THE PHOTOELECTRIC EFFECT AND PHOTONS
A few years after Planck presented his theory, scientists began to see its applicability to a great
many experimental observations. In 1888, Heinrich Hertz had discovered that electrons could be
ejected from a sample by shining light on it. This is known as the photoelectric effect-the ejection
of electrons from a metal surface upon exposure to photons of sufficiently high energy. The
classical expectation of the photoelectric effect was that the oscillating electric field of the light
wave causes the electrons in the metal to oscillate back and forth, and the electrons in the metal
respond at different frequencies, and that the energy in the light wave (its intensity) should be
transferred to the kinetic energy of the emitted electrons. That is, the number of emitted electrons
would depend upon the frequency, and their kinetic energy should depend upon the intensity of
the light wave.
However, experimental results indicated that:
i. The electrons are ejected from the metal surface as soon as the beam of light strikes the
surface.
ii. The number of electrons ejected is proportional to the intensity or brightness of light.
iii. For each metal, there is a characteristic minimum frequency, ν0 (also known as threshold
frequency) below which photoelectric effect is not observed.
iv. At a frequency ν >ν0, the ejected electrons come out with certain kinetic energy. The kinetic
energies of these electrons increase with the increase of frequency of the light used and is
independent of the intensity of the radiation. In other words, the kinetic energy of the
ejected electron is proportional to the frequency of the electromagnetic radiation.
In 1905, Albert Einstein (1879-1955) explained the observations with the bold hypothesis that
energy carried by light existed in “parcels” or “packets” of an amount hν. Each packet or photon
could cause one electron to be ejected, which is like having a moving particle collide with and
transfer energy to a stationary particle. The number of electrons ejected, therefore, depends upon
the number of photons, i.e. the intensity of the light. Some of the energy in the packet is used to
overcome the binding energy of the electron in the metal. This binding energy is called the work
𝟏
function, Φ. The remaining energy appears as the kinetic energy, 𝟐 𝒎𝒗𝟐 , of the emitted electron.
That is,
𝐸𝑝ℎ𝑜𝑡𝑜𝑛 = 𝐾𝐸𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛 + 𝑊𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛
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, 𝟏
hν. = 𝟐 𝒎𝒗𝟐 + Φ
Rearranging this equation reveals the linear dependence of kinetic energy on frequency. That is,
𝟏
𝒎𝒗𝟐 . = 𝒉𝛎 − Φ
𝟐
The slope of the straight line obtained by plotting the kinetic energy as a function of frequency
𝟏
above the threshold frequency is just Planck’s constant, and the x-intercept, where 𝟐 𝒎𝒗𝟐 = 𝟎, is
just the work function of the metal, Φ= hνo. With such an analysis Einstein obtained a value for h
in agreement with the value Planck deduced from the spectral distribution of black-body radiation.
The fact that the same quantization constant could be derived from two very different experimental
observations was very impressive and made the concept of energy quantization for both matter
and light credible.
Einstein won the Nobel Prize in Physics in 1921 for his explanation of the photoelectric effect. The
idea that the energy of light depends on its frequency helps us understand the diverse effects that
different kinds of electromagnetic radiation have on matter. For example, the high frequency
(short wavelength) of X-rays causes X-ray photons to have energy high enough to cause tissue
damage and even cancer. Thus, signs are normally posted around X-ray equipment warning us of
high-energy radiation.
Dual Behaviour of Electromagnetic Radiation
The particle nature of light posed a dilemma for scientists. On the one hand, it could explain the
black body radiation and photoelectric effect satisfactorily but on the other hand, it was not
consistent with the known wave behaviour of light which could account for the phenomena of
interference and diffraction. The only way to resolve the dilemma was to accept the idea that light
possesses both particle and wave-like properties, i.e., light has dual behaviour. Depending on the
experiment, we find that light behaves either as a wave or as a stream of particles. Whenever
radiation interacts with matter, it displays particle like properties in contrast to the wavelike
properties (interference and diffraction), which it exhibits when it propagates. This concept was
totally alien to the way the scientists thought about matter and radiation and it took them a long
time to become convinced of its validity.
Page 2 of 8
A few years after Planck presented his theory, scientists began to see its applicability to a great
many experimental observations. In 1888, Heinrich Hertz had discovered that electrons could be
ejected from a sample by shining light on it. This is known as the photoelectric effect-the ejection
of electrons from a metal surface upon exposure to photons of sufficiently high energy. The
classical expectation of the photoelectric effect was that the oscillating electric field of the light
wave causes the electrons in the metal to oscillate back and forth, and the electrons in the metal
respond at different frequencies, and that the energy in the light wave (its intensity) should be
transferred to the kinetic energy of the emitted electrons. That is, the number of emitted electrons
would depend upon the frequency, and their kinetic energy should depend upon the intensity of
the light wave.
However, experimental results indicated that:
i. The electrons are ejected from the metal surface as soon as the beam of light strikes the
surface.
ii. The number of electrons ejected is proportional to the intensity or brightness of light.
iii. For each metal, there is a characteristic minimum frequency, ν0 (also known as threshold
frequency) below which photoelectric effect is not observed.
iv. At a frequency ν >ν0, the ejected electrons come out with certain kinetic energy. The kinetic
energies of these electrons increase with the increase of frequency of the light used and is
independent of the intensity of the radiation. In other words, the kinetic energy of the
ejected electron is proportional to the frequency of the electromagnetic radiation.
In 1905, Albert Einstein (1879-1955) explained the observations with the bold hypothesis that
energy carried by light existed in “parcels” or “packets” of an amount hν. Each packet or photon
could cause one electron to be ejected, which is like having a moving particle collide with and
transfer energy to a stationary particle. The number of electrons ejected, therefore, depends upon
the number of photons, i.e. the intensity of the light. Some of the energy in the packet is used to
overcome the binding energy of the electron in the metal. This binding energy is called the work
𝟏
function, Φ. The remaining energy appears as the kinetic energy, 𝟐 𝒎𝒗𝟐 , of the emitted electron.
That is,
𝐸𝑝ℎ𝑜𝑡𝑜𝑛 = 𝐾𝐸𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛 + 𝑊𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛
Page 1 of 8
, 𝟏
hν. = 𝟐 𝒎𝒗𝟐 + Φ
Rearranging this equation reveals the linear dependence of kinetic energy on frequency. That is,
𝟏
𝒎𝒗𝟐 . = 𝒉𝛎 − Φ
𝟐
The slope of the straight line obtained by plotting the kinetic energy as a function of frequency
𝟏
above the threshold frequency is just Planck’s constant, and the x-intercept, where 𝟐 𝒎𝒗𝟐 = 𝟎, is
just the work function of the metal, Φ= hνo. With such an analysis Einstein obtained a value for h
in agreement with the value Planck deduced from the spectral distribution of black-body radiation.
The fact that the same quantization constant could be derived from two very different experimental
observations was very impressive and made the concept of energy quantization for both matter
and light credible.
Einstein won the Nobel Prize in Physics in 1921 for his explanation of the photoelectric effect. The
idea that the energy of light depends on its frequency helps us understand the diverse effects that
different kinds of electromagnetic radiation have on matter. For example, the high frequency
(short wavelength) of X-rays causes X-ray photons to have energy high enough to cause tissue
damage and even cancer. Thus, signs are normally posted around X-ray equipment warning us of
high-energy radiation.
Dual Behaviour of Electromagnetic Radiation
The particle nature of light posed a dilemma for scientists. On the one hand, it could explain the
black body radiation and photoelectric effect satisfactorily but on the other hand, it was not
consistent with the known wave behaviour of light which could account for the phenomena of
interference and diffraction. The only way to resolve the dilemma was to accept the idea that light
possesses both particle and wave-like properties, i.e., light has dual behaviour. Depending on the
experiment, we find that light behaves either as a wave or as a stream of particles. Whenever
radiation interacts with matter, it displays particle like properties in contrast to the wavelike
properties (interference and diffraction), which it exhibits when it propagates. This concept was
totally alien to the way the scientists thought about matter and radiation and it took them a long
time to become convinced of its validity.
Page 2 of 8
