Look at the curcuit diagram. There is a big gap in the circuit so no current should flow. There are two metal plates on the sides of the gap. One is made of gauze so we can shine light through it to hit the other plate.
A weird thing happens:
1. If we start with a low frequency of light we get no current, but if we increase the frequency we suddenly get to a point where there is a current. Light releases electrons to cross the gap.
2. If we keep increasing the frequency above the threshold frequency but keep the brightness the same, we get exactly the same ammeter reading but the electrons can be measured to have more kinetic energy.
3. If I pick a fixed frequency below the threshold and keep making it brighter, there will still be no current. Yes, there will be more energy and the bright light might even feel hot, but there will still be no electrons.
4. If I pick a fixed frequency above the threshold and keep making it brighter, the current will go up and up. However, the measured kinetic energy will stay the same.
Points 3 and 4 are evidence for the particle nature of light - that it is made of lumps of energy called photons.
Photons are like getting money from a cash machine. I can have as much as I like but it will be made up of £20 notes. So I can't get a £100 note, but I can get five £20s. I can't get a £280 note but I can get fourteen £20s.
Brighter light is like this. Below the threshold I get more lumps of insufficient energy. An electron can only collide with one photon at a time so it can never get enough energy.
Above the threshold, there will be more lumps of energy available so more electrons will be able to absorb the necessary energy at once so more electrons will escape and the current goes up.
