All matter that we see is made of tiny particles called atoms. A long time ago, Democritus thought of
an entity that was indivisible, he called it the final stage of all matter. He imagined that what would
he get if he keeps on cutting an item with a very sharp knife? Will there be an ending stage? He stated
that yes there is an end and he named this an atom.
So, according to Democritus, all things were made of tiny indivisible particles called atoms.
According to him, one could not further divide atoms into smaller particles. That would be
impossible according to him. John Dalton was an English school teacher whose theory of atoms was
the first to leave an impact on the scientific community.
He stated that different atoms represented
different elements. He stated that atoms could neither be created, destroyed nor be transformed into
other atoms. In a chemical reaction, according to him, there is only a rearrangement of atoms.
Dalton’s theory assumed atoms to be indivisible and spherical, which we now reject.
Joseph Thomson, discovered electron, a sub-atomic particle contradicting Democritus and Dalton.
His experiment was a consequence of photoelectric effect.
He found that all things were made of
atoms, but in turn, all atoms had a negative charged particle which he named electron. Not only that,
Thomson also predicted the existence of a positive particle – since atoms are neutral as a whole.
Proton, a positive particle was soon discovered.
Proton has equal charge
Proton has charge exactly equal to that on an
electron but the polarity is opposite. An electron is negative, on the other hand, a proton is positive.
Two sub-atomic particles were discovered and the name atom, which meant indivisible in Greek was
contradicting the then theory.
Later experiments showed the possible existence of a third sub-atomic particle. Obviously this
particle had to be electrically neutral otherwise it would make an atom charged up (an atom is neutral
by birth) Rutherford found that masses of atoms found experimentally were not agreeing with those calculated
on paper. On paper, we were considering the masses of protons and electrons, yet we were very short
of the masses found in spectrometer experiments.
Rutherford modified Thomson’s theory by stating that there was another particle inside an atom. To
illustrate this, he had designed an experiment. Locations of protons and electrons were not stated
clearly in Thomson’s model of atom. Rutherford had planned to bombard high-speed alpha particles
(by the way, alpha particle is a charged particle which has charge exactly double of that on a
proton) on a very thin gold-foil. He was actually measuring interaction of alpha particle and gold
atom. He found amazing stuff! The results were so astonishing that he had to totally change the
Thomson had created a model like that of a sphere – so he assumed atoms to be spheres, and he also
assumed that all the mass of atom was uniformly distributed in this sphere – but Rutherford disproved
Rutherford found that most of the alpha particles passed straight through the foil. This meant that most
of the atom was empty space. If it were uniformly distributed with mass, the alpha should have
rebounded, like a light particle hitting a mirror.
Another thing that Rutherford found was that a few particles deflected from their usual path at small
angles, say 15 degrees or so. This meant, that somewhere inside the atom, a few locations were
occupied by electrons, which were applying an electric force on alpha particles, causing them to
He found that one in 20000 alpha particles rebounded exactly at 180 degree angle. He assumed that
the particle must have had hit a very dense solid. This was named nucleus. Rutherford stated in his
atomic theory that most of the mass was located at extremely small and extremely dense area called
the nucleus, located at the centre of the nucleus and that electrons were revolving around the centre
just as planets revolve around the sun. Nucleus contained protons and neutrons, (the neutral particles)
and electrons revolved around the nucleus.
There is one way so that you can experience Rutherford’s experiment. This experiment is discussed in
the revision and I hope that you do this one, it is really interesting.
Now we will try to know how colours are made. The theory of atoms is not yet over. There is a lot of
understanding that is left, you would be amazed but Rutherford’s model was falsified by Neils Bohr,
whose model was again falsified by the collective efforts of Heisenberg and Schrodinger. You see
science and especially physics is about checking and validating.
No matter who you are, if one single
experiment disagrees with your theory, your theory is falsified and no longer valid. For the sake of
simplicity, we will discuss about colours in the simplest way possible, for a detailed and simplified
explanation, you should also invest your time on one of these books:
1. Light, colours and the physics behind, this book is devoted to the physics of colours
2. Let us discover modern physics, if you want to know more about atomic theory by Bohr and
Heisenberg’s modification in the simplest language possible
Colours are actually made by atoms. So imagine a yellowcoloured wall. Why is it yellow? It is yellow because light falls on its surface and it reflects back
only the frequency corresponding to yellow. When it is very dark in the room you are in, you do not
see the wall. So you see the wall, when light falls upon it.
According to Einstein, light is made of
particles which he named photons, so when these photons fell on the wall, having some energy, they
excite atoms of the wall. A neutral and not so excited atom has no absorbed energy. As we already
know, when light falls on a rough surface, most of the incident energy is absorbed and only a
particular frequency is reflected back, which we see as its colour.
When light falls on a very smooth
and very shiny surface, such as a mirror, almost all (sometimes 100%) the light is reflected back, thus
you see your reflection, mirror does not have a colour because all the light that falls on it (which is
white light of sun) is reflected back (which is again a white light)
So what we see as colours are actually frequencies, visible frequencies.
So when light falls on the
atoms of wall, these atoms get excited, an excited atom is the one which has absorbed energy. But
atoms are not stable at all in their excited states. They want to get normal. How can nature make them
normal? It is done by re-emission of the energy that they had absorbed. This re-emission is what we
see as colour. In the dark, no light falls on the atoms of this wall, no excitation occurs and thus no
colours are seen.