The Scientist Milikon’s first measure the value of Planck constsnt(h) with a experiment **Measurements of Planks Constant and Verification of Photoelectric Equation by Millikon’s Apparatus**.

The above diagram is simplified sketch of **Millikon’s apparatus** of value of Planck’s Constant. The apartments consists of highly cylindrical glass tube having electron collecting plate ‘A’ and emiting plate ‘C’. There is a glass window with filter in the two glass tube through which radiation of different frequency are entered and incident to plate ‘C’.

A millimetre and voltmeter respectively measure magnitude of photoelectric current and stopping potential. The function of rheostat is the to change the magnitude of reverse voltage.

At first radiation of frequency as exposed to the plate ‘C’ through the glass window.A number of electron is emited and accelerated towards plate ‘A’, photoelectric current is established. if we increase the value of reverse voltage ,magnitude of photoelectric current decreases.

Again reverse voltage increases magnitude of photoelectric current decreases at the certain value of reverse voltage ,photoelectric current becomes zero.This reverse voltage which magnitude of photoelectric current becomes zero is called **stopping potential**.

At stopping potential,

**Work done by stopping potential = Gain in K.E**

**eVo = 1/2mv2 …..1**)

Where e = charge of electron

Vo = stopping potential

m = mass of electron

v = velocity of electron

From Einestein’s photoelectric equation,

**hf = hfo+ 1/2mv2**

**or, hf-hfo = 1/2mv2**

**hf-hfo = eVo**

**Vo = (h/e)f + (-hfo/e) ……2)**

Equation 2) is obtained in the form of y= mx+c with slope m= h/e and y-intercept = -hfo/e .Hence **Einestein photoelectric equation** is verified.

For measurement of value of h, let us take another radiation and allowed to exposed to plate ‘C’ .its corresponding stopping potential is found out. The experiment is repeated with different frequency of radiation and their corresponding value of stopping potential are measured.if we draw the graph of stopping potential verse frequency of Radiation we get,straight line.

From the graph,

Slope of line AD,

**m = tanΦ**

**h/e = AB/BC**

**h/e = ΔVo/Δf**

**h = e(ΔVo/Δf) ….3)**

## FAQ

### How do you calculate Planck’s constant?

We can easily calculate the value of Planck’s constant by Millikon’s apparatus by formula h = e(ΔVo/Δf) in lab.

### How do you find the Planck constant using photoelectric method?

By photoelectric method we can calculate the value of planck constant by using Millikon’s apparatus of formula h = e(ΔVo/Δf).

### what is Threshold frequency?

Threshold frequency is the minimum frequency of incident radiation on the metal surface below which photoelectrons emitted from the surface.

### What is stopping potential?

The minimum value of retaring potential by the application of which in the metallic surface .The ejection of photoelectron become just stop is called stopping potential.

### What is Work function?

Work function of metal or non-metal is defined as the minimum energy absorbed by the surface to eject in electron without impacting any K.E of photoelectron.

Measurement of Planck’s Constant and Verification of Photoelectric Equation is that of all above