, the average number of photons for that mode is 1.7 x 10. This indicates that the number of photons per mode in the visible region for a spectral width of 10 s (in the order of the Doppler width) is a very small nnmber compared to unity. This has important conseqnences, as we will see in Section 2.3. 

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