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ENERGY OF A PHOTON:
A photon is the smallest division of a light beam that retains the properties of the beam. The characteristics of a photon include its frequency, its wavelength, and its energy. A photon should not be visualized as a particle that has physical dimension or a specific location in space. More accurately, a photon can be considered a "wave packet" that has a specific energy content.
PLANCK S CONSTANT:
The energy of a photon is directly proportional to the frequency of the photon and is given by Equation 1.
Equation 1
E = h?
where: E = Energy of photon in joules (J).
? = Frequency of the photon in hertz.
h = Planck s constant = 6.625 × 10–34 joule-seconds.


Given: The frequency of a photon of HeNe laser light is 4.74 × 1014 Hz.
Find: The energy of the photon.
Solution: E = h?
E = (6.625 × 10–34 J.sec) (4.74 × 1014/sec)
E = 3.14 × 10–19 J
The frequency of a photon is determined by the relation described by Equation 2.
Equation 2

where: c = The speed of light in vacuum, namely 3 × 108 m/sec.
? = Wavelength in meters.
Substitution of Equation 2 into Equation 1 yields Equation 3.
Equation 3

EXAMPLE B: CALCULATION OF THE ENERGY OF A PHOTON OF A GIVEN WAVELENGTH
Given: The wavelength of a He-Ne laser light is 633 nm.
Find: The energy of the photon of this light.
Solution:
Equations 1, 2, and 3 can be used to determine the frequency, wavelength, or energy of a photon if any one of these quantities is known. Example C is a further illustration of the use of these equations.
EXAMPLE C: CALCULATION OF WAVELENGTH AND FREQUENCY OF A PHOTON OF GIVEN ENERGY
Given: A photon has an energy of 1.875 × 10–19 J.
Find: The frequency and the wavelength of the photon.
Solution: From Equation 1

From Equation 2—

ENERGY UNITS:
The unit of energy used in Planck s constant and in the preceding examples is the joule (J). The joule is the basic energy unit in the International System of Units (sometimes referred to as the MKS system—meters, kilograms, seconds) and is the amount of energy necessary to raise one kilogram mass (2.2 lb) to a height of 10.2 cm against the pull of gravity.