Light is made up of tiny particles called photons.

A photon is like an isolated ripple of electromagnetism. If you remember back to an earlier post, we imagined a long rope tethered at one end. If you get the other end of the rope and twitch it once, you can send a single ripple travelling down the rope to the other end. If you continue twitching the rope you can send a continuous wave down the rope, with one ripple following after another. What you can't do, however, is to send a fraction of a ripple- vibrations don't work that way. It's the same with light. A photon is the smallest unit of light that can be made.

Do you remember earlier we talked about frequency and energy? We said that a rapid vibration contains more energy than a slow one (all other things being equal). That is true of photons too. The higher the frequency of the photon, the more energy it has.

The energy of a photon can be calculated by multiplying its frequency (ie the number of times it vibrates in a second), by a number called Planck's constant. It is named after Max Planxk who was a famous physicist.

Planck's constant is a tiny number- it is just under seven divided by ten thousand million million million million million, so the energy in most photons is tiny. That said, elecrttrons are tiny too, so a single photon can make an electron move.

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