Photons
WordPress-ready reconstruction from the supplied 5-page handwritten notes.
Original Scanned PDF – View Notes
Photon
Planck’s Quantum Theory of Radiations
According to Planck’s, blackbody radiations are emitted in the form of small packets of energy called quantum. Each quanta carry energy which is given by:
Where, E → energy of quanta, h → Planck’s constant, f → frequency of radiation.
Photoelectric Effect
The process of emission of electrons from a metal surface when a light of suitable frequency strikes the metal is called photoelectric effect.
The emitted electrons are called photoelectrons and current due to photoelectrons is called photoelectric current.
Threshold Frequency (f0)
It is the minimum frequency of light required for photoelectric effect.
c = speed of light.
Threshold wavelength (λ0)
It is the maximum wavelength of light required for photoelectric effect.
Work Function (φ)
It is the minimum energy of incident light required for photoelectric effect.
Einstein’s Photoelectric Equation
According to Einstein light contains small packets of energy called photons. Each photon carry energy E = hf. When light of suitable frequency strikes the metal, the photon give energy to the electron in the metal. These energy is used as work function and provides K.E. to the photo electrons.
Mathematically
Stopping Potential (Vs or V0)
It is the negative or retarding potential that makes the photoelectric current zero.
At stopping potential, work done by potential = K.E. of photoelectrons.
Vs = stopping potential
Millikan’s Experiment to Determine Value of Planck’s Constant
(Verification of Einstein’s Photoelectric Equation)
The experimental setup is as shown in figure. It consists of an evacuated glass tube having a rotating wheel at its center in which alkali metals like K, Li, Na are attached. A collector is used to collect the emitted electrons and a knife is used to make the metals fresh.
When a light falls on the metal, electrons are emitted. The emitted electrons are collected by the collector and photoelectric current is produced. The negative potential is applied between metal and collector with the help of variable D.C. source. The negative potential is adjusted such that photoelectron just stop to reach the collector and photoelectric current becomes zero. This potential is stopping potential.
At stopping potential
From Einstein’s equation:
The equation is similar to y = mx + c which is equation of straight line.
When a graph between stopping potential and frequency plotted, a straight line graph as shown in figure.
From graph:
Discussion
Share a helpful question, idea, or explanation with other students.