Why doesn't brighter light make the electrons faster?

That time, that place

Shine light on a metal and electrons come out. That much was known.

But something was odd. Make the light brighter and the electrons do not come out faster — only more of them. Shift the colour towards blue and they do come out faster. A dim blue beats a bright red.

Why this question

If light is a wave this cannot happen. As a bigger wave rocks a boat harder, brighter light should push electrons harder.

Einstein's account: suppose light arrives in lumps, and the punch of one lump is set by its colour. Blue lumps hit hard, red ones softly.

Then brightening means sending more lumps, not making each one stronger. No number of weak lumps will kick an electron hard.

What was found

This made real what Planck had assumed under duress five years earlier. Planck supposed energy is exchanged in lumps but took it for a computational trick. Einstein said light itself is built that way.

Millikan spent a decade trying to disprove it. The result went the other way: his precise measurements confirmed Einstein's equation.

Einstein's Nobel came for this, not for relativity. And solar panels, the image sensor in a digital camera, and the sensor on an automatic door are all this effect.

Terms on this card

The photoelectric effect