Quantum mechanics begin with a conflict of what exactly light is ?

One experiment say that it is a wave in nature and another experiment says it is particle.

After some experiment it come with a strange concept that light was both (particle and wave).

Such a thought had started new thing in called quantum mechanics.

One of the most important clues that light had to be more than just a wave was what’s known as

The ultraviolet catastrophe .

Now new thought begin how object radiate heat .Basis of this thought new law came out

Stefan-Boltzmann law,  that the total radiant heat energy emitted from a surface is proportional to the fourth power of its absolute temperature.

Object actually radiate energy that covers a whole range of frequencies on the electromagnetic spectrum all different kind of light .on the basis of above law :-

Black bodies –means a body which absorb light without reflecting any and radiate energy accordingly. not  all the energy coming from a blackbody has the same intensity .

Only we can predict intensity of energy based on its temperature .on the basis of this intensity physists made equation and said light is a wave a new law comes out Rayleigh–Jeans Law higher is the frequency  of the radiation shoter wavelength .

But limitation of above frequency of light got into the uv range or higher . the Rayleigh-jeans didn’t

find fit this experiment result .

at certain frequency the light would be at its most intense and after that the intensity would actually drop as the frequency increased .

result shows a certain frequency the light would be at its most intense and after that ,the intensity would actually drop as the frequency increased .

it means warmer the object ,higher the frequency of the peak intensity . but there was always a peak

it wasn’t supposed to be this way .even worse if you summed up the contributions of higher and higher frequencies to calculate.

The total power emitted by a blackbody ,the Rayleigh law predicted that you’d find infinite power.

But which contradicts the principal of conservation of energy .This was ultraviolet catastrophe.

Now after this catastrophe an equation that basically led to quantum mechanics.

A new concept is given by planks ;-

Electromagnetic energy takes packets called quanta. Simply means you cannot divide energy into anything smaller than these packets.

E=h*

E-energy of photon=plank constant ,= frequency

Energy in each quantum is equal to the frequency of light times a very small .with equation we can calculate the radiation of black body. that shows experimental result perfectly, as the peak of intensities as earlier .with these experiment and theory smallest particle is quanta and we cannot divide it into any further.

But new hero came into play called  Einstein.

He said light energy travelled in packets which is called photons.which essentially make light behave like a particle .but it is weird  because many experiment shows light is wave .

But Einstein theory based on photoelectric effect.

When you shine a beam electron leave the plate and hit a nearby collector creating a current .by studying this he found electron left plate

And propose both wave and particle theory

Wave theory ;- when a light wave hits an electron ,it exerts a force on the electron that ejects it from the metal.

Means if anyone increase the strength of electric field hitting the electrons, higher intensity means you eject more electron have maximum kinetic energy .

Such KE MAX leaving plate ,wave theory say the frequency of light shouldn’t make a difference .

Only intensity matters.

Particle theory ;- says that electron get ejected from the metal when they are hit by individual photon .The photon transfer its energy to the electron which pos out of the metal. photon destroyed in the process. But there’s a minimum energy that the photon needs to transfer, in order to get the electron to overcome its attraction to the metal and pop out

That minium energy is called work function (w˳)

If the photon has less energy than the work function ,the electron won’t go anywhere .if the photon has more energy than the work function ,then some of photon’s energy will be used up to tear tear  the metal and the rest will give the electron KE.

And some electrons attracted to the metal to the metal than others.
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