2 Particles as Building Blocks of Matter
11
showed a peculiar characteristic of emitting only a discrete set of colors, the
kind of colors depending on the material. For example, if the atoms giving light
are hydrogen atoms, the light is red and for other atoms the color is different. A
Swiss mathematics teacher named Johann Balmer, who at age 60 was studying
the pattern of hydrogen light, concluded that a mathematical formula using
only integers could fit the observed colors. This became known as the Balmer
series. This formula was later generalized by Johannes Rydberg. A continuous
spectra would hardly require only integers to describe them, and the presence
of the integers in the formula for atomic emission was quite a mystery, until
Niels Bohr made a single simple but radical hypothesis to solve this puzzle.
In 1913, Bohr wrote three papers totaling 70 pages in the Philosophical
Magazine. He suggested that the electrons inside atoms move in fixed orbits
characterized by integers. Note that there are gaps between integers (for
example, between one and two, you can have many fractional numbers, but in
the absence of the fractional numbers, there will be a gap). To get the radius
of the orbit of an electron in the atom, Bohr realized that out of a mass and
electric charge, he could not get a length. So he invoked the Planck’s constant,
suggested by Max Planck in 1900, to understand the properties of Black body
radiation. He suggested that the angular momentum of an electron (which
characterizes the electron’s rotational speed around the proton) in an orbit
must be an integer times Planck’s constant. The second fundamental proposal
of Bohr was that the light would be emitted when an electron jumps from a
higher to a lower radius orbit. The frequency of light would have nothing to
do with the orbital frequency of the electron but would be the difference in the
energies of the electron in the two orbits. These were revolutionary and bold
suggestions and were quite contrary to the classical thinking at the time which
would have related the frequency of emitted light to the orbital frequency. The
same integer that was used to describe the orbital angular momentum of the
electron describes the spectral characteristic found by Balmer. Bohr’s theory
proposal was quite different and indeed quite radical.
For atoms, however, the idea of using discrete set of orbits was quite foreign
to scientific thinking in those days. In classical physics before the time of Bohr,
continuity was central to all physical laws except for two discoveries made the
previous decade. The idea of things being discrete in physics was entertained
by Albert Einstein in describing photoelectric effect, the emission of electrons
from certain metals when light impinges on it. The second example was by
Max Planck who described radiation from objects known as black bodies,
which absorb all radiation in discrete units. Not too many physicists working
on the atomic theory of matter paid much attention to these ideas as being
relevant in the world of atoms, until Bohr made his monumental suggestion to
11
showed a peculiar characteristic of emitting only a discrete set of colors, the
kind of colors depending on the material. For example, if the atoms giving light
are hydrogen atoms, the light is red and for other atoms the color is different. A
Swiss mathematics teacher named Johann Balmer, who at age 60 was studying
the pattern of hydrogen light, concluded that a mathematical formula using
only integers could fit the observed colors. This became known as the Balmer
series. This formula was later generalized by Johannes Rydberg. A continuous
spectra would hardly require only integers to describe them, and the presence
of the integers in the formula for atomic emission was quite a mystery, until
Niels Bohr made a single simple but radical hypothesis to solve this puzzle.
In 1913, Bohr wrote three papers totaling 70 pages in the Philosophical
Magazine. He suggested that the electrons inside atoms move in fixed orbits
characterized by integers. Note that there are gaps between integers (for
example, between one and two, you can have many fractional numbers, but in
the absence of the fractional numbers, there will be a gap). To get the radius
of the orbit of an electron in the atom, Bohr realized that out of a mass and
electric charge, he could not get a length. So he invoked the Planck’s constant,
suggested by Max Planck in 1900, to understand the properties of Black body
radiation. He suggested that the angular momentum of an electron (which
characterizes the electron’s rotational speed around the proton) in an orbit
must be an integer times Planck’s constant. The second fundamental proposal
of Bohr was that the light would be emitted when an electron jumps from a
higher to a lower radius orbit. The frequency of light would have nothing to
do with the orbital frequency of the electron but would be the difference in the
energies of the electron in the two orbits. These were revolutionary and bold
suggestions and were quite contrary to the classical thinking at the time which
would have related the frequency of emitted light to the orbital frequency. The
same integer that was used to describe the orbital angular momentum of the
electron describes the spectral characteristic found by Balmer. Bohr’s theory
proposal was quite different and indeed quite radical.
For atoms, however, the idea of using discrete set of orbits was quite foreign
to scientific thinking in those days. In classical physics before the time of Bohr,
continuity was central to all physical laws except for two discoveries made the
previous decade. The idea of things being discrete in physics was entertained
by Albert Einstein in describing photoelectric effect, the emission of electrons
from certain metals when light impinges on it. The second example was by
Max Planck who described radiation from objects known as black bodies,
which absorb all radiation in discrete units. Not too many physicists working
on the atomic theory of matter paid much attention to these ideas as being
relevant in the world of atoms, until Bohr made his monumental suggestion to