2.2 The Quantum Nature of Light
11
2.2 The Quantum Nature of Light
Photons are emitted when there is a change in the energy state of a system. For
example, the model often used when the atom is introduced to students is the idea of
the electron orbiting the nucleus. Although this is strictly not true in all situations,
it is a good enough model for most purposes. Electrons that are orbiting close to the
nucleus have less energy than those orbiting farther away. Hence when an electron
gains sufficient energy, it moves to a higher orbit, and when an electron emits a
photon, losing energy, it moves to a lower one.
In the late 1800s, Heinrich Hertz studied the photoelectric effect. He noticed
that certain materials would emit electrons when exposed to light—but the release
occurred only when they were exposed to certain wavelengths. If the wavelength was
wrong, it didn’t matter how intense the light was. No current would be observed.
This was something of a puzzle—how did the materials “know” that they were being
exposed to the “wrong” sort of light? Why would they seem to care about anything
except luminous intensity?
It was Albert Einstein who suggested a solution (and it was actually this solution
that earned him his Nobel Prize). He suggested that light is quantised, that is, that
its energy is delivered in discrete packets. These packets are, of course, the particles
that we call photons. Also, Einstein theorised that instead of there being a uniform
continuum of energy levels in which electrons can exist, there are actually just a few
discrete ones—that is, electron energy levels are quantised too. For an electron to
move between these fixed levels, an incoming or outgoing photon has to have just
the right energy. If it is wrong, then the photon cannot absorb it and thus will not
respond. This is why the materials in Hertz’s experiments reacted only to specific
wavelengths of light.
As to why electrons display this behaviour, imagine them for a moment not as
particles, but rather as waves that are wrapped around an atomic nucleus. For a
wave to wrap neatly, its orbital circumference has to correspond to a multiple of its
wavelength. If the circumference doesn’t follow this pattern, then some of the wave’s
peaks and troughs will overlap each other, and the wave will interfere with itself.
Applying this to an electron, we can see that a situation would arise in which an
electron could interfere itself out of existence, thus strongly violating the principle
of conservation of energy. This has the effect of forbidding electrons from occupying
such orbits.
These insights, along with others, led to the emergence of quantum mechanics.
It is worth noting that although quantum mechanics has many weird aspects, it is
nonetheless one of the most successful and important scientific theories ever developed. It underlies not just the multibillion-dollar fields of electronics and computing
but has practical applications throughout the whole of industry and commerce. It
even has implications for seemingly unrelated fields such as biology and medicine,
through applications such as imaging machines and analyses of protein folding.
Whilst quantum theory is certainly weird, it is a necessary weirdness.
11
2.2 The Quantum Nature of Light
Photons are emitted when there is a change in the energy state of a system. For
example, the model often used when the atom is introduced to students is the idea of
the electron orbiting the nucleus. Although this is strictly not true in all situations,
it is a good enough model for most purposes. Electrons that are orbiting close to the
nucleus have less energy than those orbiting farther away. Hence when an electron
gains sufficient energy, it moves to a higher orbit, and when an electron emits a
photon, losing energy, it moves to a lower one.
In the late 1800s, Heinrich Hertz studied the photoelectric effect. He noticed
that certain materials would emit electrons when exposed to light—but the release
occurred only when they were exposed to certain wavelengths. If the wavelength was
wrong, it didn’t matter how intense the light was. No current would be observed.
This was something of a puzzle—how did the materials “know” that they were being
exposed to the “wrong” sort of light? Why would they seem to care about anything
except luminous intensity?
It was Albert Einstein who suggested a solution (and it was actually this solution
that earned him his Nobel Prize). He suggested that light is quantised, that is, that
its energy is delivered in discrete packets. These packets are, of course, the particles
that we call photons. Also, Einstein theorised that instead of there being a uniform
continuum of energy levels in which electrons can exist, there are actually just a few
discrete ones—that is, electron energy levels are quantised too. For an electron to
move between these fixed levels, an incoming or outgoing photon has to have just
the right energy. If it is wrong, then the photon cannot absorb it and thus will not
respond. This is why the materials in Hertz’s experiments reacted only to specific
wavelengths of light.
As to why electrons display this behaviour, imagine them for a moment not as
particles, but rather as waves that are wrapped around an atomic nucleus. For a
wave to wrap neatly, its orbital circumference has to correspond to a multiple of its
wavelength. If the circumference doesn’t follow this pattern, then some of the wave’s
peaks and troughs will overlap each other, and the wave will interfere with itself.
Applying this to an electron, we can see that a situation would arise in which an
electron could interfere itself out of existence, thus strongly violating the principle
of conservation of energy. This has the effect of forbidding electrons from occupying
such orbits.
These insights, along with others, led to the emergence of quantum mechanics.
It is worth noting that although quantum mechanics has many weird aspects, it is
nonetheless one of the most successful and important scientific theories ever developed. It underlies not just the multibillion-dollar fields of electronics and computing
but has practical applications throughout the whole of industry and commerce. It
even has implications for seemingly unrelated fields such as biology and medicine,
through applications such as imaging machines and analyses of protein folding.
Whilst quantum theory is certainly weird, it is a necessary weirdness.