Photoelectric Effect: Definition, Examples & Applications

The photoelectric effect is the phenomenon of the emission of electrons from metal surfaces exposed to light energy of suitable frequency.

• In this method, light or photons of suitable are used to remove free electrons from the metal surface.
• The emitted electrons are called photoelectrons and the current so produced is called photoelectric current.
• This effect is also known as photoelectric emission or photoemission or photoelectron emission.
• Other phenomena where light affects the movement of electric charges include the photovoltaic effect, the photoconductive effect, and the photoelectrochemical effect.
• A variety of electronic devices use the effect to detect light and to emit electrons at precisely timed intervals.
• Alkali metals like cesium, lithium, potassium, and sodium show a photoelectric effect with visible light. Whereas metals like magnesium, cadmium, and zinc are sensitive only to ultraviolet light.

Key Terms: Photoelectric Effect, Einstein Photoelectric Equation, Photoelectric Emission, Photons, Light, Electrons, Wave theory of light

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What is the Photoelectric Effect?

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The Photoelectric Effect is a phenomenon in which a stream of light strikes a metallic surface, resulting in the emission of electrons from the latter. The electrons that are emitted from the surface are called Photoelectrons.

• The current produced due to the photoelectrons is called photoelectric current.
• One of the important causes of the Photoelectric Effect is the absorption of the light energy by the electrons at the metal surface.
• The electrons use this light energy to overpower the forces binding them to the nuclei of the metal.
• The photoelectric effect was discovered by the German physicist Heinrich Rudolf Hertz in 1887.
• He found that high-voltage sparks across the detector loop were enhanced when the emitter plate was illuminated by ultraviolet light.
• This observation led him to conclude that light facilitated the emission of some electrons.
• From this, it was concluded that when suitable radiation falls on a metal surface, some electrons near the surface absorb enough energy from the incident radiation to overcome the attraction of the positive ions in the material surface.
• Wilhelm Hallwachs in 1988, observed that ultraviolet light on a negatively charged zinc plate, connected to a gold leaf electroscope decreases the divergence of gold leaf.
• He concluded that a negatively charged zinc plate when exposed to ultraviolet light emits negatively charged particles.
• The emitted charged particles are photoelectrons emitted by the action of light.

Photoelectric Effect

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Experimental Study of Photoelectric Effect

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Phillip Lenard used the apparatus shown in the figure to study the effect of light on a metallic surface.

• It consists of a highly evacuated tube having two electrodes, cathode C and anode A connected by a battery.
• Cathode is a photosensitive emitter and the anode is a charge-collecting plate.
• Lenard observed that when ultraviolet light was incident on the cathode, an electric current appeared in the circuit.
• He concluded that only light of suitable frequency called threshold frequency results in photoelectric current.

This experiment is used for:

1. To study the variation of photoelectric current with the intensity of radiation
2. To study the variation of photoelectric current with the frequency of incident radiation
3. To study the variation of photoelectric current with the potential difference between the plates A and C.

Experimental arrangement for the study of the photoelectric effect

Effect of Intensity of Incident Light on the Photoelectric Current

As photoelectric current is directly proportional to the number of photoelectrons emitted per second, therefore the number of photoelectrons emitted per second is directly proportional to the intensity of incident radiations.

Effect of Potential on Photoelectric Current

Photoelectric current increases with the increase in applied positive potential. When all photoelectrons emitted by the cathode reached the anode, the photoelectric current has a maximum value. This maximum current is known as the saturation current.

• When the polarity of the battery is reversed, the photoelectric current is found to decrease rapidly and at a certain value of negative potential, it drops to zero.
• The minimum negative potential V_o for which photoelectric current becomes zero is called cut-off potential or stopping potential.
• At this stage maximum kinetic energy of photoelectrons must be equal to the energy acquired by an electron while passing through a potential difference V_o. i.e. \((K.E)_{max}={1 \over 2}mv^2_{max}=eV_o\)

Effect of Frequency of Incident Radiation on Stopping Potential

There should be a minimum cut-off frequency, called the threshold frequency of the incident radiation below which no emission takes place for a given photosensitive material. The maximum kinetic energy or equivalent stopping potential increases linearly with the increase in the frequency of the incident radiation.

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Laws of Photoelectric Emission

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The laws of Photoelectric Emission are:

1. For a given photosensitive material, there should be a minimum cut-off frequency, called the threshold frequency of the incident radiation below which no emission takes place whatever may be the intensity of incident radiation.
2. The maximum kinetic energy or equivalent stopping potential of the photoelectrons increases linearly with the increase in the frequency of the incident radiation, provided that the frequency of incident radiation is greater than the threshold frequency.
3. The photoelectric current is directly proportional to the intensity of incident radiation and the frequency of incident radiation, provided that the frequency of incident radiation is greater than the threshold frequency.
4. The Saturation current is proportional to the intensity of radiation whereas the stopping potential is independent of its intensity, for a given photosensitive material.
5. Photoelectric emission is an instantaneous process. The time lag is very small between the incidence of radiation and the emission of photoelectrons (≅ 10^-9 s or less).

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Photoelectric Effect and Wave Theory of Light

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The wave picture of light satisfactorily explained the phenomenon of interference, diffraction, and polarization. As per this picture, light is an electromagnetic wave with a continuous distribution of energy in the space where the wave is present.

• As per the wave picture of light, the amplitude of the electric and magnetic fields is more, if the intensity of radiation is more and hence the energy absorbed by each electron is greater. It is expected to increase in maximum kinetic energy of the photoelectron with an increase in intensity and there is no need for threshold frequency. These expectations of the wave theory are directly contradicted by the observations of the photoelectric effect.
• According to the wave picture of light, the absorption of energy by electrons takes place continuously over the complete wavefront of radiation. For a single electron to pick up enough energy to overcome the work function and come out from the metal will take a long time. This conclusion is again in striking contrast to the observations of the photoelectric effect i.e. photoelectric effect is an instantaneous process.
• This concludes that the wave picture of light is unable to explain the photoelectric effect.

Photoelectric Effect

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Einstein’s Photoelectric Equation

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Einstein proposed a theory in 1905 based on Planck's hypothesis of quanta to explain the photoelectric effect. According to Einstein, when a photon of energy, E = hf falls on a metal surface, the energy of the photon is sued in two ways:

1. A part of this energy is used by an electron of the metal to just cross the surface barrier so that it may come out from the metal surface. This part of the energy is equal to the work function Φ of the metal.
2. The remaining photon energy is used to provide kinetic energy to the photoelectrons.

Therefore,

\(E=hf=hf_o+{1\over2}mv^2\)

This equation is Einstein’s photoelectric equation.

Where

• E = hf, Energy of a photon in joule
• h = Planck's constant (6.626 × 10^-34 J.s)
• f = frequency of the incident photon
• f_o = Threshold frequency
• hf_o = Φ, the Work function of the metal

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Things to Remember

• The Photoelectric Effect is a phenomenon in which a stream of light strikes a metallic surface, resulting in the emission of electrons from the latter.
• The electrons that are emitted from the surface are called Photoelectrons.
• Wilhelm Ludwig Franz Hallwachs propounded the idea of the Photoelectric Effect in 1887, followed by Heinrich Rudolf Hertz proving the phenomenon through experiments.
• Max Planck propounded a law to calculate the energy of photons when their frequency is known. Planck’s equation helps us understand that photons of varying energies are carried by different frequencies of light.
• The photoelectric effect can only take place when the photons that strike the metal surface carry sufficient energy to cause the electrons to get unbound from the nuclei of the metal.
• Einstein consolidated the idea by propounding that Light is nothing but a beam of particles called Photons and their energies are tied to their frequency as per Planck's formula.

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