Download Max Planck suggested that the energy of light is proportional to its

Max Planck suggested that the energy of light is proportional to its
frequency, also showing that light exists in discrete quanta of energy.
LEARNING OBJECTIVES [ edit ]
Understand Planck's Quantum Theory
Calculate the energy element E=hv, using Planck's Quantum Theory
KEY POINTS [ edit ]
Until the late 19th century, Newtonian physics dominated the scientific worldview. However, by
the early 20th century, physicists discovered that the laws of classical mechanics do not apply at
the atomic scale.
The photoelectric effect could not be rationalized based on existing theories of light, as an
increase in the intensity of light did not lead to the same outcome as an increase in the energy of
the light.
Planck postulated that the energy of light is proportional to thefrequency, and the constant that
relates them is known as Planck's constant (h). His work led to Albert Einstein determining that
light exists in discrete quanta of energy, orphotons.
TERMS [ edit ]
photoelectric effect
The emission of electrons from the surface of a material following the absorption of
electromagnetic radiation.
electromagnetic radiation
Radiation (quantized as photons) consisting of oscillating electric and magnetic fields oriented
perpendicularly to each other, moving through space.
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In the late 18th century, great progress in physics had been made. Classical Newtonian
physics at the time was widely accepted in the scientific community for its ability to
accurately explain and predict many phenomena. However, by the early 20th century,
physicists discovered that the laws of classical mechanics are not applicable at the atomic
scale, and experiments such as the photoelectric effect completely contradicted the laws of
classical physics. As a result of these observations, physicists articulated a set of theories now
known as quantum mechanics. In some ways, quantum mechanics completely changed the
way physicists viewed the universe, and it also marked the end of the idea of a clockwork
universe (the idea that universe was predictable).
Electromagnetic radiation
Electromagnetic (EM) radiation is a form of energy with bothwave-like and particle-like
properties; visible light being a well-known example. From the wave perspective, all forms of
EM radiation may be described in terms of their wavelength and frequency. Wavelength is
the distance from one wave peak to the next, which can be measured in meters. Frequency is
the number of waves that pass by a given point each second. While the wavelength and
frequency of EM radiation may vary, its speed in a vacuum remains constant at 3.0 x
108 m/sec, the speed of light. The wavelength or frequency of any specific occurrence of EM
radiation determine its position on the electromagnetic spectrum and can be calculated from
the following equation:
c =
λν
where c is the constant 3.0 x 108 m/sec (the speed of light in a vacuum), λ = wavelength in
meters, and ν=frequency in hertz (1/s). It is important to note that by using this equation,
one can determine the wavelength of light from a given frequency and vice versa.
Wavelength of EM radiation
The distance used to determine the wavelength is shown. Light has many properties associated with its
wave nature, and the wavelength in part determines these properties.
The Discovery of the Quantum
The wave model cannot account for something known as the photoelectric effect. This effect
is observed when light focused on certain metals emits electrons. For each metal, there is a
minimum threshold frequency of EM radiation at which the effect will occur. Replacement of
light with twice the intensity and half the frequency will not produce the same outcome,
contrary to what would be expected if light acted strictly as a wave. In that case, the effect of
light would be cumulative—the light should add up, little by little, until it caused electrons to
be emitted. Instead, there is a clear-cut minimum frequency of light that triggers electron
ejection. The implication was that frequency is directly proportional to energy, with the
higher light frequencies having more energy. This observation led to the discovery of the
minimum amount of energy that could be gained or lost by an atom. Max Planck named this
minimum amount the "quantum," plural "quanta," meaning "how much." One photon of light
carries exactly one quantum of energy.
Planck is considered the father of the Quantum Theory. According to Planck: E=hν , where h
is Planck's constant (6.62606957(29) x 10-34 J s), ν is the frequency, and E is energy of an
electromagnetic wave. Planck (cautiously) insisted that this was simply an aspect of the
processes of absorption and emission of radiation and had nothing to do with the physical
reality of the radiation itself. However, in 1905, Albert Einstein reinterpreted Planck's
quantum hypothesis and used it to explain the photoelectric effect, in which shining light on
certain materials can eject electrons from the material.
More Evidence for a Particle Theory of Energy
When an electric current is passed through a gas, some of the electrons in the gas molecules
move from their ground energy state to an excited state that is further away from their nuclei.
When the electrons return to the ground state, they emit energy of various wavelengths. A
prism can be used to separate the wavelengths, making them easy to identify. If light acted
only as a wave, then there should be a continuous rainbow created by the prism. Instead,
there are discrete lines created by different wavelengths. This is because electrons release
specific wavelengths of light when moving from an excited state to the ground state.
Emission spectrum of nitrogen gas
Each wavelength of light emitted (each colored line) corresponds to a transition of an electron from one
energy level to another, releasing a quantum of light with defined energy (color).