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Yaya, if you shine light on metals, electrons may start jumping out.
Oh? Are electrons afraid of light? like vampires?
Photoelectric effect is the emission of electrons from a metal surface, when illuminated by light of sufficiently high frequency.
The electrons ejected in this manner are called photoelectrons.
To escape from the surface, the electron must absorb enough energy, to overcome the attraction of positive ions in the metal lattice.
The minimum amount of energy each electron has to gain, is called the work function.
If light were indeed a wave, energy should be supplied continuously to all the electrons,
and the electrons should be “gathering up” the energy continuously, until they are energetic enough to escape.
Do the electrons care about the color of light?
The rate at which a wave propogates energy is directly proportional to both amplitude square and frequency square.
If I were an electron stuck in the metal, I don’t care whether you are red, orange, yellow, green, blue, indigo or violet.
As long as your amplitude is large enough so that you supply the energy to me fast enough, I shall escape.
What if we increase the light intensity, by increasing the amplitude of the wave?
The electrons stuck in the metal will be so thrilled if the light beam becomes brighter,
because it means energy arriving at a faster rate for all the electrons.
They will escape not just in greater numbers, but as more energetic photoelectrons as well.
What if we use really low intensity light?
A dim light is like recession times for the electrons stuck in the metal,
because they will be scrimping and saving energy at such a low rate,
we can expect a long wait before the emergence of the first photoelectron.
Guess what? All these predictions turned out to be wrong!
Mystery #1 Photoelectric effect occurs only when the frequency of light used is above certain
threshold.
Below this threshold, photoelectric effect does not occur, regardless of intensity.
Mystery #2 To all supporters of the wave theory out there, good and bad news.
The Good news is increasing the light intensity did result in more photoelectrons emitted per unit time.
The Bad news: the kinetic energy of the photoelectrons remained the same.
Instead, it was found that their kinetic energy could only be increased by increasing the frequency of light.
Mystery #3
No matter how low the light intensity is, photoelectric effect, when it does occur, always occurs instantaneously with zero delay.
Papa, I think I get it.
Physicists thought that since light is a wave, photoelectric effect should be like boiling water using fire.
The color of the flame should not be the crucial factor, as long as there is enough of the fire.
So when the water didn’t boil when it was supposed to boil, and boiled when it was not supposed to boil,
physicists knew there is something wrong with the wave theory.
You got it.