The Photon
Simple Explanation
A photon is a single, discrete packet (or "quantum") of light energy. Light is not delivered in a smooth, continuous stream, but in these individual packets, each carrying an energy of E = hf — proportional to the frequency of the light. Higher-frequency light (like ultraviolet or X-rays) delivers much more energetic photons than lower-frequency light (like infrared or radio waves).
Why Do We Need It?
The photon concept was revolutionary — it showed that light has particle-like properties as well as its well-established wave properties, launching the idea of wave-particle duality that underlies all of quantum physics.
Formula
Photon Energy
E = hf = hc/λ
Light delivers its energy in discrete packets called photons. The energy of a single photon depends only on the frequency (or, equivalently, the wavelength) of the light — higher-frequency light delivers more energetic photons.
- E
- — energy of one photon, in joules (J)
- h
- — Planck's constant, 6.63 × 10⁻³⁴ J·s
- f
- — frequency of the light, in hertz (Hz)
- c
- — the speed of light, 3 × 10⁸ m/s
- λ
- — wavelength of the light, in metres (m)
When to use it: Whenever the energy carried by a single photon of light needs to be found from its frequency or wavelength.
Worked Example
Find the energy of a photon
Find the energy of a single photon of green light with a frequency of 5.5 × 10¹⁴ Hz. Using h = 6.63 × 10⁻³⁴ J·s.
Why Does This Work?
Since Planck's constant is a fixed proportionality factor between frequency and energy, multiplying it by the frequency of any light source directly gives the fixed energy carried by every single photon of that light — the same for every photon of the same color.
Real-Life Example
Why ultraviolet light causes sunburn but visible light does not
Sitting in the sun causes sunburn from UV rays, but the visible light reaching your eyes at the same time does not burn your skin.
UV light has a much higher frequency (and shorter wavelength) than visible light, so its photons carry far more energy — enough to damage skin cells — while visible-light photons carry too little energy to cause the same harm.
Practice
Find the energy of a photon of radio waves with a frequency of 1 × 10⁶ Hz. Using h = 6.63 × 10⁻³⁴ J·s. (Enter the answer as the coefficient × 10⁻²⁸, e.g. for 6.63×10⁻²⁸, enter 6.63.)
MediumCommon mistake
Confusing the intensity (brightness) of light with the energy of each individual photon — a very BRIGHT red light still delivers only low-energy (red) photons, just many more of them per second, while even a very DIM ultraviolet light delivers high-energy photons, just fewer of them.
Quick Review
- E = hf = hc/λ
- Light delivers energy in discrete packets called photons.
- Higher frequency (shorter wavelength) means more energetic photons.