Light carries energy in tiny packets called photons. Across the electromagnetic spectrum, the energy of a single photon depends on just one thing: its frequency.
Key idea
A photon's energy is proportional to its frequency: double the frequency, double the energy per photon. So one ultraviolet or X-ray photon carries far more energy than one visible or radio photon. A brighter light sends more photons, not stronger ones.
Why it works
To break a molecule in a living cell, a single photon must deliver enough energy at once. An ultraviolet photon has enough to break some of the bonds in skin cells' molecules, which is what causes sunburn. A red or radio photon doesn't, and adding more of them doesn't help, because each one acts alone. It is like trying to knock over a bowling pin with ping-pong balls: throwing more of them won't do it.
Worked example: Compare photons
Which carries more energy per photon: a blue photon or a red photon?
- Blue light has a higher frequency (shorter wavelength) than red.
- Higher frequency means more energy per photon.
Answer: The blue photon
Worked example: Scale it
A photon of frequency $f$ carries 2 units of energy. How much does a photon of frequency $3f$ carry?
- Energy is proportional to frequency: three times the frequency, three times the energy.
- $3 \times 2 = 6$ units.
Answer: 6 units
Common mistake
Thinking brighter light means more energetic photons, so a bright red lamp should burn skin sooner than dim ultraviolet light. Brightness is the number of photons; each red photon still has too little energy to break those molecules. Check: ask about the frequency of each photon, not how bright the light is.