Table of Contents
Photon Energy
A photon is a quantum of electromagnetic radiation carrying a discrete amount of energy determined by its frequency (or equivalently, wavelength). The energy of a photon is given by the Planck-Einstein relation: E = hf = hc/lambda. Higher frequency (shorter wavelength) photons carry more energy. This quantization of light energy is one of the cornerstones of quantum mechanics.
Photon energy ranges from femto-electronvolts for radio waves to mega-electronvolts for gamma rays. Visible light photons have energies between about 1.65 eV (red, 750nm) and 3.1 eV (violet, 400nm). Understanding photon energy is essential in spectroscopy, photochemistry, solar energy, lasers, and medical imaging.
Planck-Einstein Relation
Where h = 6.626 × 10⁻³⁴ J·s (Planck's constant), f is frequency, c = 3 × 10⁸ m/s, and λ is wavelength.
Electromagnetic Spectrum Energies
| Type | Wavelength | Energy (eV) |
|---|---|---|
| Radio | > 1 mm | < 0.001 |
| Infrared | 700 nm - 1 mm | 0.001 - 1.77 |
| Visible | 400 - 700 nm | 1.77 - 3.1 |
| Ultraviolet | 10 - 400 nm | 3.1 - 124 |
| X-ray | 0.01 - 10 nm | 124 - 124,000 |
Frequently Asked Questions
What is an electron-volt?
An electron-volt (eV) is the energy gained by one electron accelerated through 1 Volt of potential difference: 1 eV = 1.602 × 10⁻¹⁹ Joules. It is a convenient unit for photon and particle energies because the values are human-readable numbers rather than extremely small exponentials.
Can you add photon energies?
Not in the usual sense. Two photons of energy E do not combine into one photon of energy 2E under normal conditions. However, in nonlinear optics, processes like second-harmonic generation combine two photons into one with double the frequency and energy. This requires special crystals and intense laser light.