Physics
Photon Energy
Quick fact
A single photon of red light carries about 3 × 10⁻¹⁹ joules — roughly a billionth of a billionth of the energy needed to lift a grain of sugar one millimeter.
Why this is interesting
When sunlight warms your skin, you're feeling energy carried by countless tiny particles. But how much energy does each one of those particles actually hold?
Read the full explanation
Understanding Photon Energy
Light is not a continuous stream but comes in discrete packets called photons. Each photon is the smallest possible unit of light at a given frequency. The energy of a photon is determined solely by its frequency (or equivalently, its wavelength). Higher-frequency light (like blue or ultraviolet) has more energetic photons than lower-frequency light (like red or infrared). Think of photons as 'energy packets' — like individual coins in a currency where the coin's value changes with color. When light interacts with matter, it transfers energy one photon at a time, which explains why dim blue light can trigger a photochemical reaction while bright red light cannot.
A deeper explanation
The relationship between photon energy and frequency is given by the Planck-Einstein equation: E = hf, where E is energy, f is frequency, and h is Planck's constant (6.626 × 10⁻³⁴ J·s). This simple formula reveals the quantum nature of light: energy is quantized, not continuous. A photon's energy can also be expressed in terms of wavelength λ: E = hc/λ, where c is the speed of light. This means shorter wavelengths (like X-rays) carry immensely more energy than longer wavelengths (like radio waves). The concept of photon energy is critical to understanding the photoelectric effect, where electrons are ejected from a metal only when the photon energy exceeds a threshold — regardless of light intensity. This principle is harnessed in solar cells, photodetectors, and even in medical imaging. In quantum mechanics, photon energy is also the key to atomic transitions: atoms absorb or emit photons with energies equal to the difference between their energy levels, producing characteristic spectra.