Experimental Study of Photoelectric Effect
Last Updated : 15 May, 2024
Heinrich Hertz discovered the phenomena of photoelectric emission in 1887. He noticed that when the spark gap is open, it conducts electricity more easily. The ultraviolet light from the arc lamp illuminates the emitter.
During the years 1886-1902, Hallwachs and Lenard studied photoelectric emission in depth. When ultraviolet radiation is permitted to fall on the emitter plate of an evacuated glass tube containing two electrodes, current flows through the circuit, according to Lenard. The current flow stopped as soon as the UV radiations were turned off. These findings suggest that when ultraviolet light strikes the emitter plate, electrons are expelled from it, and the electric field attracts them to the positive collecting plate.
As a result, light falling on the emitter's surface causes current to flow through the external circuit. Hallwachs and Lenard looked at how this photocurrent changed with collector plate potential, as well as light frequency and intensity. Certain metals, such as zinc, cadmium, and magnesium, were discovered to respond only to UV light with a short wavelength that caused electron emission from the surface. However, some alkali metals, such as lithium, sodium, potassium, cesium, and rubidium, are visible light sensitive. When these photosensitive compounds are irradiated by light, they all emit electrons.
What is Photoelectric Effect?
The phenomenon of emission of electrons by certain substances (say metals), when exposed to radiations of suitable frequencies is called photoelectric effect and emitted electrons are called photoelectrons.
The photoelectric effect is a one photon-one electron phenomenon. One photon cannot eject more than one photo-electron.
Experimental study of the Photoelectric effect
Heinrich Hertz discovered the phenomena of photoelectric emission in 1887. The Hertz experimental arrangement for the study of the photoelectric effect is shown in the below image.
Photoelectric CellIt consist of,
- Evacuated glass tube
- Photosensitive plate (C)
- Metal plate (A)
- Light source (S) [Mon
- chromatic light]
- Quartz window (W)
Monochromatic light from the source of sufficiently short wavelength enters the tube through quartz window and falls on photosensitive plate C which acts as an emitter. The electrons are emitted by the plate C and are collected by the plate A (collector), the electric field is created by the battery.
The potential difference applied between the plates can be changed by the potential divider arrangement. By means of commutator key the plate A can be maintained at desired positive or negative potential with respect to Plate C.
Important Observations are,
- When light is incident on the emitter plate the photo electrons are emitted.
- The photo electrons are attracted by positive plate A.
- The emission of electrons causes flow of electric current in the circuit.
- The potential difference between the emitter and collector plates is measured by the voltmeter (V) whereas the photoelectric current flowing in the circuit is measured by microammeter (UA).
The variation in the photoelectric effects depends upon the following factors
- Frequency.
- Intensity.
- The potential difference between plates A and C.
- The nature of the material of plate C.
Effect of frequency on photoelectric current
Collector plate A is given an appropriate positive potential. The frequency of incident light is gradually increased from its smallest value while the intensity of incident radiation remains constant. It is observed that until a specific frequency u is reached, no photoelectric current is recorded [Refer below image].
Variation of Photoelectric current with a frequency of incident radiationThis frequency is a property of emitter plate C's material, known as the threshold frequency for that metal, and is denoted by v.
Threshold frequency
The threshold frequency is the lowest frequency of incoming radiation for which photoelectrons are merely released from photosensitive material.
The threshold frequency of emitter plate C varies depending on the material. The threshold wavelength is the wavelength that corresponds to the threshold frequency (v). If λ> λo, photoemission is not possible.
Effect of intensity of light on the photoelectric current
The collector plate A is kept at a positive potential in relation to the emitter plate C, attracting electrons emitted from C to A. The intensity of light is modified while the frequency of incident radiation and accelerating potential remains constant, and the resulting photoelectric current is monitored. Photoelectric current is observed to rise linearly with light intensity, as shown in the below image.
Variation of photoelectric current with the intensity of lightThe number of photoelectrons emitted per second is exactly proportional to the photoelectric current. This means that the rate at which photoelectrons are emitted is proportional to the intensity of incident energy.
Note: More photos, not more energetic photons, are produced by a brighter light.
Effect of potential difference on photoelectric current
The incident radiation threshold frequency and intensity are both kept constant at a reasonable value. Plate A's positive potential is steadily increased, and the resulting photoelectric current is measured each time. It has been discovered that as the positive (accelerating potential) grows, so does the photoelectric current. All of the released electrons are captured by plate A at some point for a particular positive potential, and photoelectric current peaks. The photoelectric current does not rise as the positive potential of plate A is increased further. Saturation current refers to the highest value of photoelectric current.
The commutator key is now inverted, and plate A is given a negative (retarding) potential in relation to plate C. The negative potential of Plate A is gradually increased till the photoelectric current reduces to zero. The minimum negative potential V_{0} given to the plate A for which photoelectric current stops or becomes zero is called cut-off or stopping potential.
Variation of photoelectric current with collector plate potential for different intensities of incident radiationThe energy of all photoelectrons released from a metal surface is not the same. When the stopping potential is adequate to reject even the most energetic photoelectrons with the highest kinetic energy, the photoelectric current becomes zero, and
1 / 2 mv2max = eV0.
where
vmax is the maximum velocity of the photoelectron,
m is the mass of the electron, and
e is the magnitude of the charge on the electron.
When the incident radiation frequency is maintained constant and the experiment is repeated with varying strengths of the incident beam, it is discovered that V remains constant in all circumstances see above image. The stopping potential and maximum kinetic energy of photoelectrons are thus independent of incoming radiation intensity for a certain frequency of incident radiation.
Variation of photoelectric current with collector plate potential for different frequencies of incident radiationAdjusting the same intensity of incoming radiation at various frequencies to study the fluctuation of photoelectric current with collector plate potential is illustrated in the figure. For incoming radiation of various frequencies, different values of stopping potential but the same values of saturation current are achieved. The energy of a photoelectron released is proportional to the frequency of incoming light. When the frequency of incoming radiation is increased, the stopping potential becomes more negative.
As shown, the below graph between incoming radiation frequency and related stopping potential for various metals is made up of straight lines.
Variation of stopping potential with the frequency of incident radiationsThe graph shows that, for a particular photosensitive material, the stopping potential varies linearly with the frequency of incoming radiation, and there is a minimum cut-off frequency v0 below which the stopping potential is zero.
Sample Questions
Question 1: Define threshold frequency and photoelectric work function?
Answer:
Threshold frequency: The threshold frequency is the lowest frequency of incoming radiation for which photoelectrons are merely released from photosensitive material.
Photoelectric work function: The photoelectric work function is the threshold energy required by the radiation or photons that are incident on the surface of the metal. We can use the term radiation interchangeably with photons in this case.
ϕ=hν0
where h is the Planck's constant and ν0 is the threshold frequency.
Question 2: What is the photoelectric effect? State and explain its characteristics?
Answer:
The phenomenon of emission of electrons by certain substances (say metals), when exposed to radiations of suitable frequencies is called photoelectric effect and emitted electrons are called photoelectrons. The photoelectric effect is a one photon-one electron phenomenon. One photon cannot eject more than one photo-electron.
Characteristics:
- For a given photosensitive material, there exists a certain minimum cut-off frequency of the incident radiation, called threshold frequency v0 below which no emission of photoelectrons takes place. The threshold frequency is different for different metals.
- For a given photosensitive material and frequency of incident radiation (above threshold frequency), the photoelectric current is directly proportional to the intensity of incident light.
- Above the threshold frequency v0, the maximum kinetic energy of the emitted photoelectrons increases linearly with the frequency of the incident radiation, but is independent of intensity of incident radiation.
- The emission of photo electron is an instantaneous process. There is no time lag between the irradiation of the metal surface and P emission of photoelectrons.
Question 3: Define threshold wavelength?
Answer:
The wavelength (λ0) corresponding to the threshold frequency v0, is called the threshold wavelength.
If c is the velocity of light, then
v0 = ϕ / h
For the photoelectric emission the wavelength of incident light must be less than λ0.
Question 4: What is the mass of a photon?
Answer:
The photon’s rest mass is zero, which indicates that if the photon is moving, it will have some momentum, which is equivalent to mass, but at rest, the photon’s mass will be zero.
Question 5: Define stopping potential?
Answer:
The magnitude of the retarding potential for which the photoelectric current is zero is called the stopping potential (Vs)
The value of the stopping potential is a measure of the maximum kinetic energy for the photoelectrons.
Work done on electron by,
Vs = Max K.E. for the photoelectrons.
eVs = 1/2 mv2max
Question 6: What is the effect of kinetic energies on stopping potential in photoelectric emission?
Answer:
The value of the stopping potential is a measure of the maximum kinetic energy that can be possessed by a photoelectron.
The different photoelectrons have different kinetic energies. At the stopping potential, the work done by the electron against the stopping potential V is equal to the K.E. of this electron having maximum K.E.
eV = 1/2 mv2max
where V max is the maximum velocity.
Similar Reads
CBSE Class 12 Physics Notes 2023-24 CBSE Class 12 Physics Notes are an essential part of the study material for any student wanting to pursue a career in engineering or a related field. Physics is the subject that helps us understand our surroundings using simple and complex concepts combined. Class 12 physics introduces us to a lot o
10 min read
Chapter 1 - ELECTRIC CHARGES AND FIELDS
Electric Charge and Electric FieldElectric Field is the region around a charge in which another charge experiences an attractive or repulsive force. Electric Field is an important concept in the study of electrostatics which is the branch of physics. Electric Field despite its invisible nature, powers our homes with electricity, tra
15+ min read
Electric ChargeElectric Charge is the basic property of a matter that causes the matter to experience a force when placed in a electromagnetic field. It is the amount of electric energy that is used for various purposes. Electric charges are categorized into two types, that are, Positive ChargeNegative ChargePosit
8 min read
Conductors and InsulatorsWhen humans remove synthetic clothing or sweater, especially in dry weather, he or she often sees a spark or hear a crackling sound. With females' clothing like a polyester saree, this is essentially observed. Another example is Lightning a common form of electric discharge that seen in the sky duri
9 min read
Basic Properties of Electric ChargeElectric Charges are fundamental in the universe. The presence of electric charges are not only seen in the field of science but also in the daily lives of human beings. For instance, rubbing dry hair with a ruler ends up making some hair strands stand up and this happens because electric charges ar
4 min read
Coulomb's LawCoulomb’s Law is defined as a mathematical concept that defines the electric force between charged objects. Columb's Law states that the force between any two charged particles is directly proportional to the product of the charge but is inversely proportional to the square of the distance between t
9 min read
Forces Between Multiple ChargesWhen our synthetic clothing or sweater is removed from our bodies, especially in dry weather, a spark or crackling sound appears. With females' clothing like a polyester saree, this is almost unavoidable. Lightning, in the sky during thunderstorms, is another case of electric discharge. It is an ele
10 min read
Electric FieldElectric field is a fundamental concept in physics, defining the influence that electric charges exert on their surroundings. This field has both direction and magnitude. It guides the movement of charged entities, impacting everything from the spark of static electricity to the functionality of ele
14 min read
Electric Field LinesElectric field lines are a representation used to visualize the electric field surrounding charged objects. They provide a way to understand the direction and strength of the electric field at different points in space. It helps analyze electric fields in various situations, such as around point cha
5 min read
What is Electric Flux?Electric flux is a fundamental concept in physics that helps us understand and quantify the electric field passing through a given surface. It provides a means to describe the flow of electric field lines through an area. Electric flux forms the basis of Gauss's Law, to calculate the net charge encl
12 min read
Electric DipoleAn electric dipole is defined as a pair of equal and opposite electric charges that are separated, by a small distance. An example of an electric dipole includes two atoms separated by small distances. The magnitude of the electric dipole is obtained by taking the product of either of the charge and
11 min read
Continuous Charge DistributionElectric charge is a fundamental feature of matter that regulates how elementary particles are impacted by an electric or magnetic field. Positive and negative electric charge exists in discrete natural units and cannot be manufactured or destroyed. There are two sorts of electric charges: positive
7 min read
Applications of Gauss's LawGauss's Law states that the total electric flux out of a closed surface equals the charge contained inside the surface divided by the absolute permittivity. The electric flux in an area is defined as the electric field multiplied by the surface area projected in a plane perpendicular to the field. N
9 min read
Chapter 2 - ELECTROSTATIC POTENTIAL AND CAPACITANCE
Electric Potential EnergyElectrical potential energy is the cumulative effect of the position and configuration of a charged object and its neighboring charges. The electric potential energy of a charged object governs its motion in the local electric field.Sometimes electrical potential energy is confused with electric pot
15+ min read
Electric Potential Due to a Point ChargeElectric forces are responsible for almost every chemical reaction within the human body. These chemical reactions occur when the atoms and their charges collide together. In this process, some molecules are formed and some change their shape. Electric forces are experienced by charged bodies when t
7 min read
Electric Potential Of A Dipole and System Of ChargesElectric Potential is defined as the force experienced by a charge inside the electric field of any other charge. mathematically it is defined as the ratio of electric potential energy that is required to take a test charge from infinity to a point inside the electric field of any other charge with
7 min read
Equipotential SurfacesWhen an external force acts to do work, moving a body from a point to another against a force like spring force or gravitational force, that work gets collected or stores as the potential energy of the body. When the external force is excluded, the body moves, gaining the kinetic energy and losing a
9 min read
Potential Energy of a System of ChargesWhen an external force works to accomplish work, such as moving a body from one location to another against a force such as spring force or gravitational force, that work is collected and stored as the body's potential energy. When the external force is removed, the body moves, acquiring kinetic ene
11 min read
Potential Energy in an External FieldWhen an external force operates to conduct work, such as moving a body from one location to another against a force like spring force or gravitational force, the work is gathered and stored as potential energy in the body. When an external force is removed, the body moves, acquiring kinetic energy a
11 min read
Electrostatics of ConductorsWhen an external force is used to remove a body from a situation. Point to another in the face of a force like spring or gravitational force That work is stored in the body as potential energy. When the external environment When a force is eliminated, the body moves, gaining and losing kinetic energ
11 min read
Dielectrics and PolarisationHave you noticed how many of the insulators are made of wood, plastic, or glass? But why is that? When we utilise wood or plastic, why don't we receive electric shocks? Why do you only get severe shocks from metal wires? We'll look at dielectrics, polarisation, the dielectric constant, and more in t
10 min read
Capacitor and CapacitanceCapacitor and Capacitance are related to each other as capacitance is nothing but the ability to store the charge of the capacitor. Capacitors are essential components in electronic circuits that store electrical energy in the form of an electric charge. They are widely used in various applications,
11 min read
What is a Parallel Plate Capacitor?Answer: A Parallel Plate Capacitor is a capacitor with two parallel conducting plates separated by an insulating material and capable of storing electrical charge. Capacitance can be defined in Layman's terms as a physical quantity that indicates the ability of a component or circuit to collect and
8 min read
Capacitors in Series and ParallelCapacitors are special devices that can hold electric charges for instantaneous release in an electric circuit. We can easily connect various capacitors together as we connected the resistor together. The capacitor can be connected in series or parallel combinations and can be connected as a mix of
7 min read
Energy stored in a CapacitorCapacitors are used in almost every electronic device around us. From a fan to a chip, there are lots of capacitors of different sizes around us. Theoretically, the basic function of the capacitor is to store energy. Its common usage includes energy storage, voltage spike protection, and signal filt
6 min read
Chapter 3 - CURRENT ELECTRICITY
Electric CurrentElectricity has become an essential part of our everyday life, changing the way we live and work. In the past, people depended on fire for light, warmth, and cooking. Today, we can easily turn on lights, heat our homes, and charge our devices with just a switch or button. This is all possible becaus
10 min read
Electric Current in ConductorsElectric current in conductors is the movement of electric charge through a substance, usually a metallic wire or other conductor. Electric current is the rate at which an electric charge flows past a certain point in a conductor, and it is measured in amperes. When a potential difference (voltage)
8 min read
Ohm's Law - Definition, Formula, Applications, LimitationsAccording to Ohm's law, the voltage or potential difference between two locations is proportional to the current of electricity flowing through the resistance, and the resistance of the circuit is proportional to the current or electricity travelling through the resistance. V=IR is the formula for O
5 min read
Drift VelocityDrift Velocity as the name suggests refers to the slow movement of electrons in the conductor when an Electromotive force(emf) is introduced. Electrons do not move in a straight line in the conductor, but they move randomly in the conductor colliding with the other electrons and atoms exchanging ene
12 min read
Limitations of Ohm's LawOhm’s Law is a relationship between three physical phenomena: current, voltage, and resistance. This relationship was introduced by German physicist George Simon Ohm. That is why the law is well known as Ohm’s law. It states that the amount of steady current through a large number of materials is di
10 min read
ResistivityResistance is the physical property of the material which opposes the current flow in the circuit whereas resistivity is the intrinsic property that helps us understand the relation between the dimension of the substance and the resistance offered by it. In this article, we will learn about Resista
9 min read
Temperature Dependence of ResistanceDevices such as batteries, cells, etc. are essential for maintaining a potential difference across the circuit and are referred to as voltage sources. When a voltage source is connected across a conductor, it creates an electric field which causes the charges to move and this causes current. The val
5 min read
Electrical Energy and PowerElectric energy is the most important form of energy and is widely used in almost all the electrical devices around us. These devices have a rating written on them. That rating is expressed in Watts and intuitively explains the amount of electricity the device will consume. Bigger devices like AC, r
9 min read
Electromotive ForceElectromotive Force or EMF is the work done by the per unit charge while moving from the positive end to the negative end of the battery. It can also be defined as the energy gain per unit charge while moving from the positive end to the negative end of the battery. The battery or the electric gener
10 min read
Combination of Cells in Series and ParallelThere are many resistances in complex electrical circuits. There are methods to calculate the equivalent resistances in case multiple resistances are connected in series or parallel or sometimes in a combination of series and parallel. In many situations, batteries or different types of voltage sour
6 min read
Kirchhoff's LawsKirchhoff's Laws are the basic laws used in electrostatics to solve complex circuit questions. Kirchhoff's Laws were given by Gustav Robert Kirchhoff who was a famous German Physicist. He gave us two laws Kirchhoff’s Current Law and Kirchhoff’s Voltage Law which are discussed in this article.These l
8 min read
Wheatstone BridgeWheatstone bridge is a device that is used to find the resistance of a conductor, in 1842, scientist Wheatstone proposed a theory, which is called the principle of Wheatstone bridge after his name. we can prove or establish the formula for Wheatstone by using Kirchhoff laws. Wheatstone bridge is sim
10 min read
Chapter 4 - MOVING CHARGES AND MAGNETISM
Magnetic Force on a Current carrying WireWhen a charge is moving under the influence of a magnetic field. It experiences forces, which are perpendicular to its movement. This property of charge is exploited in a lot of fields, for example, this phenomenon is used in the making of motors which in turn are useful for producing mechanical for
5 min read
Motion of a Charged Particle in a Magnetic FieldThis has been already learned about the interaction of electric and magnetic fields, as well as the motion of charged particles in the presence of both electric and magnetic fields. We have also deduced the relationship of the force acting on the charged particle, which is given by the Lorentz force
9 min read
Biot-Savart LawThe Biot-Savart equation expresses the magnetic field created by a current-carrying wire. This conductor or wire is represented as a vector quantity called the current element. Lets take a look at the law and formula of biot-savart law in detail, Biot-Savart Law The magnitude of magnetic induction a
7 min read
Magnetic Field on the Axis of a Circular Current LoopMoving charges is an electric current that passes through a fixed point in a fixed period of time. Moving charges are responsible for establishing the magnetic field. The magnetic field is established due to the force exerted by the flow of moving charges. As the magnetic field is established moving
7 min read
Ampere's Circuital Law and Problems on ItAndré-Marie Ampere, a French physicist, proposed Ampere's Circuital Law. Ampere was born in Lyon, France, on January 20, 1775. His father educated him at home, and he showed an affinity for mathematics at a young age. Ampere was a mathematician and physicist best known for his work on electrodynamic
5 min read
Force between Two Parallel Current Carrying ConductorsMoving charges produce an electric field and the rate of flow of charge is known as current. This is the basic concept in Electrostatics. The magnetic effect of electric current is the other important phenomenon related to moving electric charges. Magnetism is generated due to the flow of current. M
8 min read
Current Loop as a Magnetic DipoleWhen a charge move it generates an electric field and the rate of flow of charge is the current in the electric field. This is the basic concept in Electrostatics. The magnetic effect of electric current is the other important concept related to moving electric charges. Magnetism is generated due to
11 min read
Moving Coil GalvanometerHans Christian Oersted discovered in 1820 that a current-carrying conducting wire produces a magnetic field around it. His findings from his experiments are as follows: The magnetic compass needle is aligned tangent to an imaginary circle centered on the current-carrying cable.When the current is re
10 min read
Chapter 5 - MAGNETISM AND MATTER
CHAPTER 6 - ELECTROMAGNETIC INDUCTION
Experiments of Faraday and HenryFor a long time, electricity and magnetism were thought to be separate and unrelated phenomena. Experiments on electric current by Oersted, Ampere and a few others in the early decades of the nineteenth century established the fact that electricity and magnetism are inter-related. They discovered th
5 min read
Magnetic FluxMagnetic Flux is defined as the surface integral of the normal component of the Magnetic Field(B) propagating through that surface. It is indicated by φ or φB. Its SI unit is Weber(Wb). The study of Magnetic Flux is done in Electromagnetism which is a branch of physics that deals with the relation b
6 min read
Faraday’s Laws of Electromagnetic InductionFaraday's Law of Electromagnetic Induction is the basic law of electromagnetism that is used to explain the working of various equipment that includes an electric motor, electric generator, etc. Faraday's law was given by an English scientist Michael Faraday in 1831. According to Faraday's Law of El
10 min read
Lenz's LawLenz law was given by the German scientist Emil Lenz in 1834 this law is based on the principle of conservation of energy and is in accordance with Newton's third law. Lenz law is used to give the direction of induced current in the circuit. In this article, let's learn about Lenz law its formula, e
7 min read
Motional Electromotive ForceThe process of induction occurs when a change in magnetic flux causes an emf to oppose that change. One of the main reasons for the induction process in motion. We can say, for example, that a magnet moving toward a coil generates an emf, and that a coil moving toward a magnet creates a comparable e
14 min read
Inductance - Definition, Derivation, Types, ExamplesMagnetism has a mystical quality about it. Its capacity to change metals like iron, cobalt, and nickel when touched piques children's interest. Repulsion and attraction between the magnetic poles by observing the shape of the magnetic field created by the iron filling surrounding the bar magnet will
13 min read
AC Generator - Principle, Construction, Working, ApplicationsA changing magnetic flux produces a voltage or current in a conductor, which is known as electromagnetic induction. It can happen when a solenoid's magnetic flux is changed by moving a magnet. There will be no generated voltage (electrostatic potential difference) across an electrical wire if the ma
7 min read
CHAPTER 7 - ALTERNATING CURRENT
AC Voltage Applied to a ResistorAlternating Currents are used almost as a standard by electricity distribution companies. In India, 50 Hz Alternating Current is used for domestic and industrial power supply. Many of our devices are in fact nothing but resistances. These resistances cause some voltage drop but since the voltage thi
5 min read
Phasors | Definition, Examples & DiagramPhasor analysis is used to determine the steady-state response to a linear circuit functioning on sinusoidal sources with frequency (f). It is very common. For example, one can use phasor analysis to differentiate the frequency response of a circuit by performing phasor analysis over a range of freq
10 min read
AC Voltage Applied to an InductorAlternating Currents and Voltages vary and change their directions with time. They are widely used in modern-day devices and electrical systems because of their numerous advantages. Circuits in everyday life consist of resistances, capacitors, and inductances. Inductors are devices that store energy
5 min read
AC Voltage Applied to a CapacitorAlternating Currents and Voltages vary and change their directions with time. They are widely used in modern-day devices and electrical systems because of their numerous advantages. Circuits in everyday life consist of resistances, capacitors, and inductance. Capacitors are the devices that accumula
6 min read
Series LCR CircuitsIn contrast to direct current (DC), which travels solely in one direction, Alternating Current (AC) is an electric current that occasionally reverses direction and alters its magnitude constantly over time. Alternating current is the type of electricity that is delivered to companies and homes, and
8 min read
Power Factor in AC circuitThe power factor is determined by the cosine of the phase angle between voltage and current. In AC circuits, the phase angle between voltage and current is aligned, or in other words, zero. But, practically there exists some phase difference between voltage and current. The value of the power factor
8 min read
TransformerA transformer is the simplest device that is used to transfer electrical energy from one alternating-current circuit to another circuit or multiple circuits, through the process of electromagnetic induction. A transformer works on the principle of electromagnetic induction to step up or step down th
15+ min read
CHAPTER 8 - ELECTROMAGNETIC WAVES
CHAPTER 9 - RAY OPTICS AND OPTICAL INSTRUMENTS