Objective : Determination of Planck's Constant

Work Function of Materials by Photoelectric Effect

It was observed as early as 1905 that most metals under influence of radiation, emit electrons. This phenomenon was termed as photoelectric emission

The energy of emitted photo-electrons can be measured by simple retarding potential techniques as is done in this experiment. When a retarding potential V0 is used to measure kinetic energy of electrons Ee

Ee = ½(mv2) = eV0

or

V0 = (h/e)(n-f)

So when we plot a graph V0 as a function of n, the slope of the straight line yields h and the intercept of extrapolated point n=0 can give work function f.

Apparatus consists of:

1. Photo Sensitive Device : Vacuum photo tube.

2. Light source : Halogen tungsten lamp 12V/35W.

3. Colour Filters : 3 FILTERS

4. Accelerating Voltage : Regulated Voltage Power Supply,

Output : ± 15 V continuously variable through multi-turn pot

Display : 3½ digit 7-segment LED

Accuracy : ± 0.2%

5. Current Detecting Unit : Digital Nanoammeter

It is high stability low current measuring instrument

Range : 100 mA with 100 % over ranging facility

Resolution : 1nA at 1 mA range

Display : 3½ digit 7-segment LED

Accuracy : ±0.2%

6. Power Requirement : 220V ± 10%, 50Hz.

7. Optical Bench :

The set is complete in all respect, no additional accessory required.

Typical graph of V0 (stopping potential) as a function of n0 is given below.