Particle Nature of Light: The Photon
The particle nature of light states that light is composed of discrete packets of energy called
photons.
This concept was introduced by Albert Einstein in 1905 and explains phenomena like the
photoelectric
effect, Compton effect, and blackbody radiation.
### 1. What is a Photon?
A photon is an elementary particle of light that:
- Has zero rest mass.
- Moves at the speed of light (c = 3.0 x 10^8 m/s) in a vacuum.
- Carries a quantized amount of energy given by: E = h*v
where:
- E = Energy of the photon
- h = Planck's constant (6.626 x 10^(-34) J·s)
- v = Frequency of light
- Exhibits both particle and wave properties (wave-particle duality).
### 2. Evidence for the Particle Nature of Light
#### (a) Photoelectric Effect (Einstein, 1905)
- Light ejects electrons from a metal surface when its frequency is above a certain threshold.
- The kinetic energy of emitted electrons depends on light's frequency, not its intensity.
- Classical wave theory could not explain this.
#### (b) Compton Effect (Arthur Compton, 1923)
- X-rays scatter off electrons, showing momentum transfer like particles.
- The shift in wavelength of scattered X-rays can only be explained if photons have momentum.
#### (c) Blackbody Radiation (Max Planck, 1900)
- Classical physics predicted an ultraviolet catastrophe.
- Planck solved this by proposing that energy is emitted in discrete packets (quanta), leading to the
concept of photons.
The particle nature of light states that light is composed of discrete packets of energy called
photons.
This concept was introduced by Albert Einstein in 1905 and explains phenomena like the
photoelectric
effect, Compton effect, and blackbody radiation.
### 1. What is a Photon?
A photon is an elementary particle of light that:
- Has zero rest mass.
- Moves at the speed of light (c = 3.0 x 10^8 m/s) in a vacuum.
- Carries a quantized amount of energy given by: E = h*v
where:
- E = Energy of the photon
- h = Planck's constant (6.626 x 10^(-34) J·s)
- v = Frequency of light
- Exhibits both particle and wave properties (wave-particle duality).
### 2. Evidence for the Particle Nature of Light
#### (a) Photoelectric Effect (Einstein, 1905)
- Light ejects electrons from a metal surface when its frequency is above a certain threshold.
- The kinetic energy of emitted electrons depends on light's frequency, not its intensity.
- Classical wave theory could not explain this.
#### (b) Compton Effect (Arthur Compton, 1923)
- X-rays scatter off electrons, showing momentum transfer like particles.
- The shift in wavelength of scattered X-rays can only be explained if photons have momentum.
#### (c) Blackbody Radiation (Max Planck, 1900)
- Classical physics predicted an ultraviolet catastrophe.
- Planck solved this by proposing that energy is emitted in discrete packets (quanta), leading to the
concept of photons.
