OCR Specification focus:
‘Electromagnetic radiation has a particulate nature: photons carry energy in quanta.’
Light, once thought to be a continuous wave, is now understood to also behave as discrete packets of energy called photons. This dual nature reshaped modern physics, revealing that electromagnetic radiation transfers energy in quantised amounts rather than continuously.
The Nature of Electromagnetic Radiation
Electromagnetic (EM) radiation consists of oscillating electric and magnetic fields that propagate through space. It encompasses a wide range of wavelengths and frequencies, from radio waves to gamma rays. Classical physics described light purely as a wave phenomenon, successfully explaining interference, diffraction, and polarisation. However, certain experimental results — such as the photoelectric effect and black-body radiation — could not be explained by wave theory alone. These observations necessitated a new model describing the particulate nature of light.
Emergence of the Photon Model
Wave–Particle Duality
The photon model of electromagnetic radiation proposes that light can behave both as a wave and as a stream of particles, depending on the context of observation. Each photon is a discrete quantum of electromagnetic energy that travels at the speed of light, ccc, in a vacuum. The model bridges classical and quantum physics, recognising that energy is not infinitely divisible but instead transferred in fixed quantities.
Photon: A discrete packet, or quantum, of electromagnetic energy that exhibits both wave-like and particle-like properties.
The concept of photons was introduced by Albert Einstein in 1905 while explaining the photoelectric effect. His theory extended Max Planck’s earlier proposal that electromagnetic energy is emitted or absorbed in discrete quanta.
Quantisation of Electromagnetic Energy
Planck’s Hypothesis
In 1900, Max Planck introduced the idea that energy emitted by a black-body is quantised, meaning it can only take specific discrete values. According to his hypothesis, the energy of each quantum is directly proportional to the frequency of the radiation.
EQUATION
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Photon Energy (E) = hf
E = Energy of a single photon, measured in joules (J)
h = Planck’s constant (6.63 × 10⁻³⁴ J s)
f = Frequency of the radiation, measured in hertz (Hz)
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This equation reveals that higher-frequency radiation, such as ultraviolet or X-rays, carries more energy per photon than lower-frequency radiation, such as radio waves. The quantisation of energy fundamentally distinguishes photons from classical wave energy, which would vary continuously with intensity.
The Relationship between Wavelength and Photon Energy
Electromagnetic radiation also possesses a wavelength, the distance between successive peaks of the wave. Frequency and wavelength are related through the speed of light. Substituting this into Planck’s relation gives a useful connection between wavelength and photon energy.
EQUATION
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Photon Energy (E) = hc/λ
E = Energy of a photon (J)
h = Planck’s constant (6.63 × 10⁻³⁴ J s)
c = Speed of light in vacuum (3.00 × 10⁸ m s⁻¹)
λ = Wavelength of the radiation (m)
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This relationship shows that shorter wavelengths correspond to higher-energy photons, while longer wavelengths correspond to lower-energy photons. For instance, gamma rays carry vastly more energy per photon than infrared light. The intensity of a light source depends on the total number of photons emitted per second, not on the energy of individual photons.
An annotated electromagnetic spectrum showing regions from radio to gamma rays with aligned axes of wavelength, frequency and photon energy. It demonstrates how increasing frequency (decreasing wavelength) corresponds to increasing photon energy E=hfE=hfE=hf. Extra contextual labels, such as specific radio bands, can be ignored if not needed. Source.
Understanding Energy Transfer in the Photon Model
Each photon interacts independently with matter. When a photon is absorbed, its entire energy quantum is transferred to a single particle, such as an electron. The photon is then destroyed in the process. This one-to-one interaction is a key distinction from wave models, where energy would be spread continuously over the wavefront.
Key Points on Photon Interactions
Energy transfer occurs in discrete steps, never partially.
A photon’s energy determines its ability to cause processes like electron emission or excitation.
Increasing intensity increases the number of photons, not the energy per photon.
Increasing frequency increases the energy per photon, not necessarily the total intensity.
These ideas underpin many quantum phenomena, explaining why certain frequencies can eject electrons from a metal surface while others cannot, regardless of brightness.
Experimental Evidence Supporting the Photon Model
Several key experiments provided direct evidence for the particulate nature of light:
1. The Photoelectric Effect
Light incident on a metal surface causes electrons to be emitted only if its frequency exceeds a threshold value. Classical theory predicted that increasing intensity should always lead to emission, but experiments showed otherwise. This phenomenon confirmed that energy transfer occurs via individual photons with quantised energy hfhfhf.
A labelled schematic of the photoelectric effect showing ultraviolet photons striking a solid and ejecting electrons. It illustrates the quantised, one-to-one energy transfer central to the photon model. The diagram includes minimal additional information beyond the key interaction. Source.
2. Black-Body Radiation
Classical models predicted an infinite emission of energy at high frequencies (the ultraviolet catastrophe). Planck’s quantisation of energy resolved this by assuming that energy exchange occurs in discrete quanta, matching observed spectra.
3. Compton Scattering
In experiments where X-rays scatter off electrons, the change in wavelength corresponds precisely to the conservation of energy and momentum for photon-particle collisions. This behaviour is characteristic of particles, not waves.
These results collectively reinforced the idea that electromagnetic radiation exhibits a particulate nature.
Significance in Modern Physics
The photon model has profound implications across physics. It underpins the operation of devices such as photodiodes, solar cells, and lasers, all of which rely on the controlled emission or absorption of photons. Moreover, it provides the foundation for understanding quantum phenomena like stimulated emission and energy quantisation in atoms.
Broader Quantum Implications
Energy quantisation explains atomic emission spectra, where electrons transition between discrete energy levels by absorbing or emitting photons of specific energies.
The model unifies light and matter interactions, bridging classical electromagnetism with quantum mechanics.
It establishes that energy transfer, even in light, is fundamentally granular rather than continuous.
Summary of Key Relationships
Light exhibits both wave-like and particle-like behaviour.
Each photon carries an energy quantum given by E = hf = hc/λ.
Frequency determines photon energy; intensity determines photon number.
Experimental evidence, such as the photoelectric effect, confirms the particulate nature of electromagnetic radiation.
FAQ
The wave model successfully explains interference, diffraction, and polarisation, but it fails to describe phenomena involving energy transfer at the atomic scale.
Experiments such as the photoelectric effect and black-body radiation demonstrated that energy is not emitted or absorbed continuously but in discrete packets. The photon model accounts for these quantised interactions, explaining why certain frequencies cause electron emission while others do not, regardless of intensity.
The colour of visible light corresponds to its wavelength and frequency, which determine the photon’s energy.
Red light has longer wavelengths (around 700 nm) and therefore lower photon energy.
Blue and violet light have shorter wavelengths (around 400 nm) and higher photon energy.
This energy difference explains why blue light can trigger certain photochemical reactions that red light cannot — each photon delivers more energy to the system.
Yes — in a vacuum, all photons travel at the speed of light, c = 3.00 × 10⁸ m s⁻¹, regardless of frequency or wavelength.
In materials such as glass or water, photons are absorbed and re-emitted by atoms, making light appear to slow down. However, the photons themselves always move at speed c between these interactions. The apparent reduction in speed arises from this repeated absorption and re-emission process within the medium.
Brightness corresponds to intensity, meaning more photons per second — not higher photon energy.
If each photon’s energy (E = hf) is below the threshold energy required to release an electron, no emission occurs, regardless of intensity. Once frequency exceeds the threshold, brighter light increases the number of emitted electrons, not their individual energies.
Although photons have no rest mass, they still carry momentum because of their energy. According to quantum theory, momentum (p) and wavelength (λ) are linked by the relation p = h/λ.
This property allows photons to exert a small pressure, known as radiation pressure, on surfaces. It’s measurable in phenomena such as comet tail formation and solar sails, demonstrating that massless particles can still transfer momentum through energy exchange.
Practice Questions
Question 1 (2 marks)
State what is meant by a photon and explain how the energy of a photon depends on the frequency of the electromagnetic radiation.
Mark Scheme:
1 mark: Photon correctly defined as a discrete packet (quantum) of electromagnetic energy.
1 mark: Correct relationship stated that photon energy increases with frequency (E = hf or proportional relationship described).
Question 2 (5 marks)
Describe how the photon model of electromagnetic radiation differs from the classical wave model.
Explain how one piece of experimental evidence supports the photon model.
Mark Scheme:
1 mark: States that in the classical wave model, energy is spread continuously across the wavefront and depends on wave intensity.
1 mark: States that in the photon model, energy is carried in discrete quanta (photons) each with energy E = hf.
1 mark: Identifies suitable experimental evidence (e.g. the photoelectric effect, black-body radiation, or Compton scattering).
1 mark: Explains that in the photoelectric effect, electrons are only emitted if radiation frequency exceeds a threshold value, regardless of intensity.
1 mark: Links this to photon energy being quantised — each electron absorbs a single photon’s energy, supporting the photon model.
