Simulation · Physics · Class 12
A beam of photons
From the lesson The photon in Dual Nature of Radiation and Matter. Change the values and watch what happens.
A beam of photonsPhysics · Class 12
The idea behind it
NCERT §11.7
- The photoelectric effect shows that light, when it interacts with matter, acts as if it were made of packets of energy hν.
- Einstein showed that the quantum also carries momentum hν/c. A definite energy together with a definite momentum is a strong sign of a particle, and the particle was later named the photon. Compton's experiment on X-rays scattered by electrons, in 1924, confirmed this particle-like behaviour.
- Einstein received the Nobel Prize in Physics in 1921 for his work in theoretical physics and on the photoelectric effect; Millikan received it in 1923 for his work on the elementary charge and on the photoelectric effect.
- The photon picture: (i) in its interaction with matter, radiation behaves as if made of photons; (ii) each photon has energy E = hν, momentum p = hν/c and speed c.
- (iii) All photons of a given frequency ν, or wavelength λ, have the same energy E = hν = hc/λ and the same momentum p = hν/c = h/λ, whatever the intensity. Brighter light only means more photons crossing a given area per second.
- (iv) Photons carry no charge, so electric and magnetic fields do not deflect them. (v) In a collision between a photon and a particle, total energy and total momentum are conserved, but the number of photons need not be: a photon may be absorbed or a new one created.
- Example 11.1: a laser emits 2.0 × 10⁻³ W of light at 6.0 × 10¹⁴ Hz. Each photon has E = hν = 6.63 × 10⁻³⁴ × 6.0 × 10¹⁴ = 3.98 × 10⁻¹⁹ J, and the number emitted per second is N = P/E = 2.0 × 10⁻³/3.98 × 10⁻¹⁹ = 5.0 × 10¹⁵.
- Exercise 11.1: a 30 kV electron can give at most 30 keV to an X-ray photon, so νmax = eV/h = (30 × 10³ × 1.6 × 10⁻¹⁹)/(6.63 × 10⁻³⁴) = 7.24 × 10¹⁸ Hz, and λmin = c/νmax = 0.0414 nm.