Virtual Physics Laboratory

Frank-Hertz Experiment & Photoelectric Effect Simulation

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Frank-Hertz Experiment
Photoelectric Effect

Frank-Hertz Experiment

Observe the quantized energy levels of mercury atoms by measuring the current through the tube at different accelerating voltages.

Experimental Setup

Controls

0 V

Data Table

Voltage (V) Current (µA) Peak Number Energy (eV)

Calculations & Results

Photoelectric Effect

Investigate how the kinetic energy of emitted photoelectrons varies with the frequency of incident light.

Experimental Setup

Controls

400 nm
50%
0 V

Data Table

Wavelength (nm) Frequency (×10¹⁴ Hz) Stopping Potential (V) Current (nA) Kinetic Energy (eV)

Calculations & Results

Lab Manual ▼

Frank-Hertz Experiment

Objective: To demonstrate the existence of discrete energy levels in atoms by measuring the kinetic energy lost by electrons during inelastic collisions with mercury atoms.

Theory: The Frank-Hertz experiment shows that electrons occupy only discrete, quantized energy states. When electrons are accelerated through a mercury vapor-filled tube, they undergo elastic collisions until they acquire sufficient energy to excite a mercury atom (4.9 eV). This results in a drop in current, which occurs periodically as the voltage increases.

Procedure:

  1. Begin with the accelerating voltage set to 0 V.
  2. Slowly increase the voltage while observing the current.
  3. Record the voltage and current values, especially at the peaks and troughs.
  4. Continue until you have recorded 4-5 peaks.
  5. Calculate the energy difference between peaks to determine the first excitation energy.

Calculations: The first excitation energy is calculated by finding the average voltage difference between successive current peaks and converting to electronvolts (eV).

Photoelectric Effect

Objective: To verify the particle nature of light and determine Planck's constant by measuring the stopping potential for different frequencies of light.

Theory: The photoelectric effect occurs when photons of sufficient energy strike a metal surface, ejecting electrons. The maximum kinetic energy of the ejected electrons is given by K.E. = hν - φ, where h is Planck's constant, ν is the frequency of light, and φ is the work function of the material.

Procedure:

  1. Select a wavelength of light using the wavelength control.
  2. Adjust the stopping potential until the photocurrent drops to zero.
  3. Record the wavelength (or frequency) and the stopping potential.
  4. Repeat for at least 5 different wavelengths.
  5. Plot stopping potential versus frequency and determine the slope, which gives h/e.

Calculations: Planck's constant is calculated from the slope of the stopping potential vs. frequency graph. The work function is determined from the intercept on the potential axis.