Holmarc's Photo Electric Apparatus (Model No: HO-ED-EM-02) is an instrument for studying the Photo Electric Effect and to obtain the Planck's constant.
A halogen source provides light energy to the photo tube. A filter wheel is placed in between the light source and photo tube to select different wavelengths. The filter wheel includes five narrow band interference filters to get the precise wavelength. A vacuum photo tube is kept inside the tube housing. This tube has very low anode dark current to achieve better results. The microprocessor controlled electronic interface gives the current and voltage readouts. The regulated power supply has 0 - 2V variable voltage to provide the stopping potential for electrodes. The ammeter gives accurate current output measurement facility. Toggle switch is provided to change the current range. The lamp house, filter wheel and photo tube housing are placed on an optical rail.
If we plot the Stopping potential versus frequency will get a straight line whose slope is h/e , where h is the planck’s constant and e is the electron charge.
Stopping Potential v/s Frequency
Photo electric effect is the liberation of electrons from the surface of a material by absorption of energy from incident light. For each metal, there exists a minimum binding energy for electron, which is a characteristic of the element, called the work function .When a photon strikes a bound electron, it transfers its energy to the electron. If this energy is less than the metal's work function, the photon is remitted and no electrons are liberated. If this energy is greater than an electron's binding energy, the electron escapes from the metal with a kinetic energy equal to the difference between the photon's original energy and the electron's binding energy.
The stopping potential is measured by varying the anode voltage until the current drops to zero. The slope of a plot of stopping potential versus frequency is the value of the ratio, h/e. Hence the value of Planck's constant (h) and work function of the material can be determined. This instrument also imparts students, fundamental understanding of the quantum nature of light.
The current amplifier is configured with high sensitivity and stability improve the accuracy of measurement
The optical filters are of high quality in order to reduce the error while selecting the wavelength
The photo tube has low levels of dark current and anode reverse current