Light: A Wave and a Particle

0:00Is light a wave, or is it a stream of tiny particles?

0:06First, put a screen behind two narrow slits and shine light at them.

0:12Light spreads out from each slit, like water waves through a gap.

0:17Where the waves meet in step, they add up to make bright stripes.

0:22Where they meet out of step, they cancel and leave dark stripes.

0:26This is called interference, and the stripes are an interference pattern.

0:32Interference is a wave effect, so here light behaves like a wave.

0:38Second, shine light onto a plate of potassium metal.

0:43Metals contain electrons, tiny particles with a negative charge.

0:48Light arrives in tiny packets of energy called photons.

0:53A photon of red light has too little energy to knock an electron out.

0:58Brighter red light sends more photons, but none has enough energy, so no electrons come out.

1:06Violet light has a higher frequency, more waves each second, so each photon has more energy.

1:13Even dim violet light knocks electrons out, because each photon has enough energy.

1:20This is the photoelectric effect. Packets of energy explain it, so light behaves like a particle.

1:28So light is not one or the other: it can behave like a wave or like a particle.

1:34Light spreads and interferes like a wave, and carries energy in photons like a particle.