9.Determination of Planck's constant

AIM: To determine the value of Planck’s constant ‘h’ by a photocell.
APPARATUS: Photo emissive cell mounted in a box provided with a wide slit. Regulated DC power supply, Set of filters, Light source, Digital voltmeter and Digital microammeter.
PRINCIPLE: Planck’s constant ‘h’ is given by,
h = eVl/c where ‘e’ is the electron charge, ‘V’ the stopping potential corresponding to a light of wavelength ‘l’ and ‘c’ the velocity of light.

PROCEDURE: Connections are made as shown in figure (1). Light from the light source is allowed to fall on the photo cell which is enclosed in a box. The distance between the photocell and light source is adjusted such that there is sufficient flow of current. Now a suitable filter of known wavelength l1 is placed in the path of the light. A reading corresponding to the zero anode potential is observed in the microammeter. A small negative potential is applied which is gradually increased in small steps till the microammeter reading comes to zero. This is stopping potential V1 corresponding to the wavelength l1. Planck’s constant can be calculated using l1 and V1.The experiment is repeated with other filters and corresponding stopping potentials are noted. Planck’s constant is calculated in each case and mean value is taken.

OBSERVATIONS:
Wavelength (l) Stopping potential (V) Planck’s constant h = eVl/c
Mean value of h =

RESULT: Planck’s constant =