photon
/ FOH-ton /
A photon is a single, indivisible piece of light — the smallest packet light can come in. You can't split one in half; it's all or nothing.
Photons have no mass, so they always travel at the speed of light — nothing can speed them up or slow them down in empty space. Each one carries a tiny bit of energy, and that energy depends on the light's frequency: bluer, higher-frequency light packs more energy per photon than redder light.
Strangely, light acts like both a wave and a stream of these particles at once. Spread out, it ripples and bends like a wave; up close, it arrives in countable dots — and a photon is one of those dots.
When your eye sees a faint star, it may be catching just a handful of photons that left that star thousands of years ago.