Millikan’s Experiment to explain Einstein’s photoelectric effect and to determine the value of plank’s constant



Millikan’s Experiment to explain Einstein’s photoelectric effect and to determine the value of plank’s constant

Let us consider a glass tube (discharging tube) which consists of two electrodes; anode(A) an cathode(c) . It is connected as shown in fig. The experimental setup shows the Millikan’[s experiment to determine the value of plank’s constant as well as to verify the Einstein’s photoelectric effect .

In fig. µA (micro ammeter ) , V, Rh and Rc one micro ammeter, voltmeter, rheostat (potential divider) and reverse switch respectively .

IF certain frequency of light (selected from light filter) is exposed to photo metal at the cathode electrons are emitted from the metal surface which are directed towards the anode as a result photo current can be measured by µA . Then, according to Einstein’s photoelectric equation, We have,

Millikan’s Experiment to explain Einstein’s photoelectric effect and to determine the value of plank’s constant

If anode potential is adjusted to that photocurrent becomes zero, then KE of electron becomes zero. Then KE of electron becomes equal to stopping potential i.e.

Millikan’s Experiment to explain Einstein’s photoelectric effect and to determine the value of plank’s constant

Where, e = electric charge

Vo = stopping potential

 

Now from equation i and ii we get,

Millikan’s Experiment to explain Einstein’s photoelectric effect and to determine the value of plank’s constant

This equation is the equation of straight line which is in the form y = m x + c with the slope h/e and intercept (c) =Millikan’s Experiment to explain Einstein’s photoelectric effect and to determine the value of plank’s constant

if we use different light filter to select different frequency of light, corresponding values of stopping potential varies. This variation can be realized in fig 2 and 3

Millikan’s Experiment to explain Einstein’s photoelectric effect and to determine the value of plank’s constant

fig-iii clearly indicates that with the increase of frequency of incident light corresponding stopping potential increases and the pints always gives the straight line. The slope of straight line in fig-iii is graphically equal to the slope of given equation-iii

Millikan’s Experiment to explain Einstein’s photoelectric effect and to determine the value of plank’s constant

The experimentally obtained value of h is 6.631 into 10 -34 Joule sec .Hence milliken’s experiment verifies Einstein’s photoelectric equation and determine the value of plank’s constant (h).

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