Farid parks a powerful red spotlight an inch from a zinc strip and leaves it all afternoon. Not one electron leaves. A weak ultraviolet lamp ejects them easily. Why does zinc refuse red light?
The wave model predicted that bright red light should eventually eject electrons from zinc. It never does. The photon model explains the threshold; this lesson gives the explanation.
Two taught facts do all the work: each ejection is powered by one photon, and energy per photon follows the light's frequency.
Freeing an electron from a metal takes energy. The one photon an electron absorbs must supply all of it by itself.
Below zinc's threshold, every red photon is individually too weak to eject an electron, however many arrive.
Brightness cannot rescue red light. More red photons mean more too-small lumps, and no electron can pool them.
Above the threshold, each absorbed photon's single lump is enough on its own, so ejections happen.
That answers the first broken prediction. The wave model expected any frequency to eject eventually, given enough intensity and patience. Lump-by-lump absorption forbids exactly that.
Bright red light shines on zinc all day. Explain what is ejected.
Red light sits below zinc's threshold frequency.
So every red photon individually carries too little energy to free an electron.
An electron cannot pool many red photons, so the count of photons never matters.
Nothing is ejected, however long and however bright: each red photon is individually too weak.
A metal's threshold frequency sits in violet light. Bright blue light shines on it all day. Predict the ejections.
Blue light has a lower frequency than violet light, so it sits below this metal's threshold.
Each blue photon therefore carries too little energy to free an electron.
The electrons cannot pool lumps, so brightness and time change nothing.
No ejections at all, however bright the blue light is.
Light below a metal's threshold frequency shines brightly on it for hours. How many electrons does it eject?
Why does light below the threshold frequency eject nothing, however bright it is?
Hamish says: 'Below the threshold frequency, making the light brighter still ejects nothing.' Is Hamish correct?
A metal ejects electrons under violet light but not under orange light. Which statement explains the difference?
A metal's threshold frequency lies in green light. Which lamp ejects electrons from it: a dim blue lamp or a bright red lamp?
However many red photons strike a zinc plate, none ejects an electron. Which fact about absorption makes their number useless?