Ilango Arasiyal

சனி, 7 ஜூலை, 2018

light and quantum physics (clerro's explaination)
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"What is light?", has been one of the most fundamental questions in understanding our world and intrigued a lot of scientists. A decisive answer was found in 1801 when a scientist named Thomas Young passed a beam of light through two narrow vertical slits. His experiment was called 'The Double-Slit Experiment'.

Astoundingly, the light beam passed through the two slits and created a pattern that is normally seen to be created by a wave (like a water wave). You can see below what this result looked like:

(source)

The beam split into two individual waves of light and which then interfered with each other while moving towards the screen.

When the two waves interfered, their brightness added up in some spots, and at some spots they cancelled each other out to create a dark region. This created an interesting pattern of increasing and decreasing brightness called an 'Interference Pattern', on the screen (or wall).

Scientists already knew how interference happens in other waves (like water waves) to create the interference pattern. So when Thomas Young found the same interference pattern being created by light in this experiment, it was quite clear that light is a wave.

But what is the light wave made of?

Much later (in 1873), another scientist James Clerk Maxwell published his 'Electro-Magnetic Theory of Light' where he showed mathematically that the speed of electromagnetic waves was equal to the speed of light . This proved that light must be an electromagnetic wave. Electro-magnetic waves are special kind of waves made up of varying electric and magnetic fields perpendicular to each other (as shown below).

So light is made up of electric and magnetic fields.

[This was however, just to feed your curiosity. All you need to understand here is that light is a wave.]

Chapter 3 of 10

Light Is Also Particles

In 1902, a scientist named Philipp Lenard conducted an experiment whose results raised some really puzzling questions on light for scientists.

The Experiment : "Photoelectric Effect"

The experiment involved shining light on a metal surface. It was observed that this ejects electrons from the atoms on the metal's surface. Energy contained in the light rays was transferred to the electrons pushing them out of the atoms. A part of the energy from the absorbed light is used by the electron to break free from the atom and the rest of the energy gives the electron its speed (in the form of Kinetic Energy) after it has escaped. This was called "The Photoelectric Effect".

But Two Puzzling Questions Arised

Why did the speed (kinetic energy) of the ejected electrons not increase or decrease accordingly, when the intensity (the brightness) of the light falling on the metal was increased or decreased.
If light was a wave, then increasing/decreasing the intensity should increase/decrease the energy contained in the light, thus the kinetic energy of the ejected electrons should change accordingly. Observation showed, this did not happen.
Why did the electrons not take extra time to get ejected from the atoms when the intensity of light was decreased to very low levels?
If light was a wave, then decreasing the intensity should decrease the energy contained in the light, thus it would take more time for the electron to absorb enough energy so that it can escape from the atom.

Einstein's Brilliant Answer

In 1905, in a stroke of genius, Einstein provided a radical solution to the above questions. He argued, that the puzzling observations can be explained by considering that light was made up of small packets of equal energy (we now call them 'photons' of light).

To eject an electron out of an atom, a packet of light hitting the electron must have enough energy to detach it from the atom.

If the light is made up of packets with less energy than required, doesn't matter how many packets are thrown at the electron (however much or for however long), the light would never be able to cause the electron to escape the metal surface.

This theory helped very easily understand the puzzling observations made in the experiment -

[If you're interested in knowing how, read further. But if you're already baffled at this point, all you need to keep in mind is that, Einstein's theory showed that light could also behave as a particle. And that he won a Nobel prize for this.]

Increasing/Decreasing the intensity of light actually just increases or decreases the number of photons hitting the surface at a time. It doesn't increase/decrease the energy contained within each photon, so the energy transferred to the electron remains same and thus there is no change in the kinetic energy of the ejected electrons.
An electron gets ejected from an atom, as soon as a photon with sufficient energy hits it. It doesn't absorb a photon and wait for another photon to fulfil the energy requirement. Thus there is no time delay, even if the intensity of light is low (number of photons is low).

Chapter 4 of 10

Even Particles Act Like Waves

While the idea that light was a wave could explain the interference pattern, it failed to explain the photoelectric effect. So we had to accept that it also behaved like particles in some cases. At the time, the wave nature of light was seen as an established idea and Einstein's idea of light particles was seen as a mistake by most of the scientific community.

One scientist however, Louis De Broglie, took Einstein's radical idea seriously and proposed something even more radical in 1924.

He proposed that -

all matter (from massive objects to as small as electrons) could also behave as waves and even gave a mathematical equation for the wavelength of such a wave.

Experiments conducted later in 1927, showed that -
De Broglie was right!
A number of particles including electrons, atoms and even molecules were seen to behave like waves. They created special diffraction and interference patterns traditionally seen only in waves.