Critical Questions:

What is light made of?

Why is the sky blue?

The main reason that light has always been so hard to figure out is that it follows its own rules.

For example, we’re used to things having mass. People, ferrets, churches, very small rocks – all are made up of matter and thus do things like resist acceleration (according to Newton’s Second Law) and create gravitational fields.

Light, on the other hand, has no mass. It’s made up of… nothing, it would seem. And as a result, it neither exerts a gravitational pull on massive objects nor ever slows down.

We’re also used to dealing with waves. While solid objects run into each other and generally follow Newton’s Laws of motion, waves interfere with each other and generally don’t.

But when scientists developed the technology to look more closely at light, they found that sometimes it behaved like a wave – interfering with other light, for example – and sometimes it behaved like a particle, in that it seemed to come in countable numbers of separate objects.

Once we started to make sense of all this, we had to assign light to a whole new category – neither wave nor particle but wave-particle, something which exhibits characteristics of both. And we eventually named the light wave-particle the photon.

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