Photoelectric Effect
Hertz and Lenard observations, threshold frequency, stopping potential and Einstein's equation. (Physics › Dual Nature of Matter and Radiation, NEET UG syllabus.)
What is Photoelectric Effect?
The phenomenon of emission of electrons from a metal surface when light of suitable frequency falls on it.
Key formula / rule: Energy of a photon
Key points
- Define the photoelectric effect and its key experimental observations.
- Explain the failure of classical wave theory to explain the photoelectric effect.
- State and apply Einstein's photoelectric equation.
- Define and calculate work function, threshold frequency, and stopping potential.
Common exam trap
Confusing intensity with frequency: Intensity affects the *number* of electrons, while frequency affects their *kinetic energy*.
Definitions
- Term
Photoelectric Effect
- Meaning
The phenomenon of emission of electrons from a metal surface when light of suitable frequency falls on it.
- Term
Work Function (Φ₀)
- Meaning
The minimum amount of energy required for an electron to escape from the surface of a particular metal.
- Term
Threshold Frequency (ν₀)
- Meaning
The minimum frequency of incident light below which no photoelectrons are emitted from a metal surface, regardless of the intensity of light.
- Term
Stopping Potential (V₀)
- Meaning
The minimum negative potential applied to the collector electrode with respect to the emitter electrode that is just sufficient to stop the most energetic photoelectrons from reaching the collector. It is a measure of the maximum kinetic energy of the photoelectrons.
Learning objectives
Define the photoelectric effect and its key experimental observations.
Explain the failure of classical wave theory to explain the photoelectric effect.
State and apply Einstein's photoelectric equation.
Define and calculate work function, threshold frequency, and stopping potential.
Analyze graphs related to photoelectric effect (e.g., Kmax vs. ν, photocurrent vs. intensity, photocurrent vs. potential).
Solve numerical problems based on the photoelectric effect.
Formulae
- Name
Energy of a photon
- Note
h is Planck's constant, ν is frequency, λ is wavelength, c is speed of light.
- Expression
E = hν = hc/λ
- Name
Work Function
- Note
Φ₀ is the minimum energy required to eject an electron, ν₀ is threshold frequency, λ₀ is threshold wavelength.
- Expression
Φ₀ = hν₀ = hc/λ₀
- Name
Einstein's Photoelectric Equation
- Note
hν is incident photon energy, Φ₀ is work function, Kmax is maximum kinetic energy of emitted electron.
- Expression
hν = Φ₀ + Kmax
- Name
Maximum Kinetic Energy in terms of Stopping Potential
- Note
e is charge of electron, V₀ is stopping potential.
- Expression
Kmax = eV₀
Prerequisites
Basic understanding of light as an electromagnetic wave.
Concept of energy and kinetic energy.
Basic algebra and graph interpretation.
Understanding of electric potential and charge.
Common mistakes
Confusing intensity with frequency: Intensity affects the *number* of electrons, while frequency affects their *kinetic energy*.
Forgetting the threshold frequency: Assuming electrons will always be emitted if light falls on the metal.
Incorrectly applying Einstein's equation: Not understanding that hν is incident photon energy, Φ₀ is energy to escape, and Kmax is excess energy.
Units confusion: Using electron volts (eV) for energy and joules (J) interchangeably without proper conversion (1 eV = 1.602 × 10⁻¹⁹ J).
Not relating stopping potential to kinetic energy: Kmax = eV₀.
Keywords
Photoelectric effect
photon
electron
work function
threshold frequency
stopping potential
Planck's constant
Einstein's equation
quantum theory
dual nature
photocurrent
kinetic energy
Practice preview
In Einstein's photoelectric equation, K_max = h*nu - W_0, what does W_0 represent?…
easy
When monochromatic light is incident on a photosensitive surface, the stopping potential is found to be V_0. If the intensity of the incident light is doubled while keeping the frequency constant, what will be the new st…
medium
Light of frequency nu is incident on a metal surface. If the maximum kinetic energy of the emitted photoelectrons is K, and the work function of the metal is W_0, then which of the following statements is INCORRECT?…
hard