Quantum Physics: Photons and the Photoelectric Effect

Introduction to Quantum Physics Concepts

Quantum physics marks a fundamental shift from classical mechanics in understanding the behavior of matter and energy at the atomic and subatomic scales. Two key concepts in quantum physics are the particle nature of light (photons) and the photoelectric effect.

Photons and the Planck Constant

In the late 19th century, Max Planck proposed that electromagnetic radiation is emitted and absorbed in discrete quantized amounts or 'packets' of energy called photons. The energy (E) of a photon is proportional to its frequency (f) by the relation:

E = hf

Where h is the Planck constant (6.63 x 10-34 J•s). Higher frequency photons, like X-rays and gamma rays, have more energy than lower frequency photons like radio waves.

The Photoelectric Effect

The photoelectric effect occurs when photons strike a metal surface and eject electrons from the metal. Key observations:

• Only photons above a certain threshold frequency can eject electrons. Lower frequencies have no effect, regardless of intensity.
• Increasing the light intensity increases the number of ejected electrons, not their kinetic energy.
• Ejected electrons have a maximum kinetic energy equal to the photon energy minus the metal's work function (binding energy).

These findings contradicted classical wave theory and provided evidence for the particle nature of light.

Photoelectric Effect Example

A photon of wavelength 300 nm strikes a sodium surface with work function 2.28 eV.

1. Calculate the photon energy in eV using E = hc/λ.
2. Determine if the photon can eject an electron from sodium.
3. If so, calculate the maximum kinetic energy of ejected electrons.

Applications of Quantum Physics

Understanding quantum behavior led to developments like:

• Lasers, LEDs, and photovoltaic cells
• Electron microscopes
• Quantum computing

For more, see TRH Science Blog and OCR A Level Physics specification.