It was the period of Newton when he said the light is nothing but a stream of particles, and with which the reflection, refraction and other observed phenomena are explained. Everything was good.
But after that, there were new observations which could not be explained by the particle nature of light.
The famous one was Young's double slit experiment, which deduced that light was nothing but a wave and was later known to be a type of electromagnetic wave (with the help of Maxwell's equations).
Then scientists re-explained how the wave-light could perform the reflection, refraction and other phenomena which were previously under the category of particle-light.
So people changed their perspective and agreed that light was a wave. Again, everything was good.
But with no concern about the mental peace of scientists, another observation had appeared - the photoelectric effect.
What is the photoelectric effect?
In short, you throw the light at the surface of metals, and they give out electrons.
What we should expect from the wave-light?
The wave-light has two main properties - frequency and amplitude, and the energy of the light is proportional to both.
So, if the frequency is high but the amplitude is low (or) if the frequency is low but the amplitude is high, the energy of the light could be considered the same.
Once you got the light to the required energy, and you shoot it on metal surfaces, it gives off electrons.
But what was reported by observations?
If you give the input of light with more frequency and less amplitude, electrons are coming out, but if you give it with less frequency and more amplitude, electrons are not coming!
"What the heck is happening??" - the response from the then scientists. (You can relate to this feeling, can't you?)
Then came Einstein to help us - saying that light behaves like a particle! (Yeah, back to square one...)
How did 'light as the particle' explain the photoelectric effect?
The light consists of particles and the energy of these particles solely depends on the frequency.
The amplitude of wave-light, which corresponded to the intensity (brightness, to put it simply) was mapped to the number of light particles.
So, take the case of more frequency and low amplitude(intensity):
As the particles have more frequency, means they have more energy. So if they have enough energy they can eject electrons from the surface. Job done.
Now, the case of less frequency and more amplitude:
This case is nothing but having more particles at hand, but none of them has more energy.
So, as long as the particles don't have enough energy, the number of particles you throw on metals does not matter...
This explanation matched the observations perfectly.
What happened afterwards?
Yeah so when we got the evidence for both wave-light and particle-light, we started to accept (once more) and live with the truth that light exists as both wave and particle - the wave-particle duality!!
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Thank you so much for reading up on this!
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